Pigments—Lead-based whites, reds, yellows and oranges and their alteration phases

Abstract

This review summarises the state-of-the-art of lead-based pigment studies, addressing their production, trade, use and possible alteration. Other issues, such as those related to the investigation and protection of artworks bearing lead-based pigments are also presented. The focus is mineralogical, as both raw materials and degradation products are mineral phases occurring in nature (except for very few cases). The minerals described are abellaite, anglesite, blixite, caledonite, challacolloite, cerussite, cotunnite, crocoite, galena, grootfonteinite, hydrocerussite, laurionite, leadhillite, litharge, macphersonite, massicot, mimetite, minium, palmierite, phosgenite, plattnerite, plumbonacrite, schulténite, scrutinyite, somersetite, susannite, vanadinite and an unnamed phase (PbMg(CO₃)₂). The pigments discussed are lead white, red lead, litharge, massicot, lead-tin yellow, lead-tin-antimony yellow, lead-chromate yellow and Naples yellow. An attempt is made to describe the history, technology and alteration of these pigments in the most complete manner possible, despite the topic's evident breadth. Finally, an insight into the analytical methods that can (and should) be used for accurate archaeometric investigations and a summary of key concepts conclude this review, along with a further list of references for use as a starting point for further research.

Premise

This Topical Collection (TC) covers several topics in the field of study, in which ancient architecture, art history, archaeology and material analyses intersect. The chosen perspective is that of a multidisciplinary scenario, capable of combining, integrating and solving the research issues raised by the study of mortars, plasters and pigments (Gliozzo et al. 2021).

The first group of contributions explains how mortars have been made and used through the ages (Arizzi and Cultrone 2021, Ergenç et al. 2021, Lancaster 2021, Vitti 2021). An insight into their production, transport and on-site organisation is further provided by DeLaine (2021). Furthermore, several issues concerning the degradation and conservation of mortars and plasters are addressed from practical and technical standpoints (La Russa and Ruffolo 2021, Caroselli et al. 2021).

The second group of contributions is focused on pigments, starting from a philological essay on terminology (Becker 2021). Three archaeological reviews on prehistoric (Domingo Sanz and Chieli 2021), Roman (Salvadori and Sbrolli 2021) and Medieval (Murat 2021) wall paintings clarify the archaeological and historical/cultural framework. A series of archaeometric reviews illustrate the state of the art of the studies carried out on Fe-based red, yellow and brown ochres (Mastrotheodoros et al. 2021); Cu-based greens and blues (Švarcová et al. 2021); As-based yellows and reds (Gliozzo and Burgio 2021); Pb-based whites, reds, yellows and oranges (this paper); Hg-based red and white (Gliozzo 2021); and organic pigments (Aceto 2021). An overview of the use of inks, pigments and dyes in manuscripts, their scientific examination and analysis protocol (Burgio 2021) as well as an overview of glass-based pigments (Cavallo and Riccardi 2021) are also presented. Furthermore, two papers on cosmetic (Pérez Arantegui 2021) and bioactive (antibacterial) pigments (Knapp et al. 2021) provide insights into the variety and different uses of these materials.

Introduction

The average abundance of lead in the Earth’s crust is 14 ppm (Lide 2008).

Lead is a chalcophile and amphoteric element, which can be attacked by alkalis and acids. While it is rarely found as a native element (included in the copper group), Pb-bearing minerals are rather widespread. These minerals are mostly oxides, sulfides/sulfates/sulfites and hydroxides, but several other types, such as carbonates, chlorides/chlorates, chromates, oxalates and acetates, may also be abundant. Galena is the most common Pb mineral, followed by its secondary phases, anglesite and cerussite. Lead-based minerals have been used for multiple purposes, from colouring to protective/isolating coating, cosmetics and pharmaceutics (Gunn 1973; Walter et al. 1999, 2003; Welcomme et al. 2006; Tapsoba et al. 2010; Vidale et al. 2016; Baraldi et al. 2020; Photos-Jones et al. 2020; Wang et al. 2020a).

This review intends to take stock of lead-based pigments, focusing on the various mineral phases that can be found during the investigation of paintings. This perspective facilitates the reader and implies an indication of the methods used in this field (i.e. identifying minerals as the primary objective of research aimed at characterising inorganic pigments).

With this aim in mind, this review is broken down into seven parts.

The first part deals with the main lead-based pigments, i.e. the mineral phases used as pigments or raw materials for their production. In each subsection, the macroscopic characteristics, structural parameters and production cycles are described. Although most of these pigments have resulted from a technological process, it is not yet possible to completely exclude that, in some cases, the painters also used natural correspondents. Therefore, some brief information on the formation and diffusion areas in nature¹ is provided to stimulate research dealing with the supply of raw materials. After describing the main characteristics of the mineral(s) and the corresponding pigment, alteration is addressed. Finally, in each subsection, an overview of the use and occurrence of the pigment/s in certain artworks is also provided.

The second part deals with the lead-based minerals found in paintings, as alteration phases of the pigments outlined in the first part. For each phase, mineralogical information is provided, and its occurrence in works of art is reviewed. Here, in addition to the most common or most used minerals, we also present some lesser-known phases which could be found in natural association with the former, sometimes with macroscopically indistinguishable colours or habiti. Other minor and/or rarer phases, such as rickturnerite (Pb₇O₄[Mg(OH)₄](OH)Cl₃; Rumsey et al. 2012b), shannonite (Pb₂OCO₃; Roberts et al. 1995) and fassinaite (Pb₂²⁺(S₂O₃)(CO₃); Bindi et al. 2011) have been omitted, as their structural parameters can hardly induce the analyst to confuse them with other phases; furthermore, their presence in artworks has not yet been documented. However, it is worth noting that shannonite may be present in cerussite-based compounds and, analogously to hydrocerussite, transforms to cerussite in water (Kotulanová et al. 2009). For space reasons, only those studies directly connected with the characterisation of pigments were considered; however, it is worth emphasising that a conspicuous body of literature exists on the alteration processes (especially for lead soaps²). The reader is invited to deepen this topic, as the outstanding information achieved so far on the modality and kinetics of the saponification processes represents critical knowledge for evaluating degradation and developing appropriate conservation strategies (Meilunas et al. 1990; Lazzari and Chiantore 1999; Erhardt et al. 2000; Higgitt et al. 2003; Plater et al. 2003; Saunders and Kirby 2004; Keune 2005; Van der Weerd et al. 2005; Boon and Ferreira 2006; Cotte et al. 2006, 2007; Doménech-Carbó et al. 2006; Dietemann et al. 2009; Manzano et al. 2009; van den Brink et al. 2009; Catalano et al. 2014; Hermans 2017; Baij et al. 2018; Casadio et al. 2019; Poli et al. 2019; Garrappa et al. 2020).

The third part deals with the production centres of the pigments outlined in the first part. In this brief chapter, the little information available to date is summarised.

The fourth part deals with lead-based pigments in which lead is combined with tin and/or antimony and chromium (except for natural crocoite, which is discussed in the first part). This section details the mineralogical description, production process and occurrence in artworks of lead-tin yellow I–II, lead-antimony yellow (Naples yellow), lead-tin-antimony yellow and lead chromates. Terminological issues are also addressed. Conversely, the intensive study of these materials as colouring agents in the field of glass and ceramic coatings has created such a body of literature that it would not have been possible to discuss it properly here. Therefore, the reader is referred to a short list of references that any researcher should read before approaching the archaeometric study of lead-containing glass and ceramic coatings, as follows: Turner and Rooksby (1959), Hedges and Moorey (1975), Hedges (1976), Henderson and Warren (1983), Mass et al. (1996, 2002), Mason and Tite (1997), Freestone and Stapleton (1998), Pernicka et al. (1998), Tite et al. (1998, 2008), Molera et al. (1999), Heck and Hoffmann (2000), Shortland (2002), Rosi et al. (2011) and Matin (2018).

The fifth part provides some brief notes, first on binding media and then on the diffusion of pigments. For this purpose, a literature review was performed and the list of consulted references is provided in the Supplementary Materials, Appendix 1. The geographic distribution and the association of lead-based pigments with other inorganic and organic materials are schematically described.

Based on the literature review, the sixth part provides an overview of the analytical methods used to investigate paintings. The advantages and disadvantages, with respect to the research questions, are highlighted for each technique.

The final part summarises the key points that should be known and considered in the study of lead-based pigments and their alteration phases.

Before delving into the matter, it is necessary to add an essential clarification on the chemical reaction equations reported for the various phases. The processes occurring in nature, the production cycles that lead to the creation of a pigment and the alterations arising in paintings can only take place under certain conditions. In nature, the pressure, temperature, oxygen fugacity and fluid circulation are some of the determining factors. For the production and alteration of pigments, the temperatures, relative humidity, lighting conditions, presence of carbon dioxide, water and sulfides and interactions with the binding media are the principal factors to be considered. Therefore, a reaction that may occur in nature does not necessarily happen in a painting and vice versa. For use of the reactions provided, the researcher must verify whether the reaction conditions are satisfied. A pressure of 1 atm represents the only constant for the reactions provided throughout this paper.

Main lead-based minerals, pigments and dryers

Lead white—cerussite (PbCO₃) and hydrocerussite (2PbCO₃·Pb(OH)₂)

Mineralogical phases, structure and occurrence

Named after the Latin cerussa (Table 1), cerussite is a colourless to white (sometimes with grey, blue or green hues) carbonate (Table 2; Figure 1A). It is a member of the aragonite (CaCO₃) group, together with strontianite (SrCO₃) and witherite (BaCO₃). The type locality is in the Vicenza province in Italy (Boscardin et al. 2016), and it is typically found in the oxidised zone of lead deposits (Table 3).

Table 1 Literary sources (the texts in the original language are provided in Supplementary Materials, Table S2). Translations: Rackham (1952) for Plinius; Osbaldeston and Wood (2000) for Dioscorides; Caley and Richards (1956) for Theophrastus; Morgan (1914) for Vitruvius. Other texts, personal translation. The text of (H)Eraclius is from Heraclius (1873)

Table 2 Chemical formulae, space groups and structural parameters of Pb-based minerals mentioned in the text [Pb content from Webmineral database (n.d.), if not differently specified; N/S, natural/synthetic]

Fig. 1

Cerussite (A) and hydrocerussite (B). Standard chemical reaction of formation (Franceschi et al. 2010 after Weast 1998), thermodynamic data and chemical reaction involving its transformation. Mineral photos: cerussite and hydrocerussite from the Tsumeb Mine, Namibia [Arkenstone specimens. Photo credits: Rob Lavinsky, irocks.com–Mindat.org Photo ID 18565 and 173886 – Copyrighted. Permission has been obtained for reproduction in this article]. The CC-BY licence does not supersede previously copyrighted material; therefore, these images remain under owner’s copyright]

Table 3 Occurrences and associations of the phases mentioned in the text (alphabetical order). Data from publications quoted in the dedicated sections (esp. Ibáñez-Insa et al. 2017; Siidra et al. 2018c. In addition: Spencer and Mountain 1926; Embrey and Hicks 1977; Pinch and Wilson 1977; Turner 2006) and from Anthony et al. (1990) (http://www.handbookofmineralogy.org/). Mineral names are given according to the list provided by International Mineralogical Association (2020)

Hydrocerussite is a white or light grey carbonate containing a hydroxyl group (Table 2; Figure 1B), named in 1877 by Nils Adolf Erik Nordenskiöld (Nordenskiöld 1877). Its type locality is Långban Mine in Sweden (Siidra et al. 2018b). The structure of hydrocerussite and its related phases (chiefly abellaite and grootfonteinite) is characterised by "electroneutral [PbCO₃]⁰ cerussite type layers or sheets (...) separated by the stereochemically active 6s² lone electron pairs on Pb²⁺ cations" (Siidra et al. 2018b).

Both cerussite and hydrocerussite are relatively abundant in nature (Anthony et al. 1990; Mindat.org) and are mostly found in the oxidised zone of lead deposits (Table 3). Notable occurrences are the Tsumeb and Kombat Mines in Namibia, the Monteponi and Montevecchio Mines in Sardinia (Italy), Příbram (Czech Republic), Friedrichssegen Mine in Rhineland-Pfalz (Germany), Merehead Quarry in Somerset (UK), Leadhills in Lanarkshire (Scotland, UK), Nerchinsk lead deposit in Zabaykalsky Krai (Siberia, Russia), Seh-Changi Mine near Neyband in Khorassan (Iran), Mont Saint-Hilaire (Quebec, Canada), Touissit Mine in Morocco, Broken Hill in New South Wales (Australia), Bunker Hill Mine (Idaho, USA), Leadville in Colorado (USA) and the Mammoth-St. Anthony Mine (Arizona, USA).

Most research has focused on the chemical and thermodynamical equilibria between these two phases and other lead carbonates (Figure 1), and experimental tests have been systematically performed in varying conditions (Bilinski and Schindler 1982; Flemming et al. 1984; Taylor and Lopata 1984; Petushkova and Lyalikova 1986; Mercy et al. 1998; Sánchez-Navas et al. 2013; Mendoza-Flores et al. 2017). Indeed, a conspicuous number of studies has also aimed to understand the speciation of lead in soils, groundwater and streams and to find remediation technologies (Basta and McGowen 2004). The results achieved may help to better understand the behaviour of these phases in nature (i.e. supply areas) and artworks, as well as to recover essential clues on the production technology of lead white (psimythion/psimithium - cerussa; Table 2, P3).

Of utmost importance is the fact that this pigment can be made up of a single component (i.e. cerussite or hydrocerussite), but it often includes varying amounts of both components (sometimes even including lead acetate).

The production of lead white

Not much is known about the possible exploitation of natural minerals. Plinius reported that ‘native’ cerussite was found at the estate of Theodotus’s at Smyrna (Table 1, P4), but he did not specify who the painters that employed it "in old days for painting ships" were. In the same paragraph, he also drew a distinction between the artificially produced variety, used in his time, and the natural one, which was in disuse by that time.

As for white lead recipes, Theophrastus suggested leaving a piece of lead the size of a brick in jars over vinegar for about 10 days, then to scrape off the "kind of mold" that formed, repeating the operation until the total conversion of the piece of lead initially introduced had taken place (Table 1, T1). Plinius added that production took place at the same time/place as that of lead and mentioned two different methods: suspension over vinegar and immersion in vinegar (Table 1, P3).

Similar procedures were also described by Vitruvius (Table 1, V1), Dioscorides (Table 1, D2) and (H)Eraclius. This last author described both the immersion and heating procedures in his De coloribus et de artibus Romanorum between the 7^th and 12^th centuries (Table 1, E1). He also provided some technical advice for the diversification of white and red lead production.

In summary, two main methods have been described in the literature for producing lead white: (1)suspensions—in open or sealed vessels—and immersion. The two procedures inevitably led to the formation of different compounds (due to different carbon dioxide circulation and availability and bacterial activity) and it must also be considered that the immersion procedure has not yet been experimentally tested. Simplifying matter, only lead acetate may form if only acetic acid vapours rising from the vinegar can reach the lead. The decomposition of organic matter (e.g., horse manure, tanbark, urine) described by the ancient authors (as well as by more recent documentary sources and experimental tests) could then produce both the carbon dioxide and the heating required to trigger the reaction transforming lead acetate into lead carbonates.

Photos-Jones et al. (2020) clarified that the activity of aerobic/anaerobic bacteria is crucial for the production/consumption of CO₂/O₂—i.e. the bulk of the gases within the pot—and showed that in closed systems (sealed vessels), the composition of the oxos/vinegar is fundamental. They carefully examined the recipe handed down by Theophrastus, in order to verify to what extent it is possible to obtain a psimythion of pure cerussite, as well as the role of the oxos (spoiled wine) when used in place of vinegar. Starting from the analysis of pellets from the 5^th to 4^th centuries BC, the authors performed DNA sequencing (which did not reveal the presence of microorganisms) and proposed to recognise the key driver of psimythion synthesis in the reactions between abiotic (lead phases) and biotic (e.g., Acetobacter in spoiled wine) components.

Taking a considerable leap in time, in the 15^th century, Venice was the main producer of the lead white variety named ‘Venetian white’ (Beck et al. 2020; Hendriks et al. 2020). However, according to the contemporary recipe handed down by an anonymous author in the Bologna Manuscript named Segreti per colori, the process does not seem remarkably different from that adopted in antiquity (Table 1, BM), including scraps of lead, vinegar steam and a jar cover with mud (luto) put under manure. The recipe has also been reported by Merrifield (1849) but with a slightly different translation.

During the 16^th and 17^th centuries, another method was developed for industrial applications: the ‘Dutch’ or ‘stack’ process. This procedure is similar to the ancient ones, as it involves the exposure of lead strips or 'buckles' to vinegar for 1 to 3 months (Gettens et al. 1967). The main differences, compared to the ancient procedures, seem to lie in the following:

• (a) the type of clay pot used³—although we do not know the exact shape (nor porosity) of the ancient vessels, except that they were (broad-mouthed)jar-shaped and may have had a lid (Table 1, D2, P3, P5, T1, V1);

• (b) the stacking of some hundreds or thousands of these pots in rows and their embedding in fermenting tanbark or dung (i.e. external source of CO₂), which could produce both heat and carbon dioxide (Thompson 1971).

Development of the stack process led to the widespread commercialisation of lead white. Between the 17^th and 18^th centuries, The Netherlands were the major producer, as attested by the "over 35 lead white factories" present by the end of the 18^th century (Hendriks et al. 2020 quoting Homburg and De Vlieger 1996). The development of England's production is also to be placed shortly after (Rowe 2017). From this moment on, the method remained essentially unchanged in principle; however, thanks to the discovery of carbon dioxide in the late 18^th century, it was better understood and adapted to increase its productivity.

For example, the ‘chamber method’—generally attributed to a patent awarded to James Creed in 1749—exploited heat flowing into a chamber (see Stols-Witlox 2014 on the possibility that this reconstruction is not entirely true). The ‘German method’, implemented by Franz von Herbert at the end of the 18^th century, represents the development of the ‘chamber method’ and sure proof of its use in the industrial field. It was developed following the innovations introduced by Herbert, who used fruit must vinegar produced by his own orchard in Wolfsberg (Austria) and, successively, wine lees from his Klagenfurt factory to make the famous ‘Krems white’ (Stols-Witlox et al. 2012, 2014; Hendricks et al. 2019). There is no agreement in the literature on the denomination and attribution of the various methods; however, the ‘philological’ problem does not significantly change the overall reconstruction.

In the 19^th century, white lead found a valuable competitor in zinc oxide (Roy 1993; see also Spennemann 2020), but its use did not cease.

For example, the reports compiled by Jussen—general consul of the United States in Austria (Jussen 1887; see Supplementary Materials, Appendix 2)—informed of the existence of the varieties named Kremserweiss (also Cremnitz White and Kremnitz White) and Venetianerweiss (whose history was lost in the past). He also listed several methods used to produce lead white (i.e. Lothman, Gannal, Fourmetier, Pattinson and Chenot, along with the method employed in the lead white factory of Theodor Lefebre & Co.). Over the centuries, the process was thus refined, where some treatments conducted before and/or after the synthesis, such as grinding, washing (in water or acids) and further heating (also in water), led to the production of numerous varieties of lead white (see, e.g., Stols-Witlox 2011; Stols-Witlox et al. 2012; Gonzalez et al. 2017a). Modern chemistry has also substantially modified the production process, using precipitation and hydrolysis to obtain lead white from metallic lead (both in a dry environment and by water immersion).

Excluding carbonate-based whites, white lead remained "the most important of all white pigments" (Gettens et al. 1967) until the early 20^th century, when titanium whites became predominant (Eastaugh et al. 2004).

At the beginning of the 21^st century, lead white was banned from the market, due to its toxicity (but not in all countries; since 2015 in Europe).

Archaeometric investigations have confirmed the information handed down by the authors and have clarified both the processes and the nature of the resulting products (Figure 2). Gonzalez et al. (2019b) reproduced the ‘stack’ process—although "not faithfully" as claimed by the authors—and clearly reconstructed the reaction path leading to the formation of plumbonacrite, hydrocerussite and cerussite. These authors also demonstrated that, during the ‘Dutch Golden Age’, the ‘classic ratio’ between cerussite and hydrocerussite was 30:70. On the same topic, the chemical reactions for hydrocerussite formation have also been reported by Sánchez-Navas et al. (2013; derived from previous literature). These authors used the ‘stack’ process to investigate crystal growth features that can be affected by the medium in which the precipitation occurs. They found that cerussite and hydrocerussite crystals showed different morphology and crystallinity depending on the medium in which the precipitation occurred.

Fig. 2

The production of lead white and minium starting from lead sheets and acetic acid. The pot for corroding lead white was patented by J.H. Chadwick (US Patent #: US000051018) on 21 November 1865

For example, cerussite crystals formed from gels showed dendritic or acicular habitus (or aggregates with pseudocubic forms), while hydrocerussite formed in a biomediated experiment was poorly crystalline.

A possible differentiation in the type of raw materials used for production has been suggested by Niknejad and Karimy (2019). These authors provided an in-depth analysis on the production of Persian lead white (sefidāb-i-sorb), starting from the analysis of three technical treatises: the Bayān al-sanāāt (second half of the 12^th century), the Gowhar-nāmeh (15^th century) and the Qānoun al-sovar (1533–1608). The reconstruction of the process allowed for the observation that the raw material (Murdārsanj, i.e. PbO) mixed with salt (NaCl) led to the formation of laurionite (Pb(OH)Cl), blixite (Pb₈O₅(OH)₂Cl₄) and phosgenite (Pb₂Cl₂(CO)₃), thus also providing a possibly discriminating element between European and Persian productions⁴.

Lead white in artwork

Between the 4^th century BC and the 4^th century AD, attestations of the use of lead white, regarding the Mediterranean area, are few and limited to coffin cartonnage, earthenware, wall paintings and wood portraits found in Egypt, Italy and, possibly, Ukraine (e.g., Sack et al. 1981; Smith and Barbet 1999; Scott et al. 2003; Aliatis et al. 2010; Salvant et al. 2018; de Ferri et al. 2019; Abdallah et al. 2020; Tarquini et al. 2020).

According to Plinius, white lead was indispensable, in the Roman period, for representing shadows (Table 1, P5); however, the identification of this pigment in the waterproofing caulking of the 1^st BC to the 1^st AD San Rossore shipwrecks (Pisa, Italy) testifies to its further use, beyond decorative purposes (Colombini et al. 2003).

Between the 4^th and 7^th centuries, such attestations are even fewer. Some hydrocerussite occurrences have been reported in the wall paintings of Coptic monasteries and on Sutra parchment both from Wadi El Natrun in Egypt (Moussa et al. 2009) and China (Liu et al. 2016, 2019).

Conversely, starting from the 8^th century AD, there was an increase in its use, culminating in a real boom in the 17^th century (Figure 3A). Although this reconstruction is affected by the ‘state-of-the-art effect’, it can be considered true, in general terms.

Fig. 3

A simplified overview of the use of lead white (A) and red lead (B) over the centuries. Information relating to the occurrences of the various phases in artworks was drawn from personal filing, using the references listed in Supplementary Materials, Appendix 1. The occurrences include, without distinction, all types of supports and techniques

Lead white has been found on sculptures in alabaster, limestone, sandstone, ceramic and wood, as well as on earthenware (e.g., ‘campana’ reliefs), wall paintings, oil paintings on canvas, glass and wood icons, paper (e.g., ‘lajan’), wallpaper and parchment in illuminated manuscripts and even on wooden musical instruments (e.g., a harpsichord).

The painters were perfectly aware of the instability of lead pigments in fresh lime plasters (e.g., alkaline and humid environments); therefore, they used the a secco technique for them selectively (see, e.g., Cavallo et al. 2012; Liu et al. 2016; Levstik et al. 2019; Figures 4A, B). Consequently, the combination of fresco and secco techniques has been frequently reported in wall paintings (see also Gebremariam et al. 2013; Hradil et al. 2013; Malletzidou et al. 2019).

Fig. 4

Details of the ‘Madonna and Child enthroned with Saints’ painted by Ambrogio Lorenzetti in the first half of the 14^th century AD (Damiani et al. 2014). The wall painting is preserved in the St. Augustine Church (Siena, Italy). A For the vest of the Child, white lead was painted a secco. B The vest of Saint Agatha was painted a secco using white lead. C The orange hues of the throne formed by the wings of the cherubim were painted with red lead. D Scanning electron microscope, backscattered electrons imagery, showing a colony of fungi formed on a layer of white lead. [The CC-BY licence does not supersede previously copyrighted material; therefore, these images remain under owner’s copyright.]

The distemper technique has also been found in early 3^rd to 4^th centuries AD Egyptian paintings on canvas (Sack et al. 1981), through to a 19^th-century painted plafond of a house in St. Petersburg (Russia; Petrova et al. 2019; lead white uncertain). Egg-tempera (e.g., Stanzani et al. 2016; Serafima et al. 2019a), oil tempera (Fiorillo et al. 2020), ‘tempera grassa’ (Cardell et al. 2009; Bratu et al. 2015) and modified egg-tempera (e.g., Lazidou et al. 2018) represent the most common techniques used for wood painting, especially for icons.

Typical uses of lead white also regard the preparation of the priming layers and grounds and for the application of thin foils. In the first case, the examples are very numerous between the 12^th and 19^th centuries. For example, priming layers constituted of lead white has been found (a) in medieval (~12^th – 13^th centuries) polychrome sculptures and reliefs of the Ferrara Cathedral and Parma Baptistery (Pinna et al. 2020); (b) in several oil paintings on canvas, such as those by Carlo Bononi (The Coronation of the Virgin, Ferrara 1617; Impallaria et al. 2020), Jan de Bray (Judith and Holofernes, 1659 on oak panel; Albertson et al. 2019) and several Spanish painters of the 16^th – 17^th centuries (Doménech-Carbó et al. 2019); and (c) in a 16^th-century Coptic icon, mixed with hemp fibres, animal glue and gypsum (Abdel-Maksoud et al. 2020). Two priming layers of lead white and oil (first) and lead white mixed with lead-tin yellow I (second) have even been found in the Salvador Mundi, allegedly painted by Leonardo da Vinci around 1500 (Gutman Rieppi et al. 2020).

The realisation of grounds with lead white is even more frequent. For example, (a) in oil paintings on canvas such as those by Bartolomé Esteban Murillo (linen canvas, 1618–1682, Seville; Križnar et al. 2019); (b) in the 17^th-century Martyrdom of Saint Sebastian, painted by Cecco del Caravaggio (Jasiński 2019); (c) in the polychrome plasterworks of the Alhambra complex at Granada (Spain; Arjonilla et al. 2019b); (d) in late 19^th-century paintings, such as Exit from the Theater, attributed to Honoré Daumier (1863; Smieska et al. 2019); and (e) in the 16^th-century oil paintings on wood altarpiece by the Portuguese painter Garcia Fernandes (act. 1514–1565), kept in the Goa Cathedral in India (Antunes et al. 2018).

In this regard, the use of lead white for the coating of 17^th to 20^th century Lajian (wax-coated) paper has been investigated by Li et al. (2020c); for its use in manuscripts, the reader is referred to Burgio (2021), in this TC.

As for the use of lead white in gilding, the studies carried out by Sansonetti et al. (2010) have illustrated some possible stratigraphic sequences. The authors carried out extensive research on 17^th- to 19^th-century stuccoes in four different monuments in Lombardy (Italy) and found different sequences in the gilding preparation layer⁵.

Further analytical evidence has been provided by (a)Blasco-López et al. (2016), who investigated the 14^th- to 17^th-century plasterwork of the Salón de Embajadores in the Royal Alcázar of Seville (Spain) and found cerussite and linseed oil as a preparation layer for gold foil (covering a gypsum layer); (b) Pinna et al. (2020), who investigated the medieval porch of the main portal of the Ferrara Cathedral (Italy) and found a mixture of white lead, lead-tin yellow I, red ochre and anatase in the preparation layer of gold gilding; and (c) Sansonetti et al. (2020), who investigated 17^th-century stuccoes made by the Italian ‘Artists of the Lakes’ in Lombardia⁶ (Italy) and found gold leaf applied over a 'missione' made of chrome yellow or ochre mixed with lead white.

As for its association with other inorganic materials, gypsum (Armenini 1856) and, more frequently, calcite/chalk have been reported (see, e.g., Coremans and Thissen 1962; Edwards et al. 2000; Castro et al. 2004a; Edwards et al. 2007 also with barite; Kotulanová et al. 2009; de Ferri et al. 2019 also with dolomite). The role of calcite in modifying pigments properties has been deepened by de Viguerie et al. (2018), who demonstrated that calcite addition increases the transparency, the elastic and viscous moduli and the yield stress of pure lead white. A mixture with chalk was also used by several 16^th- to 17^th-century Dutch painters, who called it ‘lootwit’, as opposed to the ‘schelpwit’ made of pure white lead (Van der Graaf 1961). Among the other pigments, lead white has been frequently associated with most of them, to lighten their colours.

As for its combined use with organic pigments, it is interesting to note the associations with carbon black, indigo and red lakes; these latter seem primarily used when minium is absent, although this is not a rule.

Certified evidence helps us to understand the distribution and methods of use of the lead white; however, the most important aspect that can be deduced from the literature is linked to this pigment's characterisation, bearing an invitation to greater accuracy in determining the phases present in artworks. Indeed, cerussite and hydrocerussite have been found both individually and in combination (Prieto et al. 2005; Gonzalez et al. 2016, 2017a, b, 2019a, b; Vanmeert et al. 2018; De Meyer et al. 2019a; van Loon et al. 2019). Archaeometric studies have highlighted the importance of their qualitative and, possibly, quantitative determination, as they have different behaviours, especially in terms of reactivity.

In this regard, the experimental studies performed by Kotulanová et al. (2009) and Vagnini et al. (2020) represent a milestone, with regard to the characterisation of both pigments and the transformations that can occur to these two phases, considered individually or combined (see below). Applied studies, such as those performed by Gonzalez et al. (2019a), are further fundamental in understanding how painters used lead-based white mixtures and how to analyse them. These authors, for example, demonstrated that the impasto used by Rembrandt van Rijn (The Netherlands, 1606–1669) was made of a mixture of cerussite and hydrocerussite (with very high HC: C ratio) and that plumbonacrite, which is stable only in alkaline conditions (pH > 10), may have resulted from the carbonation of the PbO drier.

Transformation and alteration products

Several experimental tests have been devoted to investigating the transformation of lead white into litharge, massicot and red lead. For example, Senna and Kuno (1971) obtained massicot from lead white heated at ~600 °C. Ciomartan et al. (1996) studied the thermal decomposition of basic lead(II) carbonate (static air, atmospheric conditions) and found several intermediate products between 21 and 370 °C (Figure 5). De la Roja et al. (2008) investigated the chromatic variations of lead white roasted at 330 °C as a function of time.

Fig. 5

The main steps following the exposure of lead to acetic acid (data from Ciomartan et al. 1996)

Similarly, San Andrés et al. (2008) demonstrated how time and temperature transform lead white into different lead-based compounds as follows:

• (1) a mixture of litharge, massicot and lead oxide carbonate (Pb₃O₂(CO₃)) is obtained by roasting lead white at 600 °C for 15 min;

• (2) massicot, along with other lead oxides (Pb₂O₃ and Pb₅O₈), is obtained by roasting lead white at 600 °C for at least 45 min;

• (3) litharge is obtained by using a lower temperature (~330 °C) for a longer time (~3–4 h);

• (4) red lead is obtained after about 20 h (at ~330 °C).

As for alteration products, the darkening of lead white is a typical alteration issue resulting from its conversion into lead sulfide (PbS, galena) or lead dioxide (scrutinyite α-PbO₂ and plattnerite β-PbO₂). Several authors have reported on this alteration, especially in watercolours, manuscripts, canvas and wall paintings (Augusti 1949; Plenderleith 1956; Carlyle and Townsend 1990; Giovannoni et al. 1990; Roy 1993; Clark and Gibbs 1998; Burgio et al. 1999; Saunders 2000; Andalo et al. 2001; Clark 2002; Smith and Clark 2002a, b; Goltz et al. 2003; Saunders and Kirby 2004; Franceschi et al. 2010; Gutman et al. 2014; Melo et al. 2016; Coccato et al. 2017; Vagnini et al. 2020), as well as in out-door decorations, such as polychrome ceramic statues (Pérez Rodríguez et al. 1998).

From these studies, it becomes clear that many various factors can induce the darkening and blackening of white lead. These range from pollutants (e.g., SO₂, SO₃, NO_x), alkali, salts, acids, hydrogen sulfide (H₂S) and oxidising agents to other environmental factors such as humidity, temperature and microbial activity.

In the literature, it has often been repeated that the darkening process of white lead is still not fully understood. Indeed, the basic problem is that the variables involved are so numerous and different from each other that any type of painting technique, pigment, binder and environmental condition can speed up, slow down and/or drive the transformations, in one sense or another. This aspect clearly emerged in some valuable experimental works, which are summarised below.

Goltz et al. (2003) performed a series of tests using basic lead carbonate mixed with various binders (linseed oil, gum Arabic, water and egg tempera) and evaluated their behaviour under exposure to H₂S_(g). Darkening occurred after ~24 h at 20 °C and after only 2 h at 50 °C. Using the same time frame, (a) all of the different binding agents seemed to play protective roles and (b) the mixture containing lead white and other S-containing pigments, such as cinnabar, orpiment and realgar, did not result in darkening (experiment carried out in the darkness).

Kotulanová et al. (2009) tested the behaviour of several lead-based pigments (lead white, massicot and minium) in the presence of various types of inorganic salts, both those naturally present in the environment (Na₂SO₄, MgSO₄, CaSO₄, NaCl, NaNO₃, Ca(NO₃)₂, Na₂CO₃, K₂CO₃ and urea) and those synthetically prepared for restoration (NaHCO₃, KHCO₃, (NH₄)₂CO₃ and NH₄HCO₃). The treatment of cerussite (containing 68% cerussite, 6% hydrocerussite and 26% shannonite) left only cerussite, regardless of whether natural or synthetic salts were used. The times detected for the transformations were different, and the authors observed other products (potassium basic lead carbonate, laurionite and abellaite, using K₂CO₃, NaCl and Na₂CO₃/NaHCO₃, respectively) in only three cases. In one case (using CaSO₄), the hydrocerussite remained for over a year. The lead white (containing hydrocerussite, lead acetates, lead oxide sulfate hydrate and plumbonacrite) reacted with all the salts used providing different products⁷. Only when a strong oxidising agent such as NaClO was introduced into the water suspension was darkening observed, together with the formation of plattnerite and scrutinyite (along with remnants of the pre-existing phases and cerussite). Accordingly, the same experiment conducted with hydrogen peroxide (30%) did not show any colour change.

Vagnini et al. (2020) conducted a series of experiments, using cerussite and hydrocerussite taken individually. First, they experimentally reproduced two extreme RH conditions (i.e. at 93% and 8% RH, 25 °C), in order to study the transformation of cerussite and hydrocerussite during lime carbonation. Secondly, they tested the oxidation potential of sodium hypochlorite (NaClO) when applied to these two phases.

For the first series of experiments, both cerussite and hydrocerussite were mixed with two parts of lime (to partially simulate fresco conditions); for the second series, cerussite and hydrocerussite were applied with a proteinaceous binder on 'a secco' painting mock-ups and covered for about 2 h with a cotton pack embedded with NaClO.

Investigations conducted on the first series showed the following:

• (a) at 93% RH, cerussite transformed into litharge in 18 days and disappeared after 370 days;

• (b) at 8% RH, hydrocerussite transformed into litharge and massicot in 204 days and then released massicot and minor red lead as final products at 370 days;

• (c) at 93% RH, hydrocerussite turned into massicot in 2 days, massicot and red lead formed in 98 days and, finally, only red lead and minor massicot were observed after 370 days.

Overall, the main achievements reached by these authors can be summarised as follows:

• (1) the alkaline environment favoured the oxidation of hydrocerussite (hydrocerussite Pb(II) → red lead Pb(II) and Pb(IV)) but not that of cerussite (cerussite Pb(II) → litharge Pb(II)), the reaction being accelerated by high percentages of humidity;

• (2) the alkaline environment was not sufficient to trigger the transformation of litharge into massicot in cerussite samples, while high RH percentages favoured the formation of red lead in hydrocerussite samples (maintaining a basic environment);

• (3) even under high alkalinity, the formation of plattnerite or scrutinyite (Pb(IV)) is not a spontaneous process, as it requires strong oxidising agents. These phases were obtained only by the effect of a strong reducing agent such as sodium hypochlorite;

• (4) the application of sodium hypochlorite confirmed the greater reactivity of hydrocerussite, compared to cerussite, and the preferential formation of plattnerite by cerussite and scrutinyite by hydrocerussite;

• (5) while the oxidation process triggered by NaClO is completed using either cerussite or hydrocerussite, cerussite transformed into plattnerite and minor scrutinyite (+halite); conversely, hydrocerussite transformed into cerussite, scrutinyite and minor plattnerite (+halite).

Salvadó et al. (2009) demonstrated the formation of lead carboxylates in 15^th-century oil and egg yolk tempera paintings and observed that lead and calcium carboxylates were "more crystalline with the egg yolk than with the drying oil".

Finally, Petushkova and Lyalikova (1986) investigated the role of microbial activity in the conversion of lead white to lead sulfide. They observed the formation of yellow (4 days) and brown (6 days) lead precipitates due to the growth of Arthrobacter siderocapsulatus. Petushkova and Lyalikova (1986) and Aze et al. (2008a) regarded plattnerite as an alteration product of lead white and massicot.

The few examples proposed above allow us to clarify that there is no single factor that can be deemed responsible for all cases (see also the section focused on lead soaps below). Moreover, while it is realistic that the variables to be identified as responsible for a specific case study are more than one, it can be even more difficult to ‘quantify’ the contribution of one variable over that of another.

To conclude this first part on lead white, it is also worth adding that the selective application of hydrogen peroxide can convert black lead sulfide (PbS) into white lead sulphate (PbSO₄), which is commonly used as a restoration technique (Carlyle and Townsend 1990; Daniels and Thickett 1992; McFarland 1997; Lussier 2008). As already described by Plenderleith (1956), the method was definitively tested by Giovannoni et al. (1990), using hydrogen peroxide (H₂O₂) and acetic acid (CH₃COOH), partially retracing how the lead white was originally produced. After Giovannoni and co-workers, this method was named the ‘Florentine method’ by Koller et al. (1990), who applied it to the painted vault (c. 1580 AD) of the Strechau castle in Styria (Austria). This technique is still in use, although not always successfully (see, e.g., McFarland 1997).

Red lead—minium (Pb₃O₄)

Mineralogical phase, structure and occurrence

Minium is a red lead oxide (Table 2; Figure 6) that is mainly found in association with galena and cerussite (Table 3). On the origin of the name minium (μίνιο) and its relationship with cinnabar (κιννάβαρι), see Trinquier (2013) and Becker (2021) in this TC.

Fig. 6

Minium: standard chemical reaction of formation (Franceschi et al. 2010 after Weast 1998), thermodynamic data and chemical reaction involved in its transformation. The mineral shown is from Broken Hill, New South Wales, Australia [Arkenstone specimen. Photo credits: Rob Lavinsky, irocks.com–Mindat.org Photo ID 1013485 – Copyrighted. Permission has been obtained for reproduction in this article. The CC-BY licence does not supersede previously copyrighted material; therefore, these images remain under owner’s copyright]. Mechanism of photochemical degradation of red lead, as proposed by (A) Zhou et al. (2012) and (B) Vanmeert et al. (2015).

The crystal structure of minium was first determined by Byström and Westgren (1943) and Gross (1943) and then deepened by Gavarri and Weigel (1975) and Gavarri et al. (1978). A significant number of papers have focused on the thermal behaviour of this phase and the effects of grinding (see, e.g., Dachille and Roy 1960; White et al. 1961; Aleksandrov et al. 1978; Avvakumov et al. 1983; Bohér and Garnier 1984; Boher et al. 1984; Barriga et al. 1988; Zyryanov and Gusev 2001). More recently, Zhou et al. (2012), Vanmeert et al. (2015) and Ayalew et al. (2016) have investigated the photoactivity of minium. The mechanisms proposed in the former two papers are illustrated in Figure 6A. As for the third paper, the authors experimentally tested the photoactivity of minium both in the presence and absence of bicarbonate ions and demonstrated the importance of these processes (formation of PbO or hydrocerussite) in the degradation processes. Following the studies performed by Kang et al. (1988), Guo et al. (2016) simulated a depleting chlorine regime to investigate the dissolution of minium and proposed a rational model of the process (Figure 6B).

The natural mineral occurs in the oxidised part of lead ore deposits, where it is associated with several lead minerals (Table 2), particularly galena, cerussite, litharge and massicot. Minium can be found in many places but only in low amounts (Anthony et al. 1990). Notable occurrences are Mežica (Slovenia), Leadhills in Lanarkshire (Scotland, UK), Sarrabus in Sardinia (Italy), Tsumeb (Namibia), Långban Mine (Sweden), TchahMilleh Mine (Iran), Bleialf in Rheinland-Pfalz (Germany) and Leadville in Colorado (USA).

The production of red lead

In ancient texts, the production process was handed down by various authors. Plinius specified that the discovery was accidental, following the Piraeus fire (Table 1, P5); similarly, Vitruvius handed down that it "was discovered as the result of an accidental fire" (Table 1, V1). Among the manufactured products, Plinius also included two other varieties: (1) a high-quality variety, the Asiatica (Table 1, P5), characterised by purple shades, and (2) a variety manufactured in Roma, which was obtained from the calcination and subsequent quenching with vinegar of sile marmoroso (ochre hard as marble; Table 1, P5—in this latter case, however, the reference is unclear).

Dioscorides (Table 1, D2), Vitruvius (Table 1, V1) and (H)Eraclius (Table 1, E1) similarly describe lead white roasting as the production process of red lead and some compare its colour to that of sandaraca.

Based on archaeometric research and experimental work, the various steps leading to red lead production have been clearly described by Ciomartan et al. (1996), Aze (2005) and Aze et al. (2007). As schematised in Figure 2, after the formation of hydrocerussite from the acetate formed on the surface of lead sheets, subsequent roasting induces the decarbonisation and dehydration of hydrocerussite, which leads to the production of litharge. Finally, the oxidation of litharge leads to the formation of red lead.

Indeed, red lead was mainly produced by roasting lead white (hydrocerussite) or litharge (Saunders et al. 2002); however, the decomposition of hydrocerussite can trigger the formation of intermediate phases (e.g., 4PbCO₃·3PbO and PbCO₃) and final products (e.g., only red lead or combined with other phases such as PbO), mainly depending on the temperature and oxygen fugacity conditions under which the process took place. Consequently, either litharge or (if using high temperatures) massicot may be present at the end of the process, due to incomplete reactions (Ciomartan et al. 1996; Boden 1998; Risold et al. 1998)

Considering the rarity of minium as a natural mineral and the information handed down by ancient texts, it seems appropriate to believe that almost all red lead used in antiquity was of artificial origin, although we cannot exclude the use of natural minium. In this latter case, it may also have been called by other names, considering that impurities of various types may vary the shade of the pigment. Moreover, the production of red lead may have represented a secondary product of silver smelting and, more specifically, of the waste litharge (Walton and Trentelman 2009).

Red lead in artwork

As with other lead-based pigments, red lead-based compounds were likely used first as drugs rather than as colouring pigments. It is not surprising that, in the Ebers Papyrus (~ 1550 BC), red lead is often indicated as a remedy to combat diseases such as pterygium (as well as other eye and vision problems), alopecia, tremor of the fingers and tapeworms (Bryan 1930). Its application to different types of paintings was rare from the 2^nd century BC to the 1^st century AD (Figure 3), while it recorded a very sharp and progressive increase along the following centuries (Figure 3). Conversely, its use as an anticorrosive paint, textile paint (for linen cloth; Walton and Trentelman 2009 and the references therein) and decorative paint seems to have never been interrupted, up until the modern age. At present, its toxicity has led to a sharp decline of its use, mainly being relegated to the production of batteries (exploiting its properties as an electrode material) and glass (Ayalew et al. 2016).

As far as supports and techniques are concerned, the same considerations made previously for lead white apply. However, it has some noteworthy aspects:

• it has also been used to decorate paper currencies (Shi and Li 2013), inlays (Zhao et al. 2020), lacquers (Bösiger 2019) and ancient manuscripts (Aceto et al. 2008);

• it has been less used for priming layers and grounds, although it was widely used in the Alhambra complex (Granada, Spain; Cardell et al. 2009; Arjonilla et al. 2019a);

• it has been similarly used as a substrate and mordant for gilding (Pereira-Pardo et al. 2019; Mounier et al. 2020; Zhou et al. 2020);

• it has been used, likely more frequently, in fresco painting (see, e.g., Hein et al. 2009; Almaviva et al. 2018; Khramchenkova et al. 2019), although most occurrences relate to the fresco-secco technique (see also Daniilia et al. 2000, 2008; Figure 4C);

• in one case, the encausto technique has also been reported (Gehad et al. 2015).

As for its association with other pigments, mixtures are mostly reported with lead white, calcite/chalk, cinnabar (Gliozzo 2021), red ochre and red lake. Conversely, it is rarely found alone (i.e. without lead white) in paintings where organic pigments have also been used, except for carbon black.

Transformations and alteration products

Among the most common degradation effects described in the literature, both whitening and blackening have been observed.

The whitening is due to the carbonation of lead monoxide, which is present in red lead as an impurity due to an incomplete reaction (see above for the production process). Lead monoxide reacts to form lead carbonates (PbCO₃ and Pb₃(CO₃)₂(OH)₂; see Aze et al. 2007 and the references therein) through different paths, mostly determined by the presence of moisture, CO₂ and media (Saunders et al. 2002).

The blackening is due to the formation of plattnerite and lead sulfide (PbSO₄). In the presence of dilute sulfuric acid solutions (i.e. atmospheric pollutants), this process includes the sulfation of Pb(II) atoms present in PbO impurities (Figure 7, Eq. 5), followed by the ‘solvolytic disproportionation’⁸ of Pb₃O₄ into lead dioxide (PbO₂) (Kang et al. 1988, Figure 6, Eq. 1; Aze et al. 2007, Figure 6, Eqs. 2–3).

Fig. 7

Litharge and massicot: standard chemical reaction of formation (Franceschi et al. 2010 after Weast 1998), thermodynamic data and chemical reactions involving their transformation. Mineral photo: litharge (red) and massicot (yellow) from the Thorikos Bay, Lavrion Mining District, Attica, Greece [Photo credits: Ko Jansen -Mindat.org Photo ID 997691– Copyrighted. Permission has been obtained for reproduction in this article. The CC-BY licence does not supersede previously copyrighted material; therefore, these images remain under owner’s copyright] [*Scrutinyite or plattnerite]

Aimed at unravelling the alteration path of red lead, Aze et al. (2007) carried out an experimental work on naturally aged wall paintings (built in 1977), which were exposed to different conditions (alkaline, oxidant and acidic media), environments (humid and thermo-hygrometric stressed) and lighting. Firstly, the authors observed the formation of plattnerite and anglesite (+ gypsum) in the "outer part of the pictorial layer" and validated the formation of these lead phases in dilutes sulfuric acid solutions. Secondly, they verified that light or hydrogen peroxide did not affect red lead while confirming the importance of humidity in the formation of carbonates.

Based on Ayalew et al. (2016), the presence of CO₂ is among the main factors responsible for the photodegradation of this pigment, as it is present in the atmosphere and can also be produced in the paint, through the oxidative decarboxylation of some binders. Moreover, the presence of PbO impurities (due to incomplete synthesis during production) can significantly modify the photoactivity of lead, with coexisting Pb (II) and Pb (IV) states.

The effects of applying different binding media, such as linseed, walnut, poppy-seed and stand oil to a mixture of minium, have been thoroughly investigated by Švarcová et al. (2020). These authors observed the crystallisation of lead formate (Pb(HCOO)₂), due to the evaporation of formic acid from the autoxidation of drying oils and its reaction with lead particles. This lead formate represents an intermediate phase that, while not leading to chromatic variations in the painting, contributes to the formation of lead carbonates or lead soaps; therefore, it cannot be identified in artworks.

Platania et al. (2020) have also ventured on a similar theme. Based on previous studies on the formation of carbonates, chlorides, oxides and sulfates in the aggregates of metal soaps (van der Weerd et al. 2002; Boon 2006; Boon et al. 2007), they investigated red lead formed by the mineralisation of the soaps.

Finally, Mazzeo et al. (2008) verified that red lead and litharge are likely to produce higher amounts of carboxylates than lead white, lead-tin yellow and Naples yellow.

Apart from light, temperature, humidity and binding media, other key factors in the degradation process of this pigment that have been recognised are chlorine salts (Daniilia and Minopoulou 2009), microorganisms (Petushkova and Lyalikova 1986; Giovannoni et al. 1990) and fungi (Feng et al. 1999; Figure 4D).

Litharge (α-PbO) and massicot (β-PbO)

Mineralogical phases, structure and occurrence

Litharge derives from the Greek term λιθάργνρος—meaning silver stone (from λίθος, stone and ἄργυρος, silver)—and its production was explained by both Dioscorides and Plinius (Table 1, D1 and P2). The etymology of massicot can be vaguely associated with the early Arabic martak, the Italian marzacotto, the Spanish mazacote and the late 15^th-century French massicot.

Litharge and massicot are the red (tetragonal) α-phase and the yellow (orthorhombic) β-phases of PbO, respectively (Table 2, Figure 7).

Both polymorphs crystallise in a layered arrangement and their structure (Table 2; Trinquier and Hoffmann 1984) and properties have been extensively studied (see, e.g., Radhakrishnan et al. 1983; Canepa et al. 2012). Both an α-phase (stable below 208 °K=−65 °C; Hédoux et al. 1989; Moreau et al. 1989) and a γ-phase are also known to occur along the litharge → massicot path (Adams et al. 1992).

Further studies have focused on their thermodynamic and mechanochemical phase transformations (Söderquist and Dickens 1967; Okuri and Ogo 1982; Trinquier and Hoffmann 1984; Baleva and Tuncheva 1994). The transition temperature between (low-temperature) litharge and (high-temperature) massicot (α→β) has been placed at 489 °C (Trinquier and Hoffmann 1984; Baleva and Tuncheva 1994), 540 °C (White et al. 1961) or 580 °C (Lin and Nadiv 1979) under atmospheric pressure. Conversely, the massicot→ litharge transformation (β→α) can be achieved at room temperature, upon mechanical treatment (e.g., isothermal ball milling; see Senna and Kuno 1971, Lin and Niedzwiedz 1973; Criado et al. 1982).

Their multiple and various applications of industrial importance (e.g., in photodetectors, lead–acid batteries, gasoline and integrated circuits) have chiefly stimulated recent research (see, e.g., Cao et al. 2003; Perry and Wilkinson 2007; Bhagat et al. 2018); however, the occurrence of these two phases in nature is much rarer than that of galena, as they are mainly found as secondary minerals in oxidised zones of lead deposits and/or hydrothermal environments (Table 3), such as at Cucamonga Peak in California (USA), at Mežica in Slovenia, at Långban Mine (Sweden) and in the slags related to the Lavrion ancient mines in Attica (Greece) (Mindat.org).

Historically, litharge was a typical secondary product of lead cupellation during silver smelting and, from this practice, it derived the ancient denomination of spuma argenti. Its absence or presence at smelting sites has been variously interpreted, following Rehren et al. (1999). Either it was used as a product for medical or building purposes (e.g., for waterproof lining), as a semi-finished product from which to gather residual silver, or as a tool to enhance the smelting process (added to the smelting charge). Its value was considered high enough to be worked and traded (see, e.g., Negueruela et al. 2004; Renzi et al. 2009). Within the field of archaeometallurgy, other studies have regarded the occurrence of litharge in corrosion crusts after the weathering of ancient lead objects, such as lead pipes (see, e.g., Essington et al. 2004), or as a tracer of ancient non-ferrous metallurgy processes (see, e.g., Keim et al. 2017).

As for painting, it is a widely accepted opinion that litharge was used as a siccative (i.e. drying agent) rather than an art pigment (Toniolo et al. 1998). The use of litharge as a dryer has been discussed in relation to both the palette of specific painters (e.g., for Rembrandt’s paintings, see Gonzalez et al. 2019a) and the formation of alteration products due to saponification (see below). The preparation of a lead medium using litharge cooked with linseed oil (in 1: 3/4 ratio) is commonly referred to Antonello da Messina (1430–1479) (Maroger 1948), but it has also been handed down by later authors (Gonzalez et al. 2019a).

Litharge and massicot in artwork

The correct assessment of the occurrence of these phases is affected by the relatively high number of cases in which the identification is not certain (see, e.g., Edwards et al. 2000, 2001; Wang et al. 2004; Uchida et al. 2012; Blasco-López et al. 2016; Khramchenkova et al. 2019; Malletzidou et al. 2019).

In the cases where litharge has been identified with certainty, it appears decidedly rarer than massicot. Based on data from the literature (see Supplementary Appendix 1), it is possible to list five (perhaps six) occurrences, in which it was (a) sometimes associated with massicot, as well as lead white and red lead; (b) found in paintings ranging from the 11^th to the 19^th century in Europe and India; and (c) present on all types of common supports, such as wood ceilings, miniature paintings, manuscripts, wall frescoes, wooden panels and egg-tempera icons (Burgio et al. 1999; Edwards et al. 2001; López Cruz et al. 2011; Ravindran et al. 2011; Amadori et al. 2016; Stanzani et al. 2016; Arjonilla et al. 2019a; Serafima et al. 2019b).

Massicot was identified with a slightly higher frequency—18 occurrences—in which it was:

• associated with both lead white and red lead (9 cases), red lead (6), plattnerite (4), lead-tin yellow type I (3), scrutinyite (3), anglesite (2), galena (2), lead white (2), litharge (2), PbMg(CO₃)₂(2) and chrome orange and yellow (1):

• found in paintings ranging from the 1^st to the 19^th century, in Europe, India and Iran:

• present on all types of common supports, such as wood ceilings, glass icon as (fatty tempera), illuminated manuscripts and wall paintings (fresco and secco), as well as in powdered raw pigments and wood cupboards (Burgio et al. 1999; Bruni et al. 2002; Smith and Clark 2002a; Castro et al. 2008; Kotulanová et al. 2009; Aliatis et al. 2010; Ravindran et al. 2011; Hradil et al. 2013; Lukačević et al. 2013; Duran et al. 2014; Bratu et al. 2015; Holakooei and Karimy 2015; Felix et al. 2018; Kantoglu et al. 2018; Vanmeert et al. 2018; Arjonilla et al. 2019a; Khramchenkova et al. 2019; Costantini et al. 2020b; Fioretti et al. 2020).

From case to case, the presence of litharge and/or massicot can be interpreted as a deliberately used pigment, an impurity derived from red lead or, based on Vagnini et al. (2020), a secondary phase from lead white. In this regard, it may be useful to note that only massicot was identified among the powdered pigments found in Pompeii (Aliatis et al. 2010).

Crocoite (PbCrO₄) and mimetite (Pb₅(AsO₄)₃Cl)

Mineralogical phases, structure and occurrence

Crocoite is a monoclinic lead chromate (Tables 2 and 3). The transparent prismatic to acicular crystals display bright colours, ranging from red to hyacinth red, reddish orange, orange and yellow (Figure 8A). The structure of crocoite is monazite-type, with a tetrahedral anion group XO₄, where X = Cr (Quareni and De Pieri 1965; Effenberger and Pertlik 1986a). According to Loeffler and Burns (1976), the red-orange colour of crocoite is due to the oxygen (O²⁻) to metal (Cr⁶⁺) charge-transfer within the CrO₄²⁻ group, which determines a strong absorption of the visible light in the violet-green domain.

Fig. 8

A Crocoite from Adelaide Mine, Tasmania, Australia [Arkenstone specimen. Photo credits: Rob Lavinsky, irocks.com- Photo ID: 40436]. B Mimetite from the Ojuela Mine near Mapimí, Mexico [Arkenstone specimen. Photo credits: Rob Lavinsky, irocks.com-Mindat.org Photo ID: 151580]. C Mimetite from the Guatomo Mine, Bannang Sata District, Thailand [photo credits: Enrico Bonacina –Mindat.org Photo ID: 268267]. DChromium-rich mimetite on cerussite from Nakhlak (Madan-e Nakhlak) Mine, Isfahan, Iran [specimen and photo: fabreminerals.com-Mindat.org Photo ID: 709680] [All images are subject to copyright owned by the photographers indicated above. The CC-BY licence does not supersede previously copyrighted material. Permission has been obtained for reproduction in this article]

Crocoite is a rare secondary mineral, found in the oxidation zone of lead-bearing ore associated with Cr-rich rocks (also in abandoned mines). It is associated with both primary (e.g., galena, chromite) and secondary (e.g., pyromorphite, phosgenite, cerussite, anglesite, vanadinite, opal, chrysoprase, malachite; Table 3) minerals. The first mention of crocoite (‘Siberian red lead’) and chemical studies date back to the end of the 18^th century, as related to the Tsvetnoi mine (Berezov ore deposit) in the Ural Mts., Russia (Macquart 1789; Beudant 1832b; Williams 1974). When crocoite replaces anglesite, it is closely associated with ‘parent’ galena. With advanced oxidation, crocoite will form along with cerussite, sometimes distant from galena.

In some old literary sources, this phase is incorrectly referred to as ‘red ore of lead’ instead of ‘chrome yellow’ (West FitzHugh 1986; see the section dedicated to lead chromates below).

Notable occurrences include the Dundas area in Tasmania (Australia); for example, at the Adelaide lead-silver mine and Red Lead Mine, numerous and extremely large vugs filled in with large, deep red-orange crystals of crocoite have been found (Bottrill et al. 2006; Moore and Wilson 2012). Adelaide Mine became the world's most famous producer of crocoite specimens. Other occurrences are those from the Happy Jack Mine (Lady Bee Mine) in the Comet Vale (Australia) and the Wadnaminga gold mines (South Australia). The Mindat.org database lists many occurrences distributed worldwide, such as Băița Bihor (Romania), Obercallenberg Quarry (Germany), Leadhills (Scotland, UK), Nontron (France), Goiabeira (Brazil) and Darwin in California (USA).

Mimetite is lead chloroarsenate containing 69.61 wt% Pb. It forms an efflorescence of small crystals or botryoidal crusts (Figure 12D, E). Its structure has been investigated by Baker (1966), Calos et al. (1990) and Dai et al. (1991) (Table 2). Mimetite belongs to the apatite group, having the general formula M₅(ZO₄)₃X, where M = Ca, Sr, Pb or Na, Z = P, As, Si or V, and X = F, OH or Cl. In mimetite, Pb(1) bonds to nine O atoms, whereas Pb (2), which is larger, bonds to six oxygen and two chlorine atoms, to form irregular Pb(2)-O₆Cl₂ polyhedra. As bonds to four O, in a tetrahedral arrangement. The study performed by Bajda (2010) demonstrated that, between 5 and 55 °C, it is most stable at neutral to weakly alkaline pH.

Mimetite is a secondary mineral, which forms in the oxidation zone of Pb–As ore deposits, at the expense of galena and chalcopyrite (Table 3). Under the name of ‘mimétès’, Beudant (1832b, pp. 594–595) described crusts or fibres associated to Cu-Pb deposits, located near St Day in Cornwall (England), Johanngeorgenstadt (Germany) and Champallement (France). The name comes from Greek mimethes = imitator, due to its resemblance to pyromorphite. The fibrous mineral known as ‘prixite’ was only later recognised as ‘mimetite’ (Guillemin et al. 1955). A fibrous yellowish mimetite has been found to be associated with weathered galena at Les Molérats mine in the centre of France (Thiéry 2014). Mimetite also occurs as a weathering product on Zn-smelting slags deposited near Katowice in Poland (Bril et al. 2008). Its occurrences are numerous worldwide (Mindat.org), in locations such as Tsumeb (Namibia), Příbram (Czech Republic), several places in Cornwall (England), Leadhills (Scotland), Phoenixville (Pennsylvania, USA) and Broken Hill (New South Wales, Australia).

Crocoite and mimetite in artwork

Crocoite was likely used before the introduction of artificial lead chromates. Evidence supporting this hypothesis can be summarised as follows:

• 1. it is unlikely that it formed from the alteration of other phases based on the absence of other Cr-pigments;

• 2. its intentional production is unlikely, based on the lack of chromium metallurgy in the Middle Ages;

• 3. in the few cases where crocoite was found, no traces of any later interventions were observed⁹.

The use of natural crocoite has been hypothesised in two cases only: (1) in 13^th- century mural paintings in the Siena Cathedral in Italy (Mugnaini et al. 2006) and (2) in the 13^th-century church of St. Gallus in Kuřívody (northern Bohemia), together with mimetite (Hradil et al. 2014). More doubtful is the discovery carried out in the 14^th-century wall paintings of the Dagaoxuan Taoist Temple at Beijing in China (Lei et al. 2017). The authors did not seek to solve the concerns regarding whether it was natural crocoite or a pigment imported from Europe for subsequent repairs.

The occurrences of mimetite are relatively more numerous; however, in order to correctly interpret its presence in paintings, it is worth noting that this phase may be present as a pigment or as a phase of alteration. For example, the decay of realgar and orpiment may lead to the formation of mimetite and schulténite (Vanmeert et al. 2019). In the literature, the presence of mimetite as a primary pigment or as an alteration phase is not always clarified. Consequently, in the list of occurrences collected below, this aspect is outlined only when it has been discussed by the authors:

• in the 4^th-century BC wall paintings of ancient Macedonian funerary monuments, together with vanadinite (Brekoulaki 2006);

• in the yellow-orange and green paint (mixed with Egyptian blue) of the 4^th to 2^nd century BC grave stelai from Alexandria (kept at Louvre Museum, Paris, France), Rouveret (1998), Rouveret and Walter (1998) and Kakoulli (2002) found mimetite and vanadinite. Rouveret and Walter (1998) suggested an Iranian provenance of mimetite¹⁰;

• on a Greek-Ptolemaic grave stele (limestone) with a soldier and two girls, dated to the second half of the 3^rd century BC (Leona 2009). The stele was found in 1884 in a tomb near Alexandria (Egypt) and kept at The Metropolitan Museum of Art in New York (USA), as a Gift from Darius Ogden Mills in 1904. Leona interpreted the presence of mimetite as a deliberately used yellow pigment and, hypothesising the Laurion provenance of the pigment, inferred the Athenian provenance of the painter as well;

• in the wall paintings of the 2^nd century AD tomb of Palmyra in Syria, investigated by Buisson et al. (2015);

• in the 9^th- to 10^th-century wall decorations of Masjid-iJame at Fahraj (Holakooei and Karimy 2015), whose provenance was related to the Pb-Zn deposit at Darreh Zanjir (Iran);

• on the painted decoration of the sandstone sculptures (Vairocana Buddha) in the Dazu Rock Carvings at Chongqing (China), dated to the late 12^th to mid-13^th centuries but having been retouched since the 1850s (Li et al. 2020b). These authors described in detail the spontaneous formation of mimetite + lavendulan (NaCaCu₅(AsO₄)₄Cl·5H₂O) from emerald green (Cu(C₂H₃O₂)₂·3Cu(AsO₂)₂) + cerussite;

• in the 13^th-century Church of St. Gallus in Kuřívody (northern Bohemia), where it was interpreted as a degradation product formed by the alteration of orpiment mixed with lead-based pigments (Hradil et al. 2014);

• in a 17^th-century oil on canvas (still life painting) by Jan Davidsz de Heem (1606–1684; De Keyser et al. 2017; Vanmeert et al. 2019), where schulténite (alteration product) was also found.

Lead-based minerals mainly found as alteration phases of the previous pigments

Galena (PbS)

Mineralogical phase, structure and occurrence

Named by Plinius (Table 1, P1), galena is a lead-grey (Figure 9), isometric sulfide (Table 2). In galena [MIIS-II], the M site can be substituted by several metallic trace elements, in particular, by those with I, II or III oxidation states (based on Goldschmidt's second rule), through different types of substitutions (e.g., simple and coupled; see Chutas et al. 2008; Renock and Becker 2011; George et al. 2015, 2016).

Fig. 9

Galena: standard chemical reaction of formation (Franceschi et al. 2010 after Weast 1998), thermodynamic data and chemical reaction involving its transformation. Mineral photo: galena from the Borieva Mine in the Madan ore field, Bulgaria. [Arkenstone specimen. Photo credits: Rob Lavinsky, irocks.com-Mindat.org Photo ID: 157198 – Copyrighted. Permission has been obtained for reproduction in this article. The CC-BY licence does not supersede previously copyrighted material; therefore, these images remain under owner’s copyright]

Common trace elements are Mn, Fe, Co, Cu, Zn, Ga, As, Se, Ag, Cd, In, Sn, Sb, Te, Tl, Bi and minor Au and Hg (Blackburn and Schwendeman 1977; Foord and Shawe 1989; George et al. 2015, 2016). Their partition into galena depends on several factors, such as their oxidation state, ionic radius and availability. However, the co-existence of other phases (Table 3)—chiefly sphalerite, chalcopyrite (George et al. 2016) and wurtzite (Bethke and Barton 1971)—can influence their distribution across different deposits, within a single deposit, as well as in the grains of a single specimen (George et al. 2015).

The oxidation reactions and dissolutions mechanisms of galena have been studied in detail. The dissolution of galena plays a key role in several environmental and geochemical processes; therefore, its mobility (acid rock drainage) and potential toxicity have been experimentally investigated under varying conditions (e.g., in solutions with different pH and dissolved oxygen at different temperatures) (Hsieh and Huang 1989; Ruby et al. 1994; Basta and McGowen 2004; De Giudici et al. 2005; Acero et al. 2007; Goryachev and Nikolaev 2012; Gutiérrez et al. 2016; Johnson et al. 2016; Chen et al. 2017; Zha et al. 2020 and the references therein).

Various factors, such as the pH value, the activities of both sulfur and lead and the CO₂ partial pressure of the fluid, affect the stability of these phases. The weathering of galena leads to the formation of lead and several secondary phases such as cerussite (PbCO₃), anglesite (PbSO₄), pyromorphite group minerals (e.g., pyromorphite [Pb₅(PO₄)₃Cl], mimetite [Pb₅(AsO₄)₃Cl] and vanadinite [Pb₅(VO₄)₃Cl]) and other minor phases.

Regarding the occurrence of lead-bearing minerals (Table 3), galena is undoubtedly the most common species; it is also one of the most common among overall metallic sulfides. It occurs in both terrestrial and seafloor sulfide deposits (Fallon et al. 2017) and represents the most abundant primary lead phase in lead-bearing ore deposits. The sites are so numerous that they cannot be listed exhaustively; however, galena sources which have been certainly exploited in the different periods of the past and valuable compositional information (chemistry and isotope values) for ore deposits can be gained from archaeometallurgical studies and databases (e.g., the Oxford archaeological lead isotope database OXALID from the Isotrace Laboratory, http://oxalid.arch.ox.ac.uk; and the Lead isotope ratios for Mediterranean ores, available in open document format from: http://www.brettscaife.net/lead/data/index.html, Scaife et al. 1999). However, it is worth mentioning at least a few significant galena deposits, such as Leadville in Colorado (USA), Freiberg in Saxony (Germany), Laurion (Lavrion) mines in the Lavrion Mining District (Attica, Greece), Baia Mare Mining District (Romania), Mendip Hills in Somerset (UK), Leadhills in Lanarkshire (Scotland, UK) and the ancient mines in Sardinia, at Monteponi and Montevecchio (data from Mindat.org). Moreover, a large body of useful literature also falls within the field of acid mine drainage studies, along with geological studies aimed at evaluating the mining potential of specific deposits.

Finally, it is important to note that galena can be found in cerussite aggregates (see, e.g., Mondillo et al. 2018) and could therefore be combined with lead white, if the natural mineral had been used, instead of the artificial pigment.

Galena in artwork

Galena is rarely found in artwork and is generally interpreted as an alteration product of lead white (Smith and Clark 2002b; Smith et al. 2002):

• Carlyle and Towsend (1990) found lead sulfide in a 19^th-century unvarnished oil sketch by J.M.W. Turner;

• Burgio et al. (1999) found galena in 18^th-century Javanese and Thai manuscripts;

• Smith et al. (2002) found lead sulfide in a 16^th-century manuscript illumination of the Jamnitzer manuscript;

• Franquelo et al. (2015) hypothesised the formation of PbS (due to alteration) on the 16^th-century polychrome gothic-renaissance altarpiece at Santiago Church in Écija (Spain) by Alejo Fernandez Aleman;

• Tarquini et al. (2020) found galena (degradation) on the Campana reliefs found at the Palatine Hill in Rome (Italy; before the second half of the 2^nd century);

• Costa et al. (2016) claimed that galena was present on the Trompetender Putto painting by Gustav Klimt (early 1880s), but the analyses were carried out using scanning electron microscopy.

Furthermore, Holakooei and Karimy (2015) found galena to be associated with red lead, plattnerite, mimetite and hemimorphite in the 9^th- 10^th century AD wall paintings of Masjid-i Jame in Fahraj (Yazd province, Iran).

Finally, Colombini et al. (2003) identified galena in paints applied to the 1^st BC – 1^st AD ‘Ship C’ found at the ancient harbour of San Rossore (Pisa, Italy). They indicated galena as being responsible for the blackening of the pictorial surface. The authors did not put forward any hypothesis about its presence; however, cerussite was the only lead-based pigment individuated in the association.

Anglesite (PbSO₄), palmierite (K₂Pb(SO₄)₂) and associated phases

Mineralogical phases, structure and occurrence

Anglesite is a colourless to white (sometimes with grey, yellow, green or blue hues) sulphate (Figure 10A, B), which is a member of the barite group (Tables 2 and 3). It was named in 1832 by Francois Sulpice Beudant who individuated it at the Parys Mine (Island of Anglesey, Wales, UK; corresponding to the type locality) (Beudant 1832a). Anglesite (and cerussite) occurs in lead-bearing ore oxidation zones, "either as in situ replacement of galena and/or as euhedral crystals in cavities of former, partially dissolved galena" (Keim and Markl 2015), mainly in low-temperature hydrothermal areas (Hazen et al. 2013) (Table 3). In nature, anglesite is relatively frequent (Mindat.org; Anthony et al. 1990), such as at the Tsumeb and Kombat mines in Namibia, Mežica in Slovenia, Cromford and Matlock in Derbyshire (UK), Leadhills in Lanarkshire (Scotland, UK), Bleiberg in Carinthia (Austria), Genna Zinkhütte in Nordrhein-Westfalen (Germany), Nerchinsk in Siberia (Russia), Bunker Hill Mine in Idaho (USA) and Dundas in Tasmania (Australia).

Fig. 10

A Anglesite from the Touissit Mine, Morocco [Arkenstone specimens. Photo credits: Rob Lavinsky, irocks.com- Mindat.org Photo ID: 181543]. B Anglesite from Broken Hill South Mine, New South Wales, Australia [Arkenstone specimens. Photo credits: Rob Lavinsky, irocks.com- Mindat.org Photo ID: 393034]. C Palmierite from Monte Somma, Somma-Vesuvius complex, Italy [Photo credits: Enrico Bonacina -Mindat.org Photo ID: 898863]. D Leadhillite from the Mammoth-Saint Anthony Mine, Arizona, USA [Arkenstone specimens. Photo credits: Rob Lavinsky, irocks.com-Mindat.org Photo ID: 37761]. E Macphersonite from the Tiny-Arenas Mine, Sardinia, Italy [photo credits: Antonio Gamboni -Mindat.org Photo ID: 80093]. F Susannite and lanarkite from the Susanna Mine, Leadhills, South Lanarkshire, Strathclyde (Lanarkshire), Scotland, UK [By Rob Lavinsky, iRocks.com– CC-BY-SA-3.0, CC BY-SA 3.0]. [All images are subject to copyright owned by the photographers indicated above. The CC-BY licence does not supersede previously copyrighted material. Permission has been obtained for reproduction in this article]

Palmierite is a colourless to white, trigonal sulfate (Tables 2 and 3; Figure 10C). Its observations are rare and include only eight countries, including Russia (the Tolbachich volcano in Kamtcheatka), Spain (La Cruz lead smelter slag locality in Linares) and Germany (Genna Zinkhütte in Nordrhein-Westfalen Land and Oberhütte in Sachsen). The type locality is Mt. Vesuvius (Naples, Italy), where it occurs as a fumarolic sublimate. Recognised in the early twentieth century, it has been structurally studied by various authors since the middle of the century (Bellanca 1946; Tissot et al. 2001).

As, for both these phases, their use as a pigment has not been definitively excluded, it should be added that, in nature, they are frequently associated with leadhillite, macphersonite and susannite. These are common to rare Pb hydroxide sulfate carbonates that constitute the leadhillite group (Tables 2 and 3; Figure 10D–F).

Leadhillite is typically colourless to white but may also be grey, yellowish, pale green or blue. Susannite is colourless to yellowish or greenish, while macphersonite is colourless to white but also pale amber. The type locality is the same for all three phases, that is, the Susanna Mine (Leadhills in Scotland, UK). The crystal structures have been provided by Giuseppetti et al. (1990) for leadhillite, by Steele et al. (1999) for susannite and by Steele et al. (1998) for macphersonite (the rarest of the three phases). Leadhillite pseudomorphically replaces galena and transforms into susannite at 82 °C. The structural changes accompanying this transformation have been studied by Bindi and Menchetti (2005). Leadhillite, macphersonite and susannite are very rare and can be found in small amounts, in the oxidised zone of lead ore deposits, such as at Tsumeb (Namibia), Dundas (Tasmania, Australia), Bleiberg in Carinthia (Austria) and Caldbeck Fells (Cumbria, UK). Finally, it is worth mentioning that leadhillite has been found in weathered dumps of the medieval mining district at Altemannfels in Germany by Keim et al. (2017).

Anglesite and palmierite in artwork

The use of anglesite as a white pigment is extremely rare (Eastaugh et al. 2004) and, for this reason, it was chosen to be listed in this section, instead of the previous one on the main lead-basedminerals and pigments. The numerous studies focused on the mechanisms of anglesite and cerussite formation performed by Aze et al. (2006, 2007, 2008a) have clearly demonstrated the secondary nature of this phase. As such, it has been found in the following:

• Cambodia—wall paintings (11^th/17^th century?) of the Cruciform gallery, Angkor Wat complex (Uchida et al. 2012)

• China—(a) walls, reliefs and Buddha statues (618–907 AD) in caves no. 512 and 689 at the Guangyuan Thousand-Buddha Grotto, near Guagyuan (Sichuan, China; He et al. 2012); (b) Qing dynasty (1644–1912) wall paintings of the Imperial Taidong Tomb (Hebei, China; Fu et al. 2020); and (c) 907–960 AD wall paintings of the Feng Hui Tomb in the Bin County (Shaanxi, China; Wang et al. 2004)

• Italy—Late Medieval (~15^th century) wall paintings of the St. Stephen's Chapel, near Bolzano (Costantini et al. 2020b)

• Slovakia—wall paintings (Marian cycle; ~1000 AD) of the St George Church at Kostol’any pod Tríbečom (Hradil et al. 2013)

• Spain—(a) 16^th-century(?) ceramic statues in the Seville Cathedral (Pérez-Rodríguez et al. 1998) and (b) 19^th-century redecoration of the wall paintings of the Room of the Beds (Royal Bath of Comares), Alhambra, Granada (Arjonilla et al. 2019b)

Further occurrences of anglesite, regarding oils on canvas, include the Girl with a Pearl Earring (c. 1665) by Johannes Vermeer (kept at the Mauritshuis museum in The Netherlands; De Meyer et al. 2019a, b) and two artworks by Vincent van Gogh: the Flowers in a blue vase (June 1887) and the Wheat stack under a cloudy sky (October 1889) (both kept at the Kröller-Müller Museum in Otterlo, The Netherlands; Van Der Snickt et al. 2012 and Vanmeert et al. 2015, respectively). To these, it may be possible to add Şerifaki et al. (2009), who claimed to have found anglesite, used as a priming white layer in the decorations of the 19^th-century Taxiarhis Church on Alibei-Adasi Island (Ayvalık) in Turkey.

Finally, it is worth adding that Hradil et al. (2007) found anglesite mixed with lanarkite in three 18^th to 19^th-century oil paintings on canvas, painted with Pb-Sb-Sn yellow. The authors interpreted the presence of these phases (as well as that of rosiaite) as by-products of Pb-Sb-Sn yellow production using the raw material Sb₂S₃ (see Figure 14, Eqs. 6–7).

The occurrences of palmierite are rare and limited to 17^th-century paintings on canvas (De Keyser et al. 2017; De Meyer et al. 2019a, b; Vanmeert et al. 2019) and oak wood panels (Albertson et al. 2019; Simoen et al. 2019). A further occurrence, with regard to the Voynich manuscript, has been investigated by a private company¹¹.

Scrutinyite (α-PbO₂) and plattnerite (β-PbO₂)

Mineralogical phases, structure and occurrence

Scrutinyite (Tables 2 and 3) is a red-brown mineral, first known as a synthetic material (Kameyama and Fukumoto 1946; Zaslavskii et al. 1950; Zaslavskii and Tolkachev 1952). The natural mineral was later discovered at the Sunshine#1 adit of the Blanchard Mine at Bingham in New Mexico (USA) by Taggart et al. (1988)¹². It occurs, together with plattnerite, in the oxidised parts of lead ore deposits. The name derives from the necessity of a close ‘scrutiny’ to distinguish it from plattnerite (Webmineral.com).

Plattnerite has a brownish to dark greyish black colour and a rutile-type structure (Tables 2 and 3; Figure 11A). Crystallographic studies have been performed on both natural (Paulsen et al. 2019) and synthetic (Leciejewicz and Padlo 1962; D'Antonio and Santoro 1980; Harada et al. 1981) materials, as well as under different pressure conditions (e.g., 23–140 GPa in Grocholski et al. 2014).

Fig. 11

A Plattnerite partially covered by creamy-orange calcite from Preguiça Mine in the Beja District, Portugal [Specimen and photo: fabreminerals.com-Mindat.org Photo ID: 218620]. B Plumbonacrite from the Clara Mine at Oberwolfach in the Baden-Württemberg, Germany [Photo credits: Copyright Stephan Wolfsried 2007 -Mindat.org Photo ID: 107026]. C Somersetite from the Torr Works Quarry in England, UK [Arkenstone specimens. Photo credits: Rob Lavinsky, irocks.com-Mindat.org Photo ID: 744814]. D Abellaite white crystals from the Eureka Mine in Catalonia, Spain [Collection Joan Abellai Creus. Photo credits: Matteo Chinellato – Wikipedia CC BY-SA 4.0]. [A–C images are subject to copyright owned by the photographers indicated above. The CC-BY licence does not supersede previously copyrighted material. Permission has been obtained for reproduction in this article]

Apart from the Blanchart Mine, scrutinyite and plattnerite have been found to occur at few sites in Greece (Lavrion mining district), Mexico (Durango), Namibia (Kunene) and the USA (Nevada and New Mexico) (Mindat.org).

Scrutinyite and plattnerite in artwork

Although they generally represent alteration products (Kotulanová et al. 2009), the use of plattnerite as an art pigment has been suggested by Holakooei and Karimy (2015). The hypothesis advanced by these authors was based on the prevalence of plattnerite in black paint and, on the other hand, due to the natural availability of this phase close to Fahraj (Darreh Zanjir mine, 35 km far). This is the only case in which this hypothesis has been formulated to the best of our knowledge.

Overall, these phases are fundamental in the study of paintings, as they represent alteration indicators. However, while plattnerite has been partially studied and frequently observed in wall paintings (Daniilia et al. 2008; Daniilia and Minopoulou 2009; Kotulanová et al. 2009; Hradil et al. 2013, 2014; Gutman et al. 2014; Liu et al. 2016; de Ferri et al. 2019; Costantini et al. 2020b), scrutinyite has rarely received attention (Kotulanová et al. 2009; Lytle et al. 2009; Costantini et al. 2020a; Vagnini et al. 2020).

The formation of plattnerite from lead white has been verified through experimental tests using hydrogen peroxide (H₂O₂) at high RHs (Aze 2005), ozone (Aibeo et al. 2008) and NaClO. In the last case, the experiments performed by Kotulanová et al. (2009) demonstrated that, in the presence of NaClO, lead white reacts very quickly, turning black–brown. Similarly, the experiments carried out by Vagnini et al. (2020) assessed the formation of plattnerite and minor scrutinyite (+ halite) when the starting material was cerussite, as well as the formation of scrutinyite and minor plattnerite (+ cerussite and halite) when the starting material was hydrocerussite (see above). The practical implications of these results also lie in the fact that scrutinyite is easier to remove during restoration operations due to its higher solubility than plattnerite. A mixture of both plattnerite and scrutinyite can also form in 8 days in a chlorinated water solution with pH = 8.1 (Lytle et al. 2009).

The secondary products of plattnerite are minium, litharge, massicot and another lead phase (lead oxide PbO_1.55). The transformation of βPbO₂ into Pb₃O₄ occurs spontaneously in air at T>375 °C (Clark et al. 1937); in more detail, the reaction path described by Greenwood and Earnshaw (1997) is as follows: βPbO₂ → (290 °C) Pb₁₂O₁₉ → (350 °C) Pb₁₂O₁₇ → (375 °C) Pb₃O₄ → (600 °C) PbO. The thermal treatment of plattnerite performed by Aze et al. (2008b) provided similar transformation temperatures: βPbO₂ → (300 °C) Pb₂O₃ → (375 °C) Pb₃O₄ → (589 °C) αPbO → (>589 °C) βPbO.

The degradation and conversion of plattnerite using Ar+ lasers (e.g., with a power higher than 30 mW) are crucial aspects to be taken into due account during the investigation (see the section on analytical methods below; Burgio et al. 2001; Aze et al. 2008c).

Plumbonacrite (Pb₅O(OH)₂(CO₃)₃) and associated phases

Mineralogical phases, structure and occurrence

Plumbonacrite is a lead oxide–hydroxide carbonate that, together with abellaite, grootfonteinite and somersetite (Tables 2 and 3), is related to hydrocerussite. These phases can be naturally found to be associated and, therefore, are of interest in the study of lead white.

Plumbonacrite is a lead-based ('plumbo') mineral with a pearly lustre ('nacre') (Figure 11B), individuated at the type locality Torr Works (Merehead) Quarry (East Cranmore, Somerset, UK) (Williams et al. 2012). It was first investigated by Krivovichev and Burns (2000) (Table 2 ‘a’), then revalidated in 2012 (Table 2 ‘b’) (Rumsey et al. 2012a).

Abellaite is a colourless to white (Tables 2 and 3; Figure 11D) Na/K–Pb hydroxide carbonate, first recognised in the Eureka mine in Catalonia (Spain) and structurally characterised by Ibáñez-Insa et al. (2017). It can include minor quantities of CaO [Na_0.96Ca_0.04Pb_1.98(CO₃)₂(OH)] and has a synthetic previously known analogue (Brooker et al. 1983; Krivovichev and Burns 2000; Belokoneva et al. 2002; Bette et al. 2017; Siidra et al. 2018a). Within the lead hydroxycarbonates, the parameters provided by Siidra et al. (2018d) for an unnamed species individuated in Laurion metallurgical slag (Attica, Greece) are also shown in Table 2. This phase has also been found as a reaction product of a cerussite-based pigment (containing 68% cerussite, 6% hydrocerussite and 26% shannonite) treated with Na₂CO₃ and NaHCO₃(Kotulanová et al. 2009).

Grootfonteinite is a rare, colourless Pb oxo-carbonate, which has been structurally investigated by Siidra et al. (2018a) (Tables 2 and 3). These last authors pointed out that grootfonteinite is macroscopically very similar to abellaite (as well as to hydrocerussite and plumbonacrite) and that discrimination between grootfonteinite and abellaite is only possible by means of electron microprobe analysis, provided that a preliminary XRD analysis has discriminated these phases from the others.

Somersetite (Tables 2 and 3; Figure 11C) was individuated at the same type locality of plumbonacrite. This is a green or white mineral whose crystallographic structure was resolved by Siidra et al. (2018c).

Plumbonacrite in artwork

Plumbonacrite has been found on the oil painting on canvas Wheat stack under a cloudy sky by Vincent van Gogh (Vanmeert et al. 2015).

Plumbonacrite can easily form in water solutions and in rather dry oils at ambient temperature and pressure. As demonstrated by the experiments conducted by Gonzalez et al. (2019a), a mixture of plumbonacrite and hydrocerussite can be observed after having aged PbO particles (litharge) dispersed in heated oil for three months, where PbO was no longer present at the end of the experiment.

Based on the photochemical reaction described by Zhou et al. (2012) and Vanmeert et al. (2015), plumbonacrite can form from the photoactivation of minium (Figure 6), leading to a reduction of Pb^IV to Pb^II; alternatively, PbO—which can be present as a residue due to the incomplete synthesis of minium—can trigger the transformation. As demonstrated by Kotulanová et al. (2009), both litharge and massicot can lead to the formation of cerussite, hydrocerussite and plumbonacrite due to their instability in humid and alkaline environments (Taylor and Lopata 1984). For the reaction sequence PbO ↔ Pb₁₀O(OH)₆(CO₃)₆(plumbonacrite) ↔ Pb₃(OH)₂(CO₃)₂(hydrocerussite) ↔ PbCO₃ (cerussite), the equilibrium conditions (at ~25 °C) in aqueous carbonate solutions have been investigated by Taylor and Lopata (1984).

With more difficulty, plumbonacrite can also form from lead white (Figure 1); however, being metastable with respect to both PbO and hydrocerussite, it is easily reconverted into hydrocerussite. Other possible origins are limited by the alkaline conditions (pH>10) required for its stability (Gonzalez et al. 2019a). Using a synchrotron radiation technique (μ-XRPD tomography), Janssens and Cotte (2019) identified that plumbonacrite had formed at the interface between minium and degradation products in a painting by Van Gogh.

Cotunnite (PbCl₂), laurionite (Pb(OH)Cl), blixite (Pb₈O(OH)₂Cl₄) and phosgenite (Pb₂Cl₂(CO)₃)

Mineralogical phases, structure and occurrence

Cotunnite is a lead chloride forming prismatic, short, or elongated crystals often grouped in radiating aggregates. Granular or massive masses, as well as crusts, are also common (Figure 12A). The crystallographic characteristics of cotunnite (Table 2) have been determined by Sass et al. (1963). This phase occurs both as a primary and secondary mineral, and it is most frequently associated with anglesite, cerussite and hydrocerussite (Table 3). Its stabilisation requires an acidic environment (pH <3) and high chloride activity (Humphreys et al. 1980; Edwards et al. 1992). As cotunnite forms from acid solutions, the increase in pH leads to its transformation into paralaurionite (PbCl(OH)) and, finally, mendipite (Humphreys et al. 1980). Africano et al. (2002) found cotunnite in sublimates deposited by the hot gases (below 325 °C) released by the Satsuma-Iwojima volcano (Japan), Krivovichev et al. (2002) and Pekov et al. (2015) in the incrustations associated with fumaroles of the Tolbachik volcano in the Kamchatka Peninsula (Russia) and Lacroix (1907) in the deposits of the fumaroles of Mt. Vesuvius.

Fig. 12

A Cotunnite on display at the New York Museum of Natural History [By Geckzilla - own work, Public Domain, ID=6043437]. B Laurionite from the Legrena Cove slag locality in the Lavrion district, Greece [photo credits: Ko Jansen –Mindat.org Photo ID: 677146]. C Blixite white/yellowish crystals from the Thorikos Bay slag locality in the Lavrion District, Greece [photo credits: Ko Jansen –Mindat.org Photo ID: 784361]. D Phosgenite from Monteponi Mine, Iglesias, Sardinia, Italy [specimen and photo: fabreminerals.com–Mindat.org Photo ID: 935277]. [All images are subject to copyright owned by the photographers indicated above. The CC-BY licence does not supersede previously copyrighted material. Permission has been obtained for reproduction in this article]

Lacroix (1920), Russel (1920) and Schnorrer-Köhler (1986) reported cotunnite as a natural corrosion product of lead objects placed in marine conditions (i.e. in contact with seawater containing chlorine). The Laurion mines in Attica (Greece) are famous for such an environment. Here, a wide range of alteration products, including laurionite, paralaurionite, cotunnite and blixite, had formed naturally by the reaction of seawater with Pb-rich glassy metallurgical slags obtained about 2500 years ago (Siidra et al. 2011). Similarly, Franzini and Perchiazzi (1992) described a rich association of secondary minerals, with cotunnite formed by the interaction of seawater with Etruscan iron slags spread along the Baratti Beach in Tuscany (Italy). Kutzke et al. (1997) mentioned cotunnite, laurionite and phosgenite as corrosion products formed through the prolonged contact of seawater with lead objects from a shipwreck (ca. 80 BC) sunken in the Mediterranean, near the Tunisian coast.

Other occurrences have been reported in Austria, Brazil, Canada, France, India, Iran, Israel, Namibia, New Zealand, Norway, Peru, Poland, Romania, Spain, Tunisia and the USA by Antony et al. (1990).

Laurionite is an orthorhombic lead oxychloride (Tables 2 and 3). It forms colourless or white small crystals, massive masses, or thin crusts (Figure 12B). Its structure was determined by Venetopoulos and Rentzeperis (1975) and Merlino et al. (1993). In its structure, each Pb atom is surrounded by five Cl atoms and three OH groups. The solubility of laurionite in water is low but increases with temperature. Köchlin (1887) described laurionite, for the first time, in the ancient metallurgical slags corroded by seawater at Laurion, in Greece (hence the name). Here, this phase was associated with other secondary Pb phases, such as anglesite, cotunnite, cerussite and phosgenite (Palache 1934; Katerinopoulos 2010; Periferakis et al. 2019). Scott (1994) also mentioned laurionite at the Endeavor Mine in New South Wales (Australia), where it formed in saline conditions, together with blixite and mendipite. Laurionite, phosgenite, anglesite, paralaurionite and cerussite were further described, by Russell and Hutchinson (1927), from the Wheal Rose Mine in Cornwall (UK). Other occurrences have been listed by Anthony et al. (1990) in Austria, Canada, Czech Republic, France, Germany, Israel, Italy, Mozambique, Norway, South Africa, Tunisia, Turkmenistan and the USA.

Blixite is a lead hydroxychloride, occurring as light green light yellow, yellow–orange, greyish–yellow massive masses and thin crusts (Tables 2 and 3; Figure 12C). It was first described by Gabrielson et al. (1960) from the Långban Mine (Sweden). The structure of the synthetic analogue of blixite has been investigated by Krivovichev and Burns (2006). The authors described it as (OPb₄) oxo-centred tetrahedra, linked by shared Pb…Pb edges to form [O₅Pb₈] sheets parallel to (100). The OH groups are attached to the sheets and form two short OH–Pb bonds. The authors observed: "it is also possible that blixite exists with differing degrees of order of the O^2–and OH^–anions, which causes different unit-cell parameters". A blixite + mendipite association was described at the Mendip Hills (UK) by Symes and Embrey (1977). At the Endeavor Mine at Elura (New South Wales, Australia), blixite and laurionite were found in a narrow zone between primary sulfide-bearing assemblages and the overlaying oxidation zone (Scott 1994). At Merehead (now Torr Works) Quarry in Somerset (UK), Edwards et al. (1992) described a deposition sequence starting with blixite→ mendipite → paralaurionite along with the decrease of the pH from ~8 to ~4 and within a temperature range of ~100 °C to a minimum of 30 °C.

Phosgenite is a tetragonal chloride-carbonate of lead (Tables 2 and 3). It forms mainly crusts and masses, with a wide range of colours (Table 1; Figure 12D). In its structure, the (Pb—2Cl₂—Pb) layers are connected by COs-groups (Giuseppetti and Tadini 1974). This is a secondary mineral, found mainly in the oxidised zone of lead deposits (Table 3). Köchlin (1887) described phosgenite for the first time from the ancient slags altered by seawater at Laurion (Greece). The type locality for phosgenite is Cromford, near Matlock, in Derbyshire (England), where it occurs together with matlockite (Greg 1851; Bannister 1934). Stanley et al. (1991) mentioned phosgenite and barstowite (Pb₄Cl₆(CO₃)·H₂O) in the cavities of lead-antimony veins exposed to the foreshore at Bound Cliffs in Cornwall (UK), where it formed as a result of the action of seawater on a galena assemblage. Phosgenite has also been found at Monteponi Mine in Sardegna (Italy), together with galena, leadhillite, minium, cerussite and anglesite (Guastoni and Pezzotta 2007).

Phosgenite is a mineral which is widespread worldwide and the occurrences listed by the Mindat.orgdatabase—in Argentina, Australia, Austria, Brazil, Chile, China, Czech Republic, France, Germany, Greece, Iran, Ireland, Italy, Mexico, Morocco, Namibia, Norway, Poland, Portugal, Russia, South Africa, Spain, Tunisia, the UK and the USA—are just the best-known ones.

Cotunnite, laurionite, blixite and phosgenite in artwork

Cotunnite, lauronite, blixite and phosgenite have been sporadically found in artwork and frequently interpreted as secondary phases. Moreover, the processes that occur in lead objects in contact with seawater (i.e. containing chlorine) can provide valuable information for the interpretation of these phases as secondary phases in paintings.

Laurionite (Pb(OH)Cl) and phosgenite (Pb₂Cl₂(CO)₃) have been produced starting from litharge, reduced into fine powders and used for cosmetics (Walter et al. 1999). It is also known that laurionite was possibly used as a medicament to treat eye illnesses (Tapsoba et al. 2010). Hence, given the frequent use of the same pigments both for cosmetics and for painting, their use as primary pigments cannot be excluded.

Their occurrences in paintings must be discussed on a case-by-case basis. For example, Ordonez and Twilley (1997) described a thin white cotunnite coating on the blue- and brown-coloured areas of both an oil portrait by Pierre-Auguste Renoir (dated to 1918) and on the painted ceiling of the Powell Library at the University of California (Los Angeles, USA). The authors presumed that cotunnite formed through a reaction between white lead and a Cl-rich fluid; in the second case, originating from the undried plaster.

The white hydrocerussite paint covering a 16^th-century terracotta sculpture by Della Robbia showed cotunnite associated with challacolloite KPb₂Cl₅ (Ma and Berrie 2020), where both phases were interpreted as secondary products.

Winter (1981) found that the main white lead-based pigments used for 12^th- to 16^th-century Japanese paintings was laurionite, while blixite was also present. The author assumed that both laurionite and blixite were most likely artificially produced, probably by precipitation.

A further occurrence of laurionite and/or cotunnite (and possibly of one or more ‘other Pb/Cl compounds’) has been reported by Salvant et al. (2018), in ~2^nd-century AD Roman mummy portraits from Tebtunis in Egypt.

Caledonite (Cu₂Pb₅(OH)₆CO₃(SO₄)₃), schulténite (PbHAsO₄) and vanadinite (Pb₅(VO₄)₃Cl)

Mineralogical phases, structure and occurrence

Caledonite is an orthorhombic lead and copper hydrous carbonate–sulfate (Table 2). It occurs as small prismatic crystals, massive masses and coatings (Figure 13A). The structure consists of CO₃ and SO₄ groups connecting lead polyhedra, and these with chains of CuO₆ square bipyramids. Three independent hydrogen atoms are bound, as hydroxyl groups, to the oxygen atoms at the corners of the CuO₆ bipyramids (Palache and Richmonds 1939; Giacovazzo et al. 1973; Schofield et al. 2009).

Fig. 13

A Caledonite from Mammoth-Saint Anthony Mine, AZ, USA [Arkenstone specimen. Photo credits: Rob Lavinsky, irocks.com–Mindat.org Photo ID: 828378]. B Schulténite from Tsumeb Mine, Oshikoto Region, Namibia [collection and photo of Bruce Cairncross –Mindat.org Photo ID: 744912]. C Vanadinite on a barite matrix from Mibladen, Midelt, Drâa-Tafilalet Region Morocco [specimen and photo: fabreminerals.com–Mindat.org Photo ID: 862414]. [All images are subject to copyright owned by the photographers indicated above. The CC-BY licence does not supersede previously copyrighted material. Permission has been obtained for reproduction in this article]

Caledonite is a secondary mineral, associated with anglesite, cerussite, leadhillite, linarite, azurite, and malachite, in the oxidation zone of Pb-Cu ore deposits (Table 3). In the oxidised zone of the galena–chalcopyrite deposit at Leadhills, caledonite (+ linarite PbCu(SO₄)(OH)₂ or leadhillite) directly replaces anglesite and cerussite. Temple (1956) observed that caledonite and brochantite (Cu₄(OH)₆(SO₄)) do not occur together, despite the fact that both minerals (a) are related to oxidised lead-copper ores, (b) are associated with linarite and (c) pseudomorphically replace galena. Caledonite forms from fluids in which lead predominates over copper, whereas brochantite forms from lead-deficient fluids. Thus, the conditions favouring the formation of brochantite inhibit the formation of caledonite and vice versa.

Apart from its occurrence at Leadhills, there are many other places where caledonite has formed, such as the Preobrazhenshy Mine in Sverdlovsk (Russia), Tchah Milleh (Iran), Mammoth Mine in Arizona (USA), Mex-Tex Mine in New Mexico (USA), Cerro Gordo Mine in California (USA), Challacollo Mine (Chile) and the Paddy’s River Mine (Australia).

Schulténite is a monoclinic hydrous lead arsenate (pre-IMA species), named in honour of August Benjamin de Schultén (1856–1912), who synthesised the artificial hydrous lead arsenate. It forms small crystals and (sometimes) crusts (Figure 12B). The structure of schulténite (Claringbull 1950; Choudhary 1982; Effenberger and Pertlik 1986b; Wilson et al. 1991; Wilson 1994) involves chains of AsO₄ tetrahedra connected by short hydrogen bonds, where Pb atoms are surrounded by the six nearest oxygen atoms (Table 2).

This is a rare secondary mineral, which forms by the hydrothermal alteration of Pb–As ores (Table 3). At the Tsumeb Mine (Namibia), this phase was found (a) as ‘pseudomorphous crusts’ and small crystals intermixed with bayldonite, on anglesite (Spencer and Mountain 1926); (b) associated with cuprian adamite and keyite in the lower oxidation zone (Falls et al. 1985); and (c) replaced by bayldonite and duftite (Magalhães et al. 1988). Other occurrences (Mindat.org; Anthony et al. 1990) are at the Schwartzwald Mts. and Black Forrest Mts. (Germany), North Bend (WA, USA), Deer Hills Mine (England) and Michael Mine in Weiler (Germany). An analogue lead arsenate has been synthesised by Effenberger and Pertlik (1986b).

Vanadinite is a hexagonal lead chloride vanadate. It occurs as short prismatic crystals, massive granular masses and crusts (Figure 12C). Its structure has been investigated by Trotter and Barnes (1958), Baker (1966) and Dai and Hughes (1989) (Table 2). The structural unit of vanadinite consists of an octahedron with a Cl atom surrounded by 6 Pb atoms. The octahedra form a continuous chain sharing opposite faces. The VO₄³⁻ groups are isolated as discrete bonding units (Loeffler and Burns 1976), where each vanadium atom is surrounded by four oxygen atoms at the corners of a tetrahedron. Three oxygen tetrahedra adjoin each of the lead octahedron along the chain (Trotter and Barnes 1958). The red–orange–yellow colour of vanadinite is due to the oxygen (O-II) to metal (VV)charge-transfer within the (VO₄₎^−II group, which determines a strong absorption of visible light in the violet–green domain (Loeffler and Burns 1976). Vanadinite is a secondary mineral, found in the oxidation zone of lead ore deposits (Table 3). It was first described by von Kobell (1838), and its type locality is Zimapan (Hildalgo, Mexico). Other notable occurrences are (Mindat.org): Tsumeb (Namibia), Bisbee (AZ, USA), Longefay Mine (Rhône, France) and Broken Hill (New South Wales, Australia).

Caledonite, schulténite and vanadinite in artwork

Caledonite has been identified as a decay product of the ultramarine blue paint in 12^th- to 13^th-century polychrome sculptures of the Parma baptistery and Ferrara Cathedral in Italy (Pinna et al. 2020). The authors discovered it together with gypsum and weddellite and specified that it formed by "the sulphation process in the presence of […] lead and copper".

Schulténite (alteration product) was found in the 17^th-century oil on canvas still life painting by Jan Davidsz de Heem (1606–1684; De Keyser et al. 2017; Vanmeert et al. 2019).

Vanadinite is rare and, based on the literature available so far, it has not been used as a pigment. A mixture of vanadinite, red ochre and yellow ochre has been determined by Levstik et al. (2019), in the 15^th-century orange–yellow paints on the walls of a Gothic church in Koper (Slovenia).

Challacolloite (KPb₂Cl₅) and the dolomite homotype PbMg(CO₃)₂

Mineralogical phase, structure and occurrence

Challacolloite is rare chloride, KPb₂Cl₅ (Tables 2 and 3). Its crystal structure has been studied by Schlüter et al. (2005) and Mitolo et al. (2009). As challacolloite is currently synthesised for laser applications, further studies on its structure and properties are available in the literature (see Mitolo et al. 2009 and the references therein). The few recorded occurrences include hydrothermal and volcanic environments, such as the leucotephrite lava produced by the 1855 AD Vesuvius eruption (Italy; Schlüter et al. 2005), the medium-high temperature (T=400 °C) fumaroles at Vulcano in the Aeolian archipelago (Italy; Mitolo et al. 2009), the sublimates of Satsuma-Iwojima volcano in Japan (Africano et al. 2002) and mineralisations in the active fumaroles of the Tolbachik volcano in Kamchatka (Russia; Pekov et al. 2015).

The unnamed phase PbMg(CO₃)₂ was first studied by Lippmann (1996) who synthesised it by leaving ca. 100 mg of pure lead carbonate (PbCO₃) at about 20 °C in a closed vessel with 250 ml of an aqueous solution of 0.01 ml of MgCl₂ and 0.02 ml of NaHCO₃. After about 6 months, he obtained crystals about 20 μm in size and examined them using XRD. The phase thus studied appeared similar to dolomite and norsethite (BaMg(CO₃)₂). Böttcher et al. (1996) investigated this phase using FTIR and Raman spectroscopy while Zheng and Böttcher (2016) studied oxygen and carbon isotope fractionations involving PbMg(CO₃)₂. Pimentel and Pina (2015, 2016) obtained this phase from mixing solutions (Pb(NO₃)₂, Mg(NO₃)₂, Na₂CO₃) at room temperature and observed different sequences of dissolution–crystallisation reactions depending on the Pb:Mg ratios and the overall composition of the slurries. The attention of scholars has mainly focused on norsethite; therefore, this phase has been little studied. However, Kotulanová et al. (2009) performed a series of experiments aimed at simulating the alteration of the wall paintings. They practically repeated Lippmann's experiments starting, however, from red lead instead of lead carbonate. After the same time (6 months), they obtained 26% PbMg(CO₃)₂, 51% unconsumed red lead, 13% plattnerite and 10% cerussite.

Challacolloite and PbMg(CO₃)₂ in artwork

Challacolloite was first found by Di Stefano and Fuchs (2011) in the middle Ptolemaic Book of the Dead of Amenemhet (papyrus; ROM 978×43.1). The authors interpreted its presence as a secondary product, formed by the interaction of hydrocerussite with the potassium chloride from the earth. A few years later, this phase was found, by Bezur et al. (2015), on a 15^th-century Virgin and Child terracotta relief by Michele da Firenze. The authors individuated hydrocerussite as the likely source of lead, and potash lye and hydrochloric acid—possibly used for restoration interventions—as potential sources of potassium and chloride ions. Salvant et al. (2018) found challacolloite in the ~2^nd-century AD mummy portraits from Tebtunis (Egypt). The authors found it to be associated with hydrocerussite and suggested its in situ formation from that pigment. The last occurrence was provided by Ma and Berrie (2020), who found challacolloite and cotunnite in the painted decoration of an early 16^th-century Pietà (terracotta sculpture) by Giovanni della Robbia. As with the previous authors, the formation of this phase was interpreted as a transformation of hydrocerussite under high chloride ion concentrations and/or low pH conditions, following the aqueous solution’s evaporation. The possibility of K-leaching by the glaze was considered less likely, given the stability of the glaze.

PbMg(CO₃)₂ has been found in a Gallo-Roman (end of 2^nd century AD) mural fragment from the Roman villa excavated at Pierre Hardie Street in Metz (France) (Dooryhée et al. 2005). The authors interpreted its presence as a secondary product, due to the interaction of massicot with Mg-containing calcium carbonate. The wall paintings in Kostol’any pod Triběcom (Slovakia) provided a further occurrence, investigated by both Hradil et al. (2013) and Kotulanová et al. (2009). This phase was associated with plattnerite, scrutinyite, cerussite, hydrocerussite and massicot and was interpreted as a secondary phase. Hradil et al. (2013) further claimed that its presence testified the use of dolomitic lime. Kotulanová et al. (2009) suggested that "Pb²⁺ions, formed by the disproportionation of red lead, reacted with aqueous magnesium ions in the porous system of the plaster and formed PbMg(CO₃)₂".

Production centres of lead-based pigments

Archaeological evidence of artisanal quarters devoted to the production of the pigments outlined above is small and geographically limited.

The production site individuated in the southern sector of the ancient agora of Kos Island (Greece) is of outstanding importance, as it provides evidence of the close relationship of pigment production with metalworking in the second half of the 1^st century BC (Kostomitsopoulou Marketou et al. 2019). The pigments produced onsite (Egyptian blue and red, brown, yellow and green earths) were found, together with litharge rods and lead lumps. The finding of a Fe- and Pb-based red pigment led the authors to hypothesise for this mix either an artificial nature or a natural origin. In the first case, the mix would have represented the product ‘of a mixing and calcination process’, described by Plinius (Naturalis Historia, 35, 30, 40) as sandyx (rubric and sandarach in 1:1 ratio, calcined with cerussa) and syricum (sinopis and sandyx in 1:1 ratio). The second case was suggested by its similarity with a geological sample taken from the ‘Metalleio’ site on Mt. Dikaios.

Precious information has also been provided by the research project carried out, in an Early to Late Cycladic I (c. 2700–1600 BC) context, at Akrotiri on Santorini Island (Greece). Here, Sotiropoulou et al. (2010) found and analysed raw pigments (in various shapes, ranging from powders to lumps and moulds) and traces of pigments on the surface of stone tools, which were likely used for pigment preparation. In more detail, the authors found the following: (a) a bright-pink layered fragment of minium (4%) mixed with calcite (92%), quartz (3%) and muscovite (1%); (b) various residual lead-based compounds on stone tools (cerussite, hydrocerussite, litharge, caracolite (Na₃Pb₂(SO₄)₃Cl), lead sulfate (Pb₂SO₅), lead hydroxide chloride, calcium lead phosphate hydroxide (Ca_0.81Pb_4.19(PO₄)₃(OH)), hydroxylapatite plumboan and drugmanite (Pb₂(Fe³⁺,Al)H(PO₄)₂(OH)₂)); and (c) mineral phases not bearing Pb such as hydroxylapatite, clinoptilolite, halite, hematite, calcite, dolomite, kaolinite, illite, albite, labradorite, hornblende and quartz. While the authors were able to correlate the production of these pigments to the processing of masses of litharge, the use of the pigments remained obscure. The researchers found no evidence of lead-based pigments in the local wall paintings; therefore, pending further discoveries, a likely hypothesis is that they were intended for cosmetics.

In this context, the pigments found in the workshops of artists or stores at Pompei in Italy (see, e.g., Augusti 1967; Giachi et al. 2009; Aliatis et al. 2010), or Delos in Greece (Karydas et al. 2009) provided suitable information for the reconstruction of the production process, as they did not contain binding media. For example, at Pompeii, the cerussite found in a mixture with calcite and quartz in the Egyptian blue powder "may have been introduced by the painter to fade the blue colour" (Aliatis et al. 2010); however, it is known that pigments were sold by weight, and therefore, cerussite could be present to increase the weight of the product. Still at Pompeii, the discovery of a powder made of malachite, Egyptian blue, hematite, goethite, cerussite, massicot, albite, calcite, quartz and copper arsenate compound (sample P13 from bowl no. 117364; Aliatis et al. 2009) testifies how different types of pigments were mixed to obtain the desired colours. Conversely, the absence of lead-based pigments in similar contexts, such as at Elephantine Island on the Nile in Egypt (Pagès-Camagna and Raue, 2016) can provide chronologically framed negative evidence of their use.

Finally, underwater findings can help to reconstruct the routes connecting the production centres with the distribution sites. In this regard, the discovery of galena, lead carbonate, minium, metallic objects and raw glass in the shipwreck of Glavat (Mljet/Meleda, Croatia) is very interesting (Auriemma 2018). According to various authors, the origin of the metals and pigments can be traced back to the silver and lead mines located in the Srebrenica area (Bosnia); that is to say, ancient Argentaria. The authors observed that the mining area was well-connected with the ports of Salona, Narona and Epidaurus, where the cargo of the sunken ship could have been purchased (Radić Rossi 2009).

Lead-tin yellow I–II, lead-antimony yellow (Naples yellow), lead-tin-antimony yellow and lead chromates

An exhaustive discussion of these pigments would require at least three separate articles. Thus, this section attempts to provide a brief overview, describing the main characteristics of these pigments and indicating the essential bibliographic references from which to start a detailed examination.

For these pigments, it should also be noted that lead stannates and antimonates are common glass colouring and opacifying agents. The intensive study of these phases in the field of ancient glass and ceramic production has created such a body of literature that it cannot be cited here in its entirety. We, therefore, limit ourselves to indicating the texts to be consulted to begin their study: Turner and Rooksby (1959); Hedges and Moorey (1975); Hedges (1976); Henderson and Warren (1983); Mass et al. (1996); Mason and Tite (1997); Freestone and Stapleton (1998); Pernicka et al. (1998); Tite et al. (1998); Molera et al. (1999); Heck and Hoffmann (2000); Mass et al. (2002); Shortland (2002); Tite et al. (2008); Rosi et al. (2011) and Matin (2018).

Lead-tin yellow types I and II

There are conflicting opinions on the terminology and production technology of lead-tin yellow pigment handed down by ancient texts, mainly because all of these pigments (as well as massicot) appeared indiscriminately under the name of giallorino or giallolino, or using analogous terms such as zalolin (the term zalo corresponds to modern ‘giallo’, meaning yellow). The mentions and recipes handed down since the 15^th century are many and, as such, we report only a few examples from the 15^th to the 17^th century, through which the origin and use of the term are understood (Table 3).

A terminological confusion (at least, as far as today's reader is concerned) persisted in the following centuries, until the analytical research conducted separately by Jacobi and Kühn shed light on the real nature of these pigments, also taking advantage of the studies carried out simultaneously by Turner and Rooksby on opacified glass (Turner and Rooksby 1959, 1963; Rooksby 1964).

Jacobi (1941) first took stock of the conflicting opinions of Raehlmann¹³(1910), who claimed to have found Naples yellow on paintings from the Renaissance period, and De Wild (1931), who, having not found it, proposed identification with massicot. He, then, analysed some samples taken from Germano-Gothic artworks of the 15^th and early 16^th centuries and from Dutch artworks from the 16^th and early 17^th centuries. The results highlighted the presence of rather high tin levels and allowed him to synthesise the lead-tin yellow ‘of the old masters’ in various shades. He used a mixture composed of about three parts of lead oxide, red lead or lead dioxide, and one part of metastannic acid for the synthesis process. The mixture was heated at a temperature of 650–800 °C, and different colours were obtained at variable temperatures: orange between 650 and 680 °C, rich yellow at 700 °C and lemon-yellow between 740 and 800 °C.

Kühn (1968) first deduced that "both the 'giallolino' mentioned in Italian literary sources and the 'massicot' ('masticot') of the northern manuscripts are identical with lead-tin yellow", based on various elements, including a comparison between Italian and foreign literary sources, the discovery of lead-tin yellow in 154 paintings and 4 polychrome sculptures, the analysis of lead oxides samples from old pharmacies and previous scientific works (Gettens 1967; Coremans et al. 1952; Sneyers and Thissen 1958; Coremans 1961). Secondly, Kühn distinguished between lead-tin yellow I (Pb₂SnO₄) and lead-tin yellow II, the latter being compositionally similar to the yellow lead-tin oxide determined by Rooksby (1964) as a glass opacifier (probably PbSn₂SiO₇, Kühn hypothesised). In fact, Rooksby (1964) synthesised this compound at 800–950 °C, starting from a mixture of lead-tin yellow I (Pb₂SnO₄) and silica. Reconstructing the history of this pigment and its studies, Kühn (1968, 1993):

• 1. suggested that this pigment was "manufactured in glass furnaces", based on the connection with the giallo di vetro (glass yellow) mentioned in various ancient texts (Table 3);

• 2. set up a correspondence between lead-tin yellow type I and the giallolino fino discussed by Merrifield (1849) as a Flanders origin product;

• 3. established a correspondence between the lead-tin yellow type II, the giallo di vetro (glass yellow) discussed by Merrifield (1849) as a Venetian origin product, and recipe 273 of the Bologna manuscript. Recipe 272 had to represent the first part and was not a recipe in its own right;

• 4. described the optical characteristics and provided numerous indications of the properties of these pigments, to facilitate their recognition through microchemical tests and instrumental analyses.

Subsequent studies have further clarified the structure of the various compounds and broadened our knowledge about the use of these compounds in painting and glass production. The studies performed by Byström and Westgren (1943), Swanson et al. (1972), Garnier et al. (1976), Gavarri et al. (1981), Gavarri (1982), Vigouroux et al. (1982), Clark et al. (1995) and Spahr et al. (2020) have gradually clarified the characteristics and the thermally induced transformations occurring in the structures.

In artworks, lead-tin yellow types I and II are typically used individually; however, cases in which both phases are used are not unheard of (Penny and Spring 1995; Aloupi et al. 2006; Seccaroni 2006; Borgia et al. 2007; Ricci et al. 2004). Lead-tin yellow type I is more widespread than type II and is almost always associated with lead white and cinnabar. Frequently, it has also been associated with azurite, followed by minium, malachite, ochres/earths, massicot, verdigris and ultramarine.

Lead-tin yellow type I

Walton and Trentelman (2009) argued that this pigment was produced "out of litharge associated with silver cupellation at the Rio Tinto site" (Figure 14, 1). Other authors have leaned towards a reconstruction starting from minium, thus synthesising a mixture of Pb₃O₄ and SnO between 650 °C and 900 °C (Figure 14, 2); in this case, it has been demonstrated that, when using higher temperature, the resulting pigment is lighter (Kühn 1968, 1993; Hradil et al. 2007). The hydrothermal synthesis developed by Wu et al. (1999) using lead acetate and sodium stannate (requiring caustic soda or sodium carbonate) has not been attested to in antiquity (Figure 14, 3). Based on Gavarri et al. (1981) and Clark et al. (1995), between 5 and 300 K (~ −268 °C and 27 °C), no phase transition is observed; at temperatures below 800 °C, the reaction is incomplete; and, at temperatures above 900 °C, PbO volatilises and the compound decomposes. Moreover, massicot may be present with an excess of Pb₃O₄.

Fig. 14

The chemical reactions illustrating the production processes of lead-tin yellow types I and II and lead-tin-antimony yellow. Equations 2 and 4 from Clark et al. (1995); Eq. 3 from Wu et al. (1999); Eqs. 5–7 from Hradil et al. (2007)

Modern lead stannate Pb₂SnO₄ (lead-tin yellow type I) is produced by heating PbCO₃ and SnO₂ (generally in a 2:1 molar ratio) for a variable time (e.g., 1 week in Swanson et al. 1972; 6–12 h in Hashemi et al. 1992), at temperatures between 750 and 1000 °C. This synthetic phase is characterised by edge-sharing SnO₆-octahedra forming "chains along the c-direction, interconnected within the (001) planes with Pb²⁺ions" (Spahr et al. 2020). Its structure was first assigned to the tetragonal space group P₄2/mbc by Byström and Westgren (1943) and Swanson et al. (1972) and then to the orthorhombic space group Pbam by Gavarri et al. (1981). Spahr et al. (2020) performed single crystal diffraction at ambient pressure on hydrothermally synthesised Pb₂SnO₄. They demonstrated that, although the Pbam space group has to be preferred, this lead stannate can be refined in both P₄2/mbc and Pbam space groups.

Regarding the chronology, Clark et al. (1995) claimed a period between 1450 and 1750 for the use of lead-tin yellow type I. In the literature published after their work, this pigment has been found in the following:

• 12^th- to 13^th (?)-century polychrome sculptures in the porch of the main and south portals of Ferrara cathedral (Pinna et al. 2020);

• 14^th-century wood ceilings of the Hall of the Abencerrages and the Hall of the Two Sisters in the Alhambra at Granada (Arjonilla et al. 2019a);

• 14^th-century (third quarter) wall paintings of the Dominican Monastery at Ptuj in Slovenia (Gutman et al. 2014)

• 15^th (also 16^th?)-century manuscripts on parchment (for the 15^th century, see, e.g., Chaplin et al. 2005; Duran et al. 2014; Bonizzoni et al. 2016; Carvalho et al. 2018; for a 15^th/16^th-century parchment see Vanmeert et al. 2018);

• 15^th- to 17^th-century panel paintings on wood or walnut (for the 15^th century, see, e.g., Martin and Duval 1990, 1998; Dunkerton and Roy, 1996; Hradil et al. 2007; Bersani et al. 2008; Ricci et al. 2004; Sodo et al. 2019; Gutman Rieppi et al. 2020; for the 16^th century, see, e.g., Penny et al. 1996; Higgitt et al. 2003; Roy et al. 2004; Mottin and Laval 2004; Plazzotta et al. 2006 (doubtful); Borgia et al. 2007; Hradil et al. 2007; Dunkerton and Howard 2009; Amadori et al. 2016; Želinska et al. 2018; for the 17^th century, see, e.g., Roy 1999; Olszewska-Świetlik et al. 2013);

• 14^th- to 17^th-century wall paintings (for the 14^th and 15^th centuries, see, e.g., Perardi et al. 2003; Gutman et al. 2014; Arjonilla et al. 2019a; Levstik et al. 2019; Pinna et al. 2020; for the 17^th century, see, e.g., Vlachou-Mogire et al. 2020);

• 17^th- and 18^th-century oil paintings on canvas (Feller 1973; Hradil et al. 2007; Duran et al. 2010; De Keyser et al. 2017; Vanmeert et al. 2019).

To study the alteration products, Kočí et al. (2019) mixed lead-tin yellow pigment with egg yolk and linseed oil and defined the structure of seven mixed lead carboxylates [Pb(C16)_2−x(C18)_x] in model tempera paints. The interactions between lead-tin yellow I and (a) egg yolk, (b) egg yolk and oil emulsion and (c) oil have been further investigated by Švarcová et al. (2019). These authors observed that, while tests containing only oil did not form lead soaps, tests containing the emulsion or egg yolk alone formed lead carboxylates, with a variable ratio of palmitates and stearates.

Lead-tin yellow type II

The lead stannate PbSn_1-xSi_xO₃, called lead-tin yellow type II, has been structurally investigated by Morgenstern Badarau and Michel (1971) and Clark et al. (1995). The crystallographic parameters were defined for PbSnO₃ by the former and for PbSn_0.76Si_0.24O₃ by the latter. As for the production, lead-tin yellow type I and silica were heated around about 900 °C (Figure 14, 4); being careful not to exceed the temperature of 950 °C, beyond which the basic compounds were obtained (i.e. tin oxide, lead oxide and silica). The proportions between the two components could vary within a certain limit, which changed the product's colour and the processing temperature. Based on Clark et al. (1995), an optimum was achieved with a 1:1 ratio. When this ratio was higher or lower, lead-tin yellow types I and II or lead-tin yellow type II and unreacted silica were obtained, respectively. Hradil et al. (2007) have also demonstrated that lead-tin yellow type II can also be obtained by a single-step synthesis, heating the single Pb, Sn and Si oxides at about 850 °C (Figure 14, 5), with a ratio of 6:3:2 of the three components). These authors further noticed that, when using even slightly different ratios, they obtained the following:

• (a) PbO forming a glassy phase and unconsumed SnO₂ with a slight excess of SiO₂;

• (b) Pb₂SnO₄ with an excess of Pb;

• (c) Pb₂SnO₄, PbSnO₃ and a glassy phase with equal amounts of PbO and SnO₂ but insufficient SiO₂.

They found the production of this pigment more complicate than that of lead-tin yellow type I and detailed the response in the tendency of lead to form a colourless glassy silicate phase, until a high temperature was reached (800–900 °C).

In artwork, it has been observed from the beginning of the 14^th century on panels (Gordon et al. 1985; Martin and Duval 1990; Kühn 1993) and wall paintings (Bandera et al. 2005; Seccaroni 2006). The same use has been attested in the 15^th century (Gordon et al. 2002; Borgia et al. 2007; Ricci et al. 2004), 16^th century (Kühn 1993; Penny et al. 1996) and the first half of the 17^th century (Seccaroni 2006). Accordingly, its diffusion, as indicated by Clark et al. (1995), spans from 1300 to 1650, which is not contradicted by the most recent literature. In this frame, the identification of lead-tin yellow type II, together with yellow chrome, in the rood screen and canopy of St. Mary's decorated by Sir Ninian Comper in 1896–98 for the Church of Our Lady of Egmanton in the UK sounds somewhat out of time (Edwards et al. 2007).

Before moving on to Naples yellow, it is worth noting that, while the chronologies provided for both types of lead-tin yellow likely highlight the period of the greatest diffusion of the pigment, the start and end periods of use may be partially invalidated by future detailed studies. For example, there already exist several papers that lack type characterisation (for various reasons) and whose dating falls within/outside the indicated chronological range. These latter studies reported the presence of lead-tin yellow pigments on artworks dated from the 6^th (Hedegaard et al. 2019) to the 16^th (Spring 2007; Espejo Arias et al. 2008; Križnar et al. 2009; Amadori et al. 2013; Franquelo et al. 2015; Aurélie et al. 2016; Martin-Ramos et al. 2017; Doménech-Carbó et al. 2019; Impallaria et al. 2020), 17^th (Križnar et al. 2019; Uhlir et al. 2019; Delaney et al. 2020) and 18^th (Cortea et al. 2020) centuries. Indeed, more extensive and detailed studies are required, in order to draw an indisputable chronological picture of the use of these pigments.

Lead-antimony yellow (or Naples yellow)

Naples yellow has been also called ‘lead antimonate yellow’, ‘Giallo di Napoli’, ‘Giallorino’ or ‘Giallolino di Napoli’. According to Laurenze and Riederer (1982), the term ‘Napples yellow’ (Luteolum Romaedicitur Luteolum Napolitanum) was used for the first time by Pozzo (1698–1702) (see also Table 3).

Naples yellow is an artificial lead–antimony oxide (lead antimonate). Compositionally, it is usually considered as isostructural with bindheimite (Table 2). However, Hålenius and Bosi (2013) recently re-examined old museum specimens collected in the 19^th century from the Harstigen Fe-Mn Mine (Sweden) and identified a new mineral species called oxyplumboroméite. This yellow-to-brown phase with the formula Pb₂Sb₂O₇ is a valid IMA species, which probably includes two questionable and poorly defined IMA mineral species, monimolite and bindheimite. Chemically, oxyplumboroméite is similar to ‘Naples yellow’.

In any case, Naples yellow is generally recognised as an artificial pigment, as the use of natural minerals has never been proven with certainty and the literary sources are not sufficiently clear in tracing the link between Naples yellow and the yellow pigment related to volcanoes (e.g., de Massoul 1797, Marcucci 1816; Merrifield 1846). In detail, Mary Merrifield (1849) mentioned that ‘giallorino’ (Naples yellow) is sometimes used for fresco painting but should be avoided where the paintings are exposed to the air too much. She also included ‘Giallolino di Fornace’ and ‘Giallolino di Napoli’ as alternative names for Naples yellow. According to Merrifield (1846, pp. clxii), there are two types of Naples yellow: (a) a natural mineral related to volcanoes, and which was called giallolino, giallolino di Napoli or jaune de Naples, and (2) an artificial pigment consisting of oxide of lead and antimony, called also ‘giallo di Napoli’, ‘jaune de Naples’, or ‘Naples yellow’ (see also Table 3 on this topic). Merrifield specified that the latter was in use in her times (i.e. the middle of the 19^th century) but was unknown to the old Italian painters.

The recipes that have been handed down to us directly (literary resources) and indirectly (reconstructed by archaeometric investigations on archaeological artefacts) show different procedures based on the place and chronology (Laurenze and Riederer 1982; Wainwright et al. 1986; Dik et al. 2005; Valeur 2010). For example, Marcucci (1816; p. 66-67) stated that ‘giallo di Napoli’ (Table 1, MA1) is a "yellow lead oxide mixed with antimony oxide", which can be prepared from a mix of white lead, antimony oxide, burnt alum, and ammonium chloride, calcined for 3 h.

Basically, however, all recipes have the calcination of a mixture of lead compounds (e.g., metallic lead, lead monoxide or lead white) and antimony compounds (e.g., metallic antimony, antimony (III) oxide Sb₂O₃, potassium antimonate (KSbO₃) and tartar emetic (K(SbO)C₄H₄O₆·1/2H₂O)) in common (Dik et al. 2002). Two important experimental researches have reconstructed the production process and studied the resulting products. In chronological order, the first was Dik et al. (2005), who estimated temperatures between 700 and 1000 °C for "historical lead antimonate production". The second was Hradil et al. (2007), who reproduced different processes, coming to definitively exclude the feasibility of some procedures tested in the past and reaching a significant reconstruction of how the past production could have taken place. In the first case, they showed that heating Pb₃O₄ and Sb₂O₃ without fluxes must reach at least 1000 °C to obtain a mixed product (bindheimite, rosiaite, Pb₂Sb₂O₇ and Pb_3+xSb₂O_8+x). In the second case, they showed that, through the use of 10–20% NaCl (a flux readily available in antiquity), it was possible to obtain a product (single-phase pyrochlore) of better quality than the previous ones at a lower temperature (700–900 °C; even if the elimination of NaCl requires a temperature of 800–850 °C for at least 5–10 h). When repeating the experiment with K₂CO₃ instead of NaCl, the product (pyrochlore with probable K in the structure) did not have the same quality; furthermore, when adding 5–10% silica gel, the product acquired a salmon colour. By replacing Sb₂O₃ with Sb₂S₃ there was no need for a flux.

Historically, lead antimonate has been used both for glassmaking, from the 16^th to the 14^th century BC (see below) and in painting. Clark et al. (1995) indicated the use of this pigment in painting in a period ranging from 1700 to 1850. Detailed accounts of the paintings containing Naples yellow have been given by Wainwright et al. (1986) and Seccaroni (2006).

In addition to the various attestations reported by these authors, further occurrences have been recently reported:

• tempera and oils on panels dated to the 16^th (Plesters 1984; Roy and Berrie 1998), 17^th (Felici et al. 2004; Moioli and Seccaroni, 2004) and 18^th- 19^th centuries (Bomford and Roy 1993; Hradil et al. 2007; Daniilia et al. 2008; Wagner et al. 2019);

• wall paintings from the second half of the 18^th century, at Moschopolis in Albania (Pavlidou et al. 2008) and Greece (Malletzidou et al. 2019).

Going back in time, it is undoubtedly worth mentioning the discovery carried out in the wall paintings of the Domus below the Basilica of Saints John and Paul on the Caelian Hill in Rome (Italy), dated to the late 1^st to 4^th centuries (original or retouched unknown).

Considering all these attestations, the period of use of Naples yellow expands from the beginning of the 16^th century to, at least, the 19^th century (see also Clark et al. 1995; Maggetti et al. 2009; Chiarantini et al. 2015; Yuryeva et al. 2018; Györkös et al., 2020). However, the reconstruction that admitted a complete replacement of lead-tin yellow pigments by Naples yellow at the beginning of the 18^th century (Laurenze and Riederer 1982; Lahlil et al. 2011) does not seems completely valid in paintings and was largely undermined by the state-of-the-art of the studies. The versatility of this pigment, both used individually and to obtain green in combination with Prussian blue (Grissom 1986), must have represented one of the advantages that made it so successful.

Lead-tin-antimony yellow

Lead-tin-antimony yellow was mentioned in the Darduin book of recipes (about 1644; Sandalinas and Ruiz-Moreno 2004; Pelosi et al. 2010). Hradil et al. (2007) experimentally reproduced the pigment and obtained Pb₂Sb_2-xSn_xO_7-x/2 (and traces of unconsumed SnO₂) through heating Pb₃O₄, Sb₂O₃, SnO₂ and 10% NaCl between 700 and 900 °C. The crystallographic structure of the resulting compounds, showing 2:1:1 and 2:1.5:0.5 ratios, was further investigated and found to be consistent with a "mixture of two pyrochlore structures with different cell sizes". The same experiment was successfully repeated using Sb₂S₃ instead of Sb₂O₃ (without NaCl) and resulted in the production of the yellow pigment + anglesite at 850 °C (Figure 14, 6) or + lanarkite at 900 °C (Figure 14, 7). Finally, it also worth underlining that these authors provided key indications to facilitate the individuation and discrimination of the various lead, tin and antimony-based pigments.

In artwork, this pigment is sporadically found. As previously observed by Hradil et al. (2007), this pigment "has not been identified in so many cases as Naples [---] and lead tin [---] yellows because it has not been searched".

The few occurrences we were able to find can be listed as follows:

• in early 16^th-century walls paintings and tempera on panel (Cauzzi 2003; Seccaroni 2006);

• in 17^th-century oils on canvas and, to a lesser extent, wall paintings (Garrido Perez 1992; Newman 1993; Ravaud et al. 1998; Roy and Berrie 1998; Santamaria et al. 2000; Leonard et al. 2001; Martin et al. 2003; Ruiz-Moreno et al. 2003; Seccaroni 2006; Pelosi et al. 2007a, b; Howard and Nethersole 2010; Impallaria et al. 2019);

• in 18^th- and 19^th-century oils on canvas (Hradil et al. 2007; Morrison 2010; Wagner et al. 2019)

Artificial lead chromates—yellow chrome and orange chrome

Artificial lead chromates include a series of yellow (PbCrO₄ and PbCrO₄·PbSO₄) and orange (PbCrO₄·PbO) pigments. Their production followed the discoveries of ‘Siberian red lead’ in the Urals (1761 by Johann Gottlob Lehmann; 1770 by Peter Simon Pallas) and, above all, the discovery of chromium—by boiling crocoite with potassium carbonate—byNicolas-Louis Vauquelin in 1797 (Kyle and Shampo 1989). In 1803, Berthollet and Vauquelin (1804) provided evidence of the chromium-based pigment while, in 1809, Vauquelin explained the method for obtaining chromates of potash, ammonia, lime, magnesia, lead, copper, silver metallic chromates and Cr-containing compounds for porcelain. Moreover, he indicated to heat half a part of nitre and one of chromate to decompose iron chromate (i.e. chromite, Fe²⁺Cr₂O₄; Table 1, VA1).

These two papers mark the beginning of the production of chromates; therefore, the year 1804 or 1809 is set as the start date for chromium-based pigments in painting. However, lead chromates were immediately used both as a pigment for painting and in leather tanning (discovered and patented in 1884 by Augustus Schultz and Martin Dennis). In the Nouveau dictionnaire d'histoire naturelle appliquée aux arts (1818), it is mentioned that Russian painters paid high prices for the lead chromate (i.e. crocoite) used for their artworks, thus attesting the very early and widespread use of artificial lead-chromates.

Artificial lead-chromates started to be produced on a large scale only in the second half of the 19^th century; however, due to their high price, they were frequently mixed with other substances, such as barite or gypsum (West, 1986).

As for the recipes, the Nouveau dictionnaire (op. cit.) reported that it is produced by decomposing potash chromate with lead acetate (Table 1, NO1) and described the main characteristics of the plomb chromaté (Table 1, NO2). More recent recipes used lead salts and chromate or bichromate solutions (Hurst 1892) while current technical specifications are provided in the updated version of the standard ASTM D211-67 (2019)¹⁴, including six types of commercial lead chromate pigments (type I—primrose chrome yellow; type II—lemon chrome yellow; type III—medium chrome yellow; type IV—light chrome orange; type V—dark chrome orange; and type VI—chrome yellow for green).

For the purposes of this review, it is important to keep in mind two aspects directly related to the production process: (1) the size of the crystals and the amount of lead are responsible for the colour variations, from light yellow to deep orange (Bell et al. 1997) and (2) incomplete reactions, different ratios of the ingredients and variable temperatures produce compounds with different characteristics, and therefore, it is common to find different phases of basic lead chromate within the same pigment (for example, Sansonetti et al. 2010).

As for the individuation of yellow lead chromates, the reader is referred to the section dedicated to crocoite for the yellow PbCrO₄. As we have seen, the production process does not give rise to a single phase but, instead, to a group of phases (solid solution) which have, as end members, the monoclinic crocroite and the orthorombic lead (II) sulfate (PbSO₄, corresponding to the natural anglesite). "The PbCrO₄-PbSO₄ system and its mineralogical significance" were investigated by X-ray powder diffraction by Crane et al. (2001), who clarified numerous aspects related to the stability of the various phases. The compositional variability of the intermediate compounds is indicated by the chemical formula PbCr_1−xS_xO₄, where x can range from 0 to 0.8; the structure varies accordingly, from monoclinic to orthorhombic with x > ~0.4–0.5 (Monico et al. 2019). The different compositions and crystalline structures of these compounds influences their stability in artworks. In this regard, the studies carried out by Monico and co-workers on the photochemical alteration of these pigments are of great importance (Monico et al. 2011, 2014, 2019). In addition to providing numerous reference Raman spectra, these authors demonstrated that the alteration is due to the photoreduction of chromium from the hexavalent to the trivalent state and that the intermediate compounds PbCr_{1− x}S_xO4 (0<x ≤ 0.8) are more unstable than PbCrO₄ (in which the S is not present).

Furthermore, they identified the formation of compounds such as viridian (Cr₂O₃•2H₂O), Cr₂(SO₄)3•H₂O or (CH₃CO₂)₇Cr₃(OH)₂ and experimentally tested the stability of the various compounds in the presence of binders (especially oil and acrylic binders) which had been identified among those responsible for the photoreduction of Cr (VI). A scheme of the reconstructed (photo) reduction mechanism was provided in Monico et al. (2019)

The orange lead chromates correspond to phoenicochroite, Pb₂O(CrO₄), which has been structurally investigated by Williams et al. (1970) (Table 2). It is notable that this lesser-known and -attested phase has been found in 9^th to 12^th century AD polychrome decorations (carved stucco, wall paintings and terracotta friezes) in Nishapur, Iran, and was deemed to be of local provenance (Holakooei et al. 2018).

In artwork, yellow lead chromates are the most attested, followed by orange and green. Their attestations are all related to the 19^th and 20^th centuries and include both contemporary works and retouches of older works. To give just a few examples, yellow (mostly) and orange (to a lesser extent) chrome pigments have been found in the following artworks:

• the stuccoes of several Italian churches at Milan (17^th-century Guastalla Chapel in the Church of San Fedele), Laino (Church of S. Lorenzo), Campione d’Italia (Church of S. Maria dei Ghirli), Como (Church of S. Maria Assunta at Puria) and Mantova (18^th- to 19^th-century Church of Sant’Andrea) (Sansonetti et al. 2010, 2020);

• the iconostasis of a small wooden Saint Nicholas Church in Şirineasa (Vâlcea County in Romania), dated to the 18^th to 19^th centuries (Huică et al. 2020);

• a 19^th-century tempera icon from northern Greece (Lazidou et al. 2018);

• the 1812 AD wallpaper Les Monuments de Paris by Dufour and Leroy (Paris) (Castro et al. 2004a, b);

• the wall tempera of the Catholicon of the Rila Monastery in Bulgaria, dated to the first half of the 19^th century (Stamboliyska et al. 2021);

• the icons kept in the Antim Monastery at Bucharest (Romania) and the One-Wood Monastery in the Valcea County (Romania), dated to the first half of the 19^th century (Serafima et al. 2019a);

• a printed Turkish red calico (cotton; Wertz et al. 2019);

• the 19^th-century restoration of the wooden polychrome sculptures by Jan Geernaert (1704–1777) (Aliatis et al. 2012);

• the oil on canvas Battle of Grunwald by Jan Matejko (1838–1893) (Wagner et al. 2019);

• 1880–1890 Turkish postage stamps (Akyuz and Akyuz 2017);

• the rood screen and canopy by Sir Ninian Comper in the Church of Our Lady of Egmanton in UK (Edwards et al. 2007);

• the 1886–1942 oils on canvas or on cardboard by Pío Collivadino (1869–1945) (Buscaglia et al. 2020);

• the wall paintings by Augusto César Ferrari of the Dome and aisles of the Church of San Miguel at Buenos Aires in Argentina (Tascon et al. 2017);

• the oils on canvas by José María Espinosa (1796–1883) and Epifanio Garay (1849–1903) (Badillo-Sanchez and Baumann 2016);

• the oils on canvas and tin-plated iron sheets by Giorgio Marini (1836–1905) (Bordalo et al. 2016);

• the circa 1900 AD Thai manuscript no. 3 (Burgio et al. 1999).

In the 20^th century, its attestations are still numerous; however, they go beyond the chronological limit observed in the review (i.e. up to 19^th century) and, therefore, are not cited herein. Conversely, more information about the first uses and early nomenclature can be found in Kühn and Curran (1986). Further occurrences have also been provided by Lei et al. (2017; based on previous literature in Chinese), who reported the finding of this pigment on a Ming Dynasty (1368–1644) sculpture from the Chunyang Temple (Shanxi, China) and in a Qing Dynasty (1644–1912) mural in the West River Site (Zhangqiu, Shandong, China). In these cases, the pigment was deemed to be of European origin.

Finally, it is worth adding that lead chromates represent only the most popular type of Cr-containing pigments. In fact, chromium can also yield yellow pigments when combined with cadmium (CdCrO₄), zinc (K₂O·4ZnCrO₄·3H₂O), strontium (SrCrO₄) and barium (BaCrO₄).

Further notes on the use of lead-based pigments

Binding media and coatings

While the binding media used with lead-based pigments were various, vegetable oils and proteinaceous materials were primarily used, although at varying rates, depending on the period.

Among oils, linseed oil was likely the most popular, followed by walnut oil (in the Middle Ages; see, e.g., Cardell et al. 2009) and poppy oil (from the 17^th century) (Gettens et al. 1967). The occurrences of oil media are numerous and span over a long chronological period. To provide just a few examples, in China, they have been attested since the 5^th century AD (see, e.g., Liu et al. 2016; Zhou et al. 2020) while in Europe, despite the presence of earlier medieval occurrences (see, e.g., Cardell et al. 2009; Pereira-Pardo et al. 2019; Pinna et al. 2020), they became increasingly popular starting from the 15^th to the 16^th centuries.

Similarly, egg has had a wide diffusion, both chronologically and geographically. It appears as the preferential medium used to create panel paintings and, especially, icons (see, e.g., Daniilia et al. 2002; Bratu et al. 2015; Lazidou et al. 2018; Serafima et al. 2019a), although there is also no lack of evidence of its use in wall paintings (Daniilia et al. 2008; Malletzidou et al. 2019; Stamboliyska et al. 2021) and woodwork (Cardell et al. 2009; Măruțoiu et al. 2017).

Among other organic binders, animal glues have been frequently attested, especially in Chinese and Japanese paintings (Gettens et al. 1967; Wei et al. 2010; Liu et al. 2016; Luo et al. 2019) and on wooden supports (Arjonilla et al. 2019a; Abdallah et al. 2020; Abdel-Maksoud et al. 2020).

Beeswax has been sporadically reported for artworks ranging from the 2^nd century AD onwards (Cardell et al. 2009; Gehad et al. 2015; Salvant et al. 2018; De Meyer et al. 2019a, b), sometimes specifically used as a topcoat (Arjonilla et al. 2019a).

Moving towards (apparently) less-used materials, diterpenic resins (mostly colophony and sandarac) have been sporadically found in 14^th- to 17^th-century artworks (Cardell et al. 2009; Sansonetti et al. 2010), while terpenic ones have been in 12^th- to 19^th-century artworks (Stanzani et al. 2016; Abdel-Maksoud et al. 2020; Pinna et al. 2020).

The use of gums (e.g., Arabic gum) has been rarely, but surely, related to either paper/wallpaper (Espejo Arias et al. 2008; Arrizabalaga et al. 2014) or wall paintings (Hradil et al. 2014).

Lastly, it is worth mentioning the use of bovine milk, as a casein-based fixative, in the drawings (1750–1785) of Thomas Gainsborough, in combination with the lead white pigment (Pozzi et al. 2020), as well as the use of heat-bodied tung oil and glue-protein in the 11^th-century polychrome statues of the Hua Yan Temple in Datong (Shanxi, China; Wang et al. 2020b).

Diffusion

Lead-based pigments have been used in combination with virtually all known pigments (Supplementary Table S3), from circa the end of the 4^th century BC to the 20^th century AD. The literature consulted for this review (Supplementary Materials, Appendix 1) has allowed for the collection of archaeometric investigations of artworks found and/or preserved in Albania, Argentina, Austria, Belgium, Bolivia, Brazil, Bulgaria, Cambodia, China, Croatia, Cyprus, Czech Republic, Egypt, Ethiopia, France, Greece, India, Iran, Italy, Malta, The Netherlands, Palestine – Israel, Poland, Romania, Russia, Serbia, Slovakia, Slovenia, Spain, Switzerland, Turkey, the UK, Ukraine and the USA. Among this collection, lead-based pigments from artworks found or preserved in Europe represent the bulk. There are still many countries for which we do not yet have any kind of information solidly based on scientific investigations; however, this does not mean that lead-based pigments did not reach them. The available data do not even allow for the proposal of any type of quantification, as they more reflect the state-of-the-art of the studies than the real diffusion of these pigments.

Despite these limits, it may be interesting to observe how the introduction was geographically diversified. For example, in the ancient ‘European’ world, lead-based pigments mostly appear as by-products of metalworking. Their use does not seem linked to particular historical-cultural events as to technical-stylistic motivations, linked to markets supply.

In Japan, in contrast, Yamasaki (1967) stated that the introduction of these pigments corresponded to that of Buddhism and, in support of this thesis, reported documentary evidence regarding the monk Koguryo, who introduced new techniques for manufacturing pigments in 610, and that of monk Kanshȏ, who received an award for preparing lead white in 692. Furthermore, the discovery of 128 paper packages containing red lead powder (for a total of ~ 100 kg) of 'superior', 'medium' and 'low' quality (as written on the packages) in the Shōsōin (i.e. the repository of Imperial Treasures at Nara) proves the importance that was attributed to these materials even in the 8^th century.

Analytical methods

A plethora of laboratory and portable instruments have been used for the characterisation of pigments and binders.

Based on our survey (Supplementary materials Appendix 1), Raman spectroscopy represents the most-used technique, especially thanks to the development of portable instruments. The possibility of identifying both inorganic and organic compounds is another advantage of this technique and for the study of pictorial works in general. On the other hand, the researcher may encounter two well-known issues. The first difficulty can arise due to fluorescence that produces broad bands and overwhelms the Raman signal. The second difficulty can arise when the compound is mixed and does not seem to allow a certain identification of each component. To solve the first problem, the following measures can be effective: use extremely long/short wavelength lasers, work at large magnifications and apply mathematical corrections to subtract the background fluorescence (Smith and Clark 2004).

The second problem, regarding identifying and quantifying mixed compounds, has been addressed through various approaches. For example, Navas et al. (2010) suggested applying principal component analysis (PCA) to first-derivative Raman spectra. Their experimental work involved the preparation of mixtures with known amounts of both pigments (azurite, chalk, cinnabar, gypsum, lapis lazuli, lead white, minium, raw Sienna and glam) and binders (egg yolk). Their results demonstrated that Raman spectroscopy was able to discriminate the composition of the tests. On the same topic, Pallipurath et al. (2014) proposed a method that combines Fourier transform Raman spectroscopy with multivariate analysis. On one hand, they were able to test the method’s effectiveness; on the other hand, they also emphasised the need for suitable reference spectra.

Concerning the use of Raman spectroscopy, it is worth repeating that some lead-based pigments can be converted into other secondary products—and hence damaged—if the analysis is not conducted within certain parameters. For example, the laser wavelength must be kept below a certain threshold, which is not the same for all pigments. In this regard, Burgio et al. (2001) investigated the transformations that can trigger the reaction path βPbO₂ → Pb₃O₄ → α-PbO → βPbO when using varying laser power and wavelengths. Similarly, the degradation of plattnerite induced by laser power increase was the subject of the experimental works carried out by Smith et al. (2001), Aze et al. (2008c) and Costantini et al. (2020a). These latter authors used 633 nm and 785 nm laser excitation and temperatures up to 600 °C to test both modern commercial samples of plattnerite and the plattnerite detected in ancient frescoes. They were thus able to observe that the photo/thermal instability of the two types of plattnerite is different (i.e. the modern one is more stable) and that the reaction path previously described by Greenwood and Earnshaw (1997) is reversible.

These and other studies investigating the effects of different types of irradiations (see, e.g., Santis et al. 2007; Gourier et al. 2018; She et al. 2020) have provided useful guides for those who want to approach the study of these pigments using this technique.

The second most-used technique is FT-IR spectroscopy. This technique allows for identifying pigments and binders, based on the characteristic absorption bands of both inorganic and organic compounds. Therefore, it can be used in a complementary or substitutive manner to Raman spectroscopy. In comparison, FT-IR does not induce fluorescence but shows a greater scattering intensity of water and hydroxyl groups, thus resulting in less effective results in the presence of species dissolved in aqueous solutions.

The third most common type of technique is represented by microscopy, namely, optical microscopy (OM) and scanning electron microscopy (SEM). The advantage of these techniques is that of observing the morphological characteristics of the pigment grains and their distribution; the disadvantage is that they require sampling in most cases. Some SEMs, such as environmental and field emission SEM, allow for analysis without prior manipulation; however, the sample must be small enough to enter the chamber.

The fourth most common technique is X-ray fluorescence (XRF). The commercialisation of inexpensive handheld XRF equipment has greatly expanded the use of this technique. The most obvious shortcomings are immediately clear when the numerous lead-based compounds found in artworks are listed. Portable or laboratory XRF cannot discriminate cerussite from hydrocerussite or other compounds composed of the same elements. It is, therefore, assumed that a characterisation carried out solely through XRF is not acceptable.

Surprisingly, X-ray diffraction (XRD) is in the fifth position. This technique is indispensable for crystallographic and mineralogical studies; therefore, its effectiveness in discriminating inorganic pigments appears too obvious to be justified. The very structure of this review proved that the investigation of these pigments largely corresponds to the analysis of minerals and that each mineral has a different story to tell. Consequently, only an accurate determination of the phases provides a reliable characterisation of both the primary pigments and their secondary products. Using XRD, the advantage is an unambiguous identification and (possibly) quantification of the phases constituting the pigment, even in the presence of mixtures. The disadvantages are related to the scarce diffusion of portable instruments and the consequent need for sampling. Even in this case, however, large facilities (e.g., synchrotrons) allow for analysis without manipulating the sample and can also provide spatial distribution maps of the components. In addition, macro XRD (MA-XRD) has been frequently used, as it consists of a mobile scanner that allows the distribution of pigments to be visualised, provided that the artwork has a quasi-flat surface. Indeed, these techniques are not particularly accurate for determining several pigments but have the advantage of providing mineralogical information in a non-destructive way.

Among the remaining techniques, we notably mention:

• all imaging techniques; in particular, hyperspectral imaging reflectance spectroscopy (HSI) has been frequently used, due to its non-invasive and non-destructive nature. It allows for the characterisation of pigments and the results can be modelled when mixtures are present (e.g., deep neural networks, as proposed by Rohani et al. 2019). In this regard, Li et al. (2020a) set up a specific and automatic hyperspectral scanning system (hyperspectral camera + a halogen light source + an automatic scanning platform + data processing software);

• digital radiography (DR) is an X-ray-based imaging technique that provides qualitative information. The lower/higher the atomic number of an element, the darker/lighter the radiographic image will be; however, there may be cases in which gold foils are so thin that they cannot produce a measurable effect (see, e.g., Serafima et al. 2019b);

• μ-computer tomography (μCT), which has been proved to be effective in obtaining virtual cross-sections of the paper (Li et al. 2020c);

• two low-frequency vibrational spectroscopy methods—terahertztime-domain (THz-TDS) and low-frequency Raman (LRS)—which have been proposed as complementary methods for pigment and mixture identification (Kleist and Korter 2020);

• cyclic voltammetry, differential pulse stripping voltammetry and voltammetry of nanoparticles/atomic force microscopy (VNP/AFM), which has been tested on lead-based pigments to assess their reliability in the unambiguous identification of pigments (Doménech-Carbó et al. 2000, 2007);

• several computational methods such as the density functional theory (DFT) and the time-dependent density-functional theory (TDDFT), as well as computer simulations, have been reviewed by Fantacci et al. (2010) and applied to the study of red lead by Amat et al. (2020).

Isotopic analyses have been used to investigate the provenance of lead white—especially for the authentication of artworks—and red lead.

As for lead white, an example is provided by the lead isotope analyses made on lead white samples taken from the Saint Praxedis (an oil on canvas with disputed attribution). As stated in the catalogue edited by Christie’s for its sale: "the results placed the lead white squarely in the Dutch/Flemish cluster of samples, establishing with certainty that its origin is north European and entirely consistent with mid-seventeenth century painting in Holland"¹⁵(Ragai 2015); however, the attribution left so much perplexity that the painting sold for only US$10,687,160. Further examples have been provided by Keisch and Callahan (1976) and Fortunato et al. (2005). These authors sampled small amounts of lead white pigments (50–200 mg) from several artworks painted by Peter Paul Rubens, Anthony van Dyck, Rembrandt van Rijn, Peter Breughel (Peter Breughel III) and A. Breughel. The authors compared the results obtained with the data available for proven "Italian paintings" (L. Giordano, A. Allori, D. Tintoretto, E. Procaccino, GB Caracciolo) and observed that the isotopic values allow for the delineation of a compositional field for the Flemish and Dutch paintings, which was clearly distinguishable from that of Italian painters.

As for red lead, Walton and Trentelman (2009) used inductively coupled plasma time-of-flight mass spectrometry (ICP–TOFMS) and determined the Spanish provenance (Rio Tinto) of the red lead used to paint seven Egyptian mummies (Roman period). Nord et al. (2015) and Nord et al. (2016) investigated 34 samples of minium and plattnerite (as well as one of lead white) through MC-ICP-MS. The samples were taken from different Swedish sites dating between the 12^th and early 16^th century, where the analyses disclosed the German origin of the pigment (Harz and Erzgebirge). In no case was a correspondence found with the Bergslagen region, representing the principal medieval mining district in Sweden.

In recent years, the application of this method has progressively expanded (see, e.g., Curley et al. 2020), and the analysis of further case studies is fundamental to the growth of the lead isotopes database.

Finally, the dating issue has also been successfully approached using radiocarbon (¹⁴C) AMS. First, the oil binders were dated by Hendriks et al. (2018); shortly after, the lead carbonate was subjected to AMS dating by Hendriks et al. (2019). The latter started from the consideration that, in the lead white produced through the stack process, the CO₂ is produced by fermenting organic materials and, therefore, bears atmospheric ¹⁴C levels, while in the lead white produced using more recent methods (e.g., the chamber process developed by Herbet in Germany), the CO₂ may come from coal fires, as well as from calcium or potassium carbonates added to vinegar, and therefore, the ¹⁴C levels are very low (resulting in radiocarbon ages thousands of years older than the real ones). The authors also pointed out that "it is not clear if the remineralization of lead carboxylates to lead carbonates will interfere with the¹⁴C dating of the carbonate"; nevertheless, they (1) suggested the adoption of a two-step method of sample preparation to date both the lead carbonate and the binder and (2) positively assessed the effectiveness of the dating method for lead white produced with the stack process. Finally, they also highlighted that this approach could discriminate between lead whites produced and not produced using the stack method.

Last year, three notable papers were published on this topic. Hendriks et al. (2020) published a new study aimed at combining radiocarbon dating and lead isotope analysis "to maximise the research output" from a single sample. They sampled Swiss and British artworks spanning from the 18^th to the 20^th century and obtained information on both lead white trading (likely from British ores until 1860s, multiple sources after 1860) and the dating of artwork ("coherent with the signed date"); however, the authors warned about the possibility of obtaining accurate dates through microsampling when the probable chronological interval ranges between 1700 and 1950.

As for the closely related papers by Messager et al. (2020) and Beck et al. (2020), the former determined the temperature at which lead white samples to be subjected to radiocarbon dating should be heated for C extraction (= 400 °C), while the latter applied the protocol previously established to perform radiocarbon dating on lead white samples taken from two medieval contexts: the 1388–1390 courtly decoration of Margaret of Bavaria’s dressing room at the Château de Germolles in Burgundy (France) and the decorations of the destroyed rood screen of the church of the Cordeliers in Fribourg (Switzerland). Among the latter, two groups of paintings were stylistically dated to 1500–1510, while another decoration was tentatively dated between 1340 and 1700. Radiocarbon results placed the French decorations between 1283 and 1419 (with a statistical combination pointing to 1297–1400), the first group of the Swiss ones between 1262 and 1630 (with a statistical combination pointing to 1426–1460) and the second group of the Swiss samples between 1430 and 1666.

Finally, Quarta et al. (2020) deepened the aspect regarding the preparation of the samples, by testing both acid hydrolysis and thermal decomposition. Their results showed that both procedures can be effective, even if thermal decomposition at 500 °C was more effective in eliminating calcium carbonate contamination.

In conclusion, this method is in the midst of its development. The tests carried out appear promising, but limitations have been highlighted in the case of (1) relatively recent samples and (2) the coexistence of other carbon-containing compounds (e.g., contamination from CaCO₃). Partially similar problems have been recognised in the study of mortars (see Gliozzo et al. 2021 in this TC for a brief description and the references quoted therein). Apparently, interference due to lead carboxylates is still possible.

Concluding summary of key concepts

This review aimed to provide the tools for an accurate study, without going into numerous details. For this reason, we focused on minerals and pigments and provided some basic information that can help in their recognition and study. The key concepts to be extrapolated from this study are outlined as below.

• Production and trade—While archaeological evidence regarding production sites is limited to very few sites, especially in ancient Greece, the discovery of bowls containing lead-based pigments provides valuable information. At present, the state-of-the-art does not allow us to outline an exhaustive picture of the production and trade of lead-based pigments. Lead isotope analyses have proved promising, but the available database is small and strongly biassed towards the almost-modern era.

• Use—Despite being known today as toxic products, lead compounds have been used both as paint pigments and as drugs and cosmetics. Red lead has also been used as an anticorrosive paint and a textile paint. The most attested techniques are a fresco, a secco and tempera, but the former is less frequent when lead white is used. As for binders, there is no evidence of selective use of any particular ones, compared to others. Vegetable oils and proteinaceous materials have been mainly observed in artworks bearing lead-based compounds.

• Protection—Lead soaps can deeply affect the visual appearance and integrity of a painting. Different levels of saponification can be observed, depending on the nature of the pigment and binder used, the temperature, the presence of water and the humidity. Overall, lead-based compounds are sensible to humidity, light (photodegradation), heating and chlorine salts. The presence of impurities can trigger/enhance their alterations. All of these factors must be considered for the correct conservation of works of art.

• Investigation—Among the analytical techniques, the most often used ones fall in the field of conventional techniques (Raman, FT-IR, XRD, XRF, OM and SEM). While using a single technique always leads to inaccurate and partial results (especially in the case of portable X-ray fluorescence), a combination of techniques is always the best approach. In this case, the combination of destructive and non-destructive techniques also represents a highly suitable choice, able to satisfy the needs for accuracy, completeness and conservation. For example, non-destructive techniques can provide large-scale preliminary analysis and guide targeted sampling for more accurate destructive analysis. Large facilities, such as synchrotrons, also offer possibilities that cannot be reached with conventional techniques. Notwithstanding the plethora of laboratory and portable instruments that can be used, it is still possible to observe a certain approximation that should be overcome. From a vague determination of lead white, it is necessary to arrive at an accurate qualitative and (possibly) quantitative determination of the cerussite and hydrocerussite ratio. Likewise, it is important to discriminate between litharge and massicot, scrutinyite and plattnerite, or lead-tin yellow types I and II. Finally, as can be seen from the long list of minerals provided, other phases could be present and it is also conceivable that the phases rarely mentioned in the literature are actually not as rare but, instead, simply rarely recognised.

The information available for the main lead-based pigments can be summarised as follows.

• Cerussite (PbCO₃) and hydrocerussite (2PbCO₃·Pb(OH)₂) are colourless to white carbonates constituting the lead white pigment. In nature, the former is more widespread than the latter; however, it is not clear whether natural minerals were used and to what extent or in which period(s). Starting from ancient sources, the use of lead whites produced through lead corrosion likely predominated. The ancient ‘Dutch’, ‘stack’, ‘chamber’ and ‘German’ methods were at the base of the production processes in different epochs. In artwork, lead white was scarcely observed from the late/final first millennium BC to the 7^th century AD. From the 8^th century AD, its occurrence increased on every type of support and technique, such as to prepare grounds and gildings, paint in white and lighten other colours. Lead white can transform into several other Pb-based compounds, through heating and/or degradation (leading to colour change and darkening). Recent studies have verified the possibility of dating the production of this pigment through AMS ¹⁴C.

• Red lead—minium (Pb₃O₄)—is a lead oxide which can be found in nature (relatively infrequently) or artificially produced by roasting lead white and/or litharge. Minium was rare in artworks until the 1^st century AD, becoming ever more widespread in the following centuries. Used similarly as lead white, to which it is often associated, minium has also been used to decorate paper currencies and inlays, but less frequently alone or for the preparation of priming layers. The alteration of minium can give rise to whitening or blackening, depending on the degradation product into which it is transformed (lead sulfide or lead carbonates/lead sulfate).

• The red (tetragonal) litharge (α-PbO) and the yellow (orthorhombic) massicot (β-PbO) can transform into each other through heating (α→β) at a temperature of 489/580 °C, or upon mechanical treatment (β→α). Their occurrences are less frequent than galena but not rare. A typical secondary product of lead cupellation during silver smelting, litharge (spuma argenti) has also found applications in medicine and as a building material. In archaeological contexts, litharge may be present in the corrosion crusts of lead objects and paintings. In these, it was likely used as a siccative, rather than an artistic pigment, which is probably why it has rarely been identified in artworks. Massicot is more frequent, generally having been used as a yellow pigment. However, both PbO compounds may be present as deliberately used pigments or impurities derived from red lead. Both litharge and massicot can lead to the formation of cerussite, hydrocerussite, minium and plumbonacrite and may form from scrutinyite and plattnerite.

• Crocoite is a lead chromate likely used as a pigment before the end of the 18^th century through the beginning of the 19^th century, later being substituted by artificial counterparts.

• Mimetite is a lead chloroarsenate showing highly variable colours. Its presence is generally interpreted as an alteration product but there exist cases in which it was supposed to be used as a pigment.

The information available for minerals mainly found as alteration phases of the previous pigments is summarised as below.

• Galena (PbS) is a widely occurring lead-grey isometric sulfide representing the main raw material for the production of lead. It may contain trace amounts of Mn, Fe, Co, Cu, Zn, Ga, As, Se, Ag, Cd, In, Sn, Sb, Te, Tl, Bi and minor Au and Hg. It is rarely found in artworks, as an alteration product of lead white.

• Anglesite, palmierite and the associated minerals are natural phases (mainly colourless to white or yellowish) that can be found in the oxidised zones of lead deposits. In artworks, only anglesite has been rarely (if any) used as a white pigment, and most occurrences have denoted its secondary nature. Palmierite has been found as a secondary phase in 17^th-century paintings.

• Scrutinyite and plattnerite are dark mineral phases, generally formed as oxidation products in hydrothermal environments. In artwork, they represent important degradation products of lead white, which can further transform into minium, litharge and massicot. The use of plattnerite as an artistic pigment has been suggested only once, so far.

• Plumbonacrite and its associated phases are carbonates (oxohydroxide) related to hydrocerussite, with which they are frequently associated. Unambiguous discrimination between these minerals may not be straightforward, depending on the technique used. In this group, an unnamed phase was also found in a Laurion metallurgical slag, as a reaction product from a cerussite-based compound. In artwork, plumbonacrite represents an important degradation product: it can easily form in water solutions and in rather dry oils from litharge or massicot or as a product of the photoactivation of minium. Its formation from lead white is more difficult.

• Cotunnite and laurionite (lead chlorides), blixite (oxychloride) and phosgenite (chloride-carbonate) occur from rarely to frequently in nature, generally as secondary phases but also primary phases (e.g., cotunnite). Laurionite and phosgenite were produced in antiquity from litharge and used for cosmetics. Laurionite probably also had a pharmaceutical use. Cotunnite, laurionite and blixite have been found on various artwork objects. However, while the formation of cotunnite is generally referred to as a process of alteration of lead white (when in contact with a Cl-rich fluid), deliberate use of the artificially produced pigment has been hypothesised for laurionite and blixite.

• Caledonite, schulténite and vanadinite are lead and copper hydrous carbonate-sulphate, hydrous lead arsenate and lead chloride vanadate, respectively, with highly variable colours. Caledonite has been identified as an alteration product in the presence of ultramarines. The occurrence of vanadinite and schulténite is currently rare and generally associated with that of mimetite.

• Challacolloite and the unnamed phase PbMg(CO₃)₂—The first phase is a chloride, which is almost more infrequent in nature than in ancient artefacts. This secondary phase is formed by the interaction of hydrocerussite with potassium chloride from the earth or with potash lye and hydrochloric acid, possibly used in restoration interventions. In any case, high chloride ion concentrations and/or low pH conditions are required for its formation. Regarding the second phase, only research carried out on synthetic materials was found, obtained from lead or red lead carbonate. In artwork, this unnamed phase was found in association with massicot, cerussite, hydrocerussite, plattnerite and scrutinyite.

Finally, the information available for lead-tin yellow types I and II, Naples yellow, lead-tin-antimony yellow and artificial lead chromates can be summarised as follows:

• Lead-tin yellow type I is an artificial pigment whose tetragonal/orthorhombic structure can be refined in both P₄2/mbc and Pbam space groups. It was produced by heating litharge or minium and SnO to between 650 and 900° C. Different temperatures and component ratios affect the final colour and composition of the pigment (e.g., residual PbO due to incomplete reaction and/or an excess of Pb₃O₄). Its period of use, to the best of our knowledge based on the available literature, ranges from the beginning of the 15^th century to, at least, the mid-18^th century. Its alteration and degradation are strongly influenced by the presence of organic binders.

• Lead-tin yellow type II is a pigment with a cubic structure, which has been less attested than type I. It could have been produced in at least two ways: (1) heating lead-tin yellow type I and silica at ~900 °C. The optimum ratio of the two components was found to be 1:1, while changes in this ratio produced a mixture of lead-tin yellow types I and II or lead-tin yellow type II and unreacted silica, and (2) a single-step synthesis by heating Pb, Sn, and Si oxides at ~850 °C. The optimum ratio of the three components was found to be 6:3:2, while variable changes from this ratio may leave unconsumed SnO₂and/or produce an excess of SiO₂ or Pb, mixed products and, especially, a glassy phase. Its period of use known so far ranges from the beginning of the 14^th century to the first half of the 17^th century. The identification of lead-tin yellow type II in 19^th-century paintings requires validation.

• Lead-antimony yellow (or Naples yellow) is isostructural with bindheimite and oxyplumboroméite; however, the use of natural minerals has not been proved. The synthesis of the artificial pigment required a mixture of lead and antimony compounds. An experimental reproduction verified that (1) heating Pb₃O₄ and Sb₂O₃ without fluxes requires at least a temperature of 1000 °C to be reached and the final product is a low-quality mixture of several phases; (2) heating the same components with 10–20% NaCl requires lower temperatures (~850 °C) and the final product is a higher quality single-phase; (3) using K₂CO₃ instead of NaCl, the product is of lower quality; (4) adding 5–10% of silica gel, the product becomes salmon-coloured; and (5) when replacing Sb₂O₃ with Sb₂S₃, a flux was not needed. Its period of use known to date (in paintings) ranges from the beginning of the 16^th century to, at least, the 19^th century AD. The identification of Naples yellow in the late 1^st- to 4^th-century wall paintings of the Domus in Rome may represent the very first attestation of its use or later retouches.

• Lead-tin-antimony yellow is an artificial cubic pigment. It may have been produced through heating Pb₃O₄, Sb₂O₃, SnO₂ and 10% NaCl to between 700 and 900 °C. Different ratios of the components affect the colour, composition and processing temperature. By replacing Sb₂O₃ with Sb₂S₃ anglesite (850 °C) or lanarkite (900 °C) were further found. Its period of use known so far ranges from the early 16^th to the 19^th century.

• Lead chromates. The lead chromates are an artificial group of pigments, ranging in composition from crocoite PbCrO₄ to PbCrO₄·PbSO₄. As for the composition, the yellow pigment corresponds to the natural crocoite, while the orange pigment corresponds to phoenicochroite. They were first produced, in 1804/1809, by decomposing potash chromate with lead acetate; more recently, they have been produced using lead salts and chromates or bichromates. The colour is mainly affected by the amount of lead, while the nature of the final product is determined by the different ingredient ratios and processing temperatures. For this reason, several phases (PbCr_1−xS_xO₄) can be individuated through analysis of the pigment. The alteration is due to the photoreduction of Cr(VI) to Cr(III), where the intermediate compounds are more unstable than PbCrO₄. The binder plays a key role in Cr photoreduction. The period of use known so far ranges from the 14^th to the 20^th century. The identification of orange lead chromate in a 9^th- 12^th-century AD polychrome decoration (carved stucco, wall painting and terracotta friezes) in Nishapur, Iran, may represent an early attestation or later retouches.