- Open Access
On the stability of mediaeval inorganic pigments: a literature review of the effect of climate, material selection, biological activity, analysis and conservation treatments
Heritage Science volume 5, Article number: 12 (2017)
This review is to be considered part of the development of the MEMORI dosimeter, to evaluate the impact of climate (relative humidity, temperature, illumination, etc., including volatile organic compounds) on moveable objects. In the framework of the MEMORI project, Ghent University was given the task to assess pigment degradation upon acetic acid exposure, and to collect information on pigments’ stability. Moreover, to obtain a wider knowledge on the stability of common pigments, the effect of a variety of parameters was reviewed from literature. Discolouration and degradation of pigments significantly alter the legibility of polychrome works of art, so that the development of monitoring methods to ensure the preservation of cultural heritage objects is of primary importance.
Colour is one of the most important properties of objects, in archaeology and art history. This review is aimed at collecting information on the stability of traditional inorganic mediaeval pigments under a variety of conditions. The pigments considered are whites (leadwhite and calcium carbonate), yellows (ochres, orpiment, massicot, lead tin yellows), orange-reds (ochres, realgar, vermillion, litharge, red lead), blues (ultramarine, blue ochre, smalt, azurite, Egyptian blue), greens (green earths, malachite, verdigris and other Cu containing materials), brown (umbers) and black (carbon black). Pigments as Naples yellow (Pb2Sb2O7/Pb3(SbO4)2), lead chromate yellows (PbCrO4/PbCr1−xSxO4), Prussian blue (KFe[Fe(CN)6]·xH2O/Fe4[Fe(CN)6]3·xH2O), zinc white (ZnO), etc., although known to degrade, are not included in this review, as they were not available during the Middle Ages . For the purpose of this review, mainly chemical and archaeometrical literature was reviewed, so that an explanation for the on-going degradation processes could be provided as revealed by advanced analytical techniques.
The starting point for such data collection has been the MEMORI project , directed at studying and understanding the impact of the air quality inside museum exhibition cases and storage premises on complex objects of cultural relevance. Relevant factors are relative humidity (RH, stable, fluctuating), temperature, illumination conditions, inorganic pollutants, and volatile organic compounds; the latter being extensively studied during the MEMORI project [3, 4]. Moreover, information on the influence of other factors was collected from literature. The results of investigations of works of art are combined, when possible, with specific studies on pigments reactivity performed in the lab using powders or mock-ups. Attention is given to the evolution of the interpretation of specific processes as a result of scientific advancement in understanding the on-going chemical reactions. This review is not limited to the application of a specific analytical technique for detecting and understanding degradation processes; on the other hand, it focusses on the so far identified alterations, their impact on the polychromy appearance, and their causes.
Centuries of practical knowledge allowed artists and artisans to select materials which were not susceptible to fading or discolouration, and recommendations on unstable pigment mixtures (therefore not recommended), or on the use of specific pigments with selected binders, are common in artistic literature and treatises (see, for example, Cennini’s “Il Libro dell’Arte” ). Artists and artisans were anyway constantly experimenting, and the material selection was performed according to their savoir faire and to practical considerations [6, 7], often using unstable mixtures during the actual production of the object, which nowadays result in hardly legible works of art. Also, the material history of the polychrome objects, being subjected to the environment, including seasonal climatic changes and extreme conditions such as in fires and floods, to biological activity, and to pollution related to the industrial revolution (sulphur and nitrogen compounds SO2, SO3, NOx [8–11]), is bound to provoke alterations on those materials sensitive to climatic factors, to biological activity, and to anthropogenic pollutants. Finally, another important aspect to consider is the human intervention on the objects, which is here reunited under the umbrella term of “conservation treatments”. Relevant activities which might alter the stability of pigments in paint layers include, but are not limited to, harsh cleaning procedures and restorations performed by using unsuitable/incompatible materials and methods, or interventions regarding (lack of) climatic control. Finally, also the scientific approach to materials characterization might modify irreversibly the sample during the analysis. It is important to mention that these factors often act in synergy.
The review is articulated in sections corresponding to the main element present in the pigment, according to increasing atomic number. When possible, a general introduction to the sensitivity of the class of pigments is given, with more details for each single pigment. An overview of the observed alterations and the main factors involved in the process (climate, material selection, biological activity, analysis and conservation treatments) is given in Table 1.
Low atomic number (Z) elements
Carbon blacks (C)
Carbon blacks (disordered C) are worldwide commonly used in arts and crafts throughout the centuries [1, 12]. They are of natural of artificial origin, and the microscopical structure of the pigment is related to its origin and manufacture process [13, 14]. They are stable to light and humidity, but burn at high temperatures. Their curing properties in oil are poor, often requiring the addition of siccatives [12, 15]. Evidence of degradation is observed upon exposure to oxidizing agents, and especially when impurities are present (e.g. presence of residual salts/uncarbonized moieties in carbonaceous pigments applied by using the fresco technique) [12, 15, 16]. Whitening is also reported for bone black [amorphous carbon with apatite Ca3(PO4)2] in oil binding medium, possibly related to the photodegradation of the aromatic structure, probably catalysed by lead (present as a siccative, or pigment) . In another study, by the same authors, a variety of issues was identified related to the causes of discolouration of carbon blacks, such as the formation of lead carboxylates from the reaction of the siccative with the binder, or the alteration of lakes present in the mixture, or the degradation of the aromatic carbon compounds. The latter process is related to both the manufacturing process of the carbon based pigment, and the presence of siccatives . Upon infrared laser irradiation, the colour of mock-ups prepared with organic and inorganic binders changed, showing evidence of darkening , while a 248 nm (UV) laser induced no changes .
Ultramarine blue (Na8[Al6Si6O24]Sn)
The term lazurite refers to the blue mineral of the sodalite group (Na8[Al6Si6O24]Sn), lapis lazuli to the rock from which it is extracted. Notwithstanding its enormous price related to its rarity and colour, natural ultramarine was extensively used throughout history, as a decorative stone [1, 20], as a colourant for ceramics [21–23], and in paintings and manuscripts both in Asia and Europe starting from the 6th century , as well as in pre-Columbian cultures . In European contexts, this extremely valuable material was mined in present-day Afghanistan and traded via Venice [1, 20], and it was reserved to specific iconographic elements of the composition, such as Christ and the Virgin’s blue cloaks [1, 20, 26, 27]. It was used both in tempera and oil, to produce either mixtures (with crimson or a white pigment for example) or pure layers, of different opacity according to the binder (e.g. oil based blue glazes painted over a cheaper blue layer [1, 26]). The high price and marvellous colour somehow promoted the development of adulterations, and finally the synthesis of artificial ultramarine in 1828 by Guimet, and in the same year, but with a different process, by Gmelin [1, 20, 28]. Natural ultramarine is stable to light, including lasers [18, 29], heat, acids and alkalis [20, 30, 31]. No oxalates were formed upon exposure of the pigment to oxalic acid . It was however observed that in presence of SO2 (pollution), the pigment discolours only when liquid water (e.g. condensation) is present . The sulphur present in the crystalline matrix of lazurite does not affect pigments that are otherwise sensitive to hydrogen sulphide (H2S), such as leadwhite . No alteration could be observed on decorative plasterworks in the Alhambra (Spain) , neither on manuscripts . However, a greyish alteration of the paint surface (“ultramarine sickness”) can be sometimes detected, but it seems that many factors can cause discolouration of lazurite paint layers, such as the oil degradation in presence of humidity , or the discolouration of smalt in case of mixtures [20, 34]. It is reported in literature that ultramarine sickness is related to an acidic attack of the pigment by pollutants, biological metabolites, or even acidity of the binder [35, 36]. This hypothesis seems to be confirmed by the fact that, in oil medium, when the basic pigment leadwhite was added to the mixture (to enhance curing, increase opacity and adjust the shade), no visible alteration can be detected, the leadwhite somehow protecting the other pigment . The zeolitic structure of the mineral allows for ion exchanges , and for trapping volatile molecules, such as the S3 − chromophore , and CO2 whose presence was recently related to natural ultramarine from Afghanistan . Artificial ultramarine discolouration revealed a variation in the aluminium coordination, resulting in the opening of the cage and release of the chromophore [31, 39, 40]. Such a cage-opening process can be initiated either by high temperatures, acids [31, 38, 39] or alkalis . Moreover, as known for sulphur-saturated zeolites , it appears that ultramarine can have a catalytic effect on the binder breakdown, and that such property can be related to a pigment pre-treatment (heating), which can explain the inconsistent appearance of degradation .
Calcium (Z = 20)
Calcium containing pigments are of mineral, biogenic, fossil or artificial origin, mainly corresponding to groups of carbonates, sulphates, phosphates, or fluorides . A variety of Ca, CaMg carbonates, and sulphates was used as white pigment (chalk, bianco di San Giovanni: CaCO3; huntite: CaMg3(CO3)4 and dolomite: CaMg(CO3)2; gypsum: CaSO4·2H2O), as well as mixed with glue to prepare ground layers in northern and southern Europe, respectively [1, 20, 43–45] and to prepare parchment for manuscripts . The calcination of bones gives a white pigment mainly containing apatite Ca3(PO4)2 . Calcium compounds were as well used as extenders (for example with leadwhite, to achieve transparency effects), and for lakes, as chemical or physical supports [20, 47]. The purple mineral fluorite (CaF) was also used as a pigment, mainly in fifteenth to sixteenth century central European artworks [1, 48, 49].
Chalk pigments [(Ca,Mg)CO3: natural/artificial, lime white, shell white]
Calcite (CaCO3) can be considered stable under normal circumstances. It decomposes to lime (CaO) and CO2 when heated, and it is dissolved by acids with release of CO2 and formation of the corresponding salt. SO2 pollution is responsible for the formation of gypsum, which is soluble and produces mechanical stress, as its volume is larger than that of the starting material [20, 45]. Gypsum (CaSO4·2H2O) is a common degradation product in wall paintings and manuscripts . Being alkaline, these pigments are not suitable for mixing with pigments such as verdigris , but they are stable to sulphide containing materials (pollutants or pigments) . The lists of occurrences reported by  and  mainly refer to calcite containing ground layers, although the use as a pigment is ascertained. Calcium carbonate pigments are not suitable for use in oil medium, as they don’t have hiding power , but they are used in mixtures with other white pigments to adjust the shade and hue, as well as in wall paintings . In proteinaceous binder, interactions are reported between pigment and medium .
Huntite and dolomite, Mg-rich carbonates having formula CaMg3(CO3)4 and CaMg(CO3)2, respectively, are also identified as white pigments in rock art paintings. As many other carbonatic materials, they are converted to Ca-oxalates (weddellite CaC2O4·2H2O and whewellite CaC2O4·H2O) in presence of oxalic acid, with the excess magnesium being included in dolomite and magnesite MgCO3 (which can be considered as intermediate degradation products, sensitive to oxalic acid), in the very soluble magnesium oxalates MgC2O4·xH2O, and finally lixiviated . The attack from polluted water can as well form gypsum CaSO4·2H2O and the extremely soluble magnesium sulphate MgSO4, and calcite. The adsorption of water negatively affects the stability of paint layers, as swelling occurs .
The use of a buon fresco technique implies the use of Ca(OH)2 as a binder, which recrystallizes into calcium carbonate, surrounding the pigments and binding them to the wall surface. The formed calcite is subjected to degradation processes involving acids, from biological activity or binders degradation (when details are added a secco) [32, 53]. Carboxylic acids from binding media are expected . However, these compounds were not always detected in samples . On the other hand, Ca-carboxylates were successfully identified at the boundary between the ground layer and the paint in egg tempera samples [56, 57]. They were as well detected on artworks (laboratory and in situ), especially underneath the varnish and on top of the ground layers [58, 59], and on manuscript samples. There, a correlation between calcium oxalates and the loss of the proteinaceous signal in green areas was highlighted, so that the binder’s degradation is catalysed by Cu-ions, present in the green pigment, and oxalic acid is released [57, 60]. Chalk is lightfast , and gypsum does not show discolouration upon Nd:YAG laser irradiation . Ion beam analysis proved to be potentially harmful to chalk, as discolouration was observed .
Iron (Z = 26)
Yellow, red, brown ochres (Fe2O3, FeOOH, Fe3O4; MnxOy)
Ochres are natural products related to rock weathering, whose predominant phases are phyllosilicates (clays). The presence of less than 2% chromophores, such as iron oxides and hydroxides, the red haematite (α-Fe2O3) and yellow goethite and lepidocrocite (α-FeOOH and γ-FeOOH respectively), is sufficient to impart a deep colour to the rock [1, 12]. Other minerals are commonly present and can help to ascertain provenance and paragenesis, such as quartz (SiO2), feldspars ((Na, K)AlSi3O8), micas and clays (complex hydrosilicates), gypsum (CaSO4·2H2O), ferrihydrite Fe10O14(OH)2, maghemite (γ-Fe2O3) and magnetite (Fe3O4), iron sulphates [jarosite group, KFe3 3+(SO4)2(OH)6], etc. [1, 12]. If manganese oxides (MnO, MnO2, Mn3O4) are present together with the iron oxides, the shade of the ochre is darker and suitable for producing various shades of brown (Siennas and umbers), and for shadow rendering [1, 12]. The shade of iron oxides and oxi-hydroxides can be adjusted by roasting processes, which produce darker shades (yellow to red-brown; red to purple-dark red) [1, 12]. In fact, haematite, the anhydrous oxide, is the most stable, being the end product of heating goethite and lepidocrocite (150–400 °C); and maghemite (γ-Fe2O3) can also be produced if organic matter is present . The transformation of magnetite (Fe3O4) to haematite is also reported [62, 63]. These materials are very stable and maintain their colour when ground to powders, moreover their occurrences are numerous: diffusion, stability, lightfastness and inertness make their use as pigments straightforward [1, 12, 64]. In oil layers, Fe-ions promote photo-oxidative reactions, while Mn-ions act as a siccative . In proteinaceous binder, interactions are reported between pigment and medium . In rabbit skin glue, haematite shows a high degree of interaction with the binder, making the paint layer more stable than for other pigments . The instability to light reported for some umbers  seems to be attributable to the presence of tarry materials and other impurities . As for the red and yellow ochres, burning produces a darker shade (burnt umber, burnt Sienna) . Ochres are stable to light, moisture, alkali, and dilute acids, and are inert in mixtures. Their stability to acids is testified upon oxalic acid exposure, which causes only the formation of Ca-oxalates, from the other components of the ochre [32, 67]. They are, however, sensitive to high temperatures, such as fires [12, 68], or local heating effects related to the use of lasers for cleaning [19, 69, 70] or for spectroscopical analysis (i.e. Raman spectroscopy, [62, 63, 71, 72]). A UV (248 nm) laser showed darkening of yellow ochre and raw Sienna, as a result of dehydration, conversion to haematite and modification of the manganese phases present , but low fluences of the same laser caused little modification of this ochre . High fluence of 355 and 633 nm laser, caused the conversion of yellow ochre to haematite [29, 72]. The effect of NIR laser irradiation (1064 nm) was as well investigated, showing significant discolouration of both yellow and red ochres when mixed with gypsum (no organic binder) , and an increase of haematite content after irradiation of ochres . Moreover, some issues are encountered on wall paintings. In fact, coquimbite/paracoquimbite Fe2(SO4)3·9(H2O) were identified in Pompeii, together with magnetite Fe3O4 and gypsum CaSO4·2H2O, as a result of the degradation of the fresco paint layer due to SO2 pollution . On wall paintings in the Tournai Cathedral, on the other hand, anhydrous haematite was found to be converted to iron oxy-hydroxides due to the humid conditions, which resulted in a visible discolouration of some red areas .
Green earths (Fe, Mn, Al, K-containing hydrosilicates)
The green coloured iron-containing silicates glauconite ((K,Na)(Fe3+,Al,Mg)2 (Si,Al)4O10(OH)2) and celadonite (K[(Al,Fe3+),(Fe2+,Mg)] (AlSi3,Si4)O10(OH)2) can be found in outcrops all over the world [1, 74]. After crushing and grinding, the material is ready to use as a pigment, showing good stability and lightfastness in all media, although in oil the hiding power is relatively poor [1, 74]. Both these characteristics supported the wide use of such pigments. Green earths are soluble in both acids and alkalis, and they turn brown upon heating, as divalent iron is oxidised to its trivalent counterpart . They were as well used as lake substrates . Green earth oil layers exposed to humidity and heat showed discolouration, pinpointing differences based on the binder composition, and ageing conditions .
Vivianite (Fe3(PO4)2·8H2O, blue)
Vivianite is an hydrated iron phosphate. Its blue colour arises from intervalency charge transfer between Fe2+/Fe3+ [1, 37], as a consequence of oxidation of some of the Fe2+ in colourless vivianite . Occurrences of vivianite deposits and of its use as pigment are listed in literature [1, 47, 76–80]. As a pigment, it only shows medium stability [47, 81]. The discolouration of this blue monoclinic phosphate to a greenish hue is attributed either to unbalanced Fe2+/Fe3+ ratio, or to its oxidation to green triclinic metavivianite Fe3(PO4)2(OH)2·6H2O , and finally to yellowish brown amorphous santabarbaraite Fe3(PO4)2(OH)3·5H2O [76, 82, 83]. Oxidation is expected to be faster in air, and to slow down once the pigment is embedded in a binder . On top of these oxidation reactions, heat related damages can be observed starting already at 70 °C, causing colour changes in both pure vivianite  and oil paint layers containing it . The degradation seems to preferentially affect larger grains .
Cobalt (Z = 27)
Smalt (CoO·nSiO2, blue)
Smalt is a synthetic cobalt-doped potash glass. More details on smalt fabrication and (early) occurrences are given elsewhere [1, 20, 86–88]. It was not only used as a pigment, but as an additive to improve oil curing as well [34, 47, 86]. Coarse grinding of the blue glass was required in order to obtain a satisfactory colour, which seems to have affected the degree of alteration [34, 89]. The discolouration of smalt can be observed in 25–50 years, and it can be related to various phenomena: on one hand, the similarity of optical properties (refraction index) of the glass and of the oil, on the other the instability of potassium glass, Co and K ions being the two species considered as responsible [1, 20, 34, 90]. In fact, smalt use is recommended in wall paintings (secco technique) and in aqueous media only . Environmental moisture proved harmful to oil paint layers, to potash glasses , and to wall paintings . The first theories on the cause of smalt discolouration in oil assume the role of Co as a catalyst in the binder’s oxidation, with the formation of an organometallic compound at the grain boundary [34, 92]. Moreover, a change in cobalt coordination occurs and it is attributed to the oxidation of cobalt . Later studies [89, 93, 94], however, demonstrated that the distribution of potassium in degraded particles of smalt is not homogeneous, while cobalt is confined to the particles, and that K ions are leached out of the glass affecting its K:Co ratio, the pH, and the Co(II) coordination, which result in colour change. As a consequence of K+ leaching, the coordination of Co2+ changes from tetrahedral to octahedral (X-ray absorption spectroscopy studies (XAS) [90, 94], vibrational spectroscopies and elemental analysis ), causing the colour loss; simultaneously, the glass network is modified as the Q3 component related to alkali content decreases, and hydration is observed . The change in pH might additionally damage the glass network. The leached K+ ions interact with the aged oil, increasing its water sensitivity, forming soaps and causing blanching [35, 47, 89, 95, 96]. The paint composition might affect the discolouration, by providing chemical species to buffer the potassium leaching. If calcium is present in the glass network, the pigment appears to be less sensitive [47, 89, 90, 96]. When smalt is mixed with leadwhite in oil, lower degrees of alterations are observed, probably because lead soaps are favoured compared to the potassium ones, so that leaching of alkali is not as relevant as in pure smalt paint layers [34, 90, 96, 97]. The discolouration of smalt and the rough surface of degraded particles significantly alter the appearance of oil paintings where it was used for the sky, or other parts intended to be bright blue [34, 89, 94, 96, 98–101]. Smalt egg tempera samples proved sensitive to environmental conditions in museums, so that such a paint layer might be successfully used as a dosimeter to evaluate the air quality in terms of conservation issues . In glue binder, the role of humidity and airborne pollutants in accelerating glassy pigments degradation was demonstrated, where leaching of both potassium and cobalt ions occurred. No evident effect of SO2 and NOx synergy was observed though . In fresco wall paintings, smalt is expected to deteriorate due to the very alkaline conditions, to the presence of liquid water (including condensation, capillary rise and infiltrations), to the small particle size increasing the surface reaction, and to the possible contamination by pollutants. Again, leaching of alkali is observed, and in some strongly degraded smalt particles showing cracks, cobalt and other divalent ions are leached as well, probably due to aggressive environmental conditions (humidity, basic pH) [88, 91, 96]. On top of ions lixiviation and weathering of the glass, examples of heat degraded smalt are reported in wall paintings affected by fire .
Copper (Z = 29)
It was recently observed that historical copper-based pigments are not only limited to malachite, azurite, verdigris and copper resinate. In fact a variety of salts (organic acids salts such as copper citrate , silicates, phosphates, sulphates, chlorides, etc.) were as well used as pigments [105, 106], and should not be regarded anymore as degradation products only. Moreover, the situation is complicated by inconsistent nomenclature use in artistic literature [1, 105]. It is well known that malachite and azurite are not stable in fresco, and that they tend to discolour in oil . Studies on metallic copper exposed to a museum-like environment demonstrated its sensitivity to organic acids, including from the binder, so that Cu(II) compounds are always formed [107–110]. More details on the reactivity of copper salts in acidic conditions are given for each pigment in the next sections.
Azurite (2CuCO3·Cu(OH)2, blue)
Azurite (2CuCO3·Cu(OH)2) was the most diffused blue pigment during Middle Ages, and before [1, 20]. It appears to be stable to light and atmosphere, and shows good performances both in oil and tempera mediums [111, 112], although its poor hiding power in oil is reported in literature . Degradation of azurite in frescoes seems related to pH and grain size [91, 114]. Azurite degrades to green compounds: malachite (CuCO3·Cu(OH)2) and paratacamite/atacamite (Cu2Cl(OH)3) are some examples [28, 64, 111, 112, 115–118]. Humidity and chloride ions from various sources cause the formation of black copper oxides (CuO) and green chlorides (nantokite CuCl, paratacamite/atacamite or botallackite Cu2Cl(OH)3 [103, 116, 118–121]). Azurite degrades to black tenorite CuO when exposed to heat in presence of alkali [20, 68, 91, 103, 113, 114, 121–123], while cold alkaline conditions might not affect it , or cause conversion to malachite , or the formation of tenorite via formation of copper hydroxide Cu(OH)2 [35, 64, 114]. On the other hand, it is decomposed by acids, such as oxalic acid to form oxalates (CuC2O4·nH2O, mooloite) [32, 55, 116]. It has been reported that the combination of oxalic acid and chlorides in wall paintings results in Cu-hydroxychlorides Cu2Cl(OH)3 and Ca-oxalates ; and that no Cu-oxalates were observed in azurite paint layers . On the other hand, oxalates attributable to the biodegradation of an organic binder were found in both a gypsum preparation (weddellite/whewellite) and in the overlying azurite-containing paint layer . Cu-carboxylates are rarely formed, as leadwhite, often mixed with azurite, is more reactive; moreover, as azurite is preferred in a tempera medium, carboxylic acids are less present . In proteinaceous binder, other interactions are reported . Verdigris xCu(CH3COO2)·yCu(OH)2·zH2O might be formed when high relative humidity interacts with an azurite containing paint layer, degrading the oil binder . Also, the yellowing of the binder/varnish can alter the blue shade to a green one in oil paintings . Small particles are reported to have less colouring power and to be more sensitive to chemical interaction , and to dissolve in organic binding media . In egg tempera, azurite is not affected by light, while thermal ageing, high relative humidity, and pollutants affect the paint film (oxidative processes) . Also, the copper interacts with the proteinaceous moieties to form metalloproteins . Azurite turns bluish-black when exposed to H2S vapours , especially in frescoes , as covellite CuS is formed. The effect of laser irradiation on azurite is the formation of black CuO, which depends on the particle size and therefore on the temperature increase [112, 114, 125]. Selective biological activity is observed towards lead pigments .
Malachite (CuCO3·Cu(OH)2, green)
Malachite (CuCO3·Cu(OH)2) is more stable than azurite (2CuCO3·Cu(OH)2) and verdigris (xCu(CH3COO2)·yCu(OH)2·zH2O), therefore showing less or slower reactivity towards many factors [119, 123]. It is known to be permanent in all binding media, lightfast and alkali proof. Its deeper colour is obtained by coarse grinding, and as a consequence of having relatively low refractive index, it shows better performances in tempera than in oil [50, 126]. Due to its chemical composition, malachite is subject to interactions with acids, bases, humidity, temperature and circulating ions. In presence of humidity, malachite stains can be observed, which are actually caused by proteinaceous binders degradation . Moreover, ions such as Cl− present in the mortar, sand or in the bricks, can react with the basic carbonate to form copper hydroxychlorides ((Cu2Cl(OH)3) atacamite, clinoatacamite, paratacamite botallackite) [119, 121, 123, 126–128] and the copper chloride nantokite . Sulphate ions are also likely to be present in wall paintings, especially from the degradation of calcite to gypsum, from gypsum preparation layers , or from SO2/SO3 pollution.
Copper basic sulphates (CuSO4·yCu(OH)2·zH2O: brochantite, langite, posnjakite, antlerite) are formed according to pH, humidity and temperature [119, 127, 129]. Copper chlorides and sulphates can further react to form other salts . In tempera medium, it seems that the carbonate function is affected, more than in fat media, and more when no varnish is present to protect the paint layer from ageing . Malachite particles in the form of spherulites were identified in brownish tempera layers, the discolouration being caused by the pigment-binder reaction . In oil or oil/resin medium, it can turn into copper resinates  and other organometallic compounds [131, 132]. Copper carboxylates, originating from the binding media interacting with the pigment’s cation, were observed for both tempera and oil binders [55, 133, 134]. Acidic conditions are commonly found in, and surrounding, polychrome objects, as a result of biological activity on the object  or of degradation of the organic binders (e.g. oxalic acid [129, 135]). Diluted acids decompose malachite (for example acetic, hydrochloric, nitric acid ), causing the release of Cu2+ ions, and the formation of the most stable phase according to pH and other present ions. The release of acetic and formic acid by the organic building materials of the display cases has been ascertained [136–138]. The copper oxalate mooloite was studied  and identified in various works of art [53, 135]. Bluish Cu acetates, corresponding to the well-known pigment verdigris, were identified on pure pigment powders exposed to acetic acid atmospheres . H2S vapours cause darkening by formation of the bluish sulphide covellite, which is formed on the pure malachite pigment [126, 139], although in paint layers this has not yet been identified [124, 126]. On the other hand, it seems that Cu act as a biocide towards microorganisms that produce sulphuric acid, leading to a selective H2S attack on non-copper pigments . In fresco wall paintings, malachite can discolour due to the alkaline pH of the lime binders, especially when the particle size is small. The exposure to high temperature (fires) can lead to the formation of black copper oxides [1, 91, 103, 121, 139, 140], especially in frescoes due to alkaline conditions . On exposure to laser light, pure malachite darkens , as a consequence of reduction of Cu2+ to form dark cuprite Cu2O and black tenorite CuO [141, 142]. The same reactions take place when heating malachite above ca. 360 °C .
Verdigris (copper acetates: xCu(CH3COO2)·yCu(OH)2·zH2O, bluish green)
Verdigris is currently used to indicate a variety of blue-green compounds resulting from the exposure of copper to various acidic media (vinegar, for example) [1, 143]. The instability of verdigris is well known . Improved performances seem related to oil/oil-resin rich paint layers, or to the selection of the neutral respect to the basic form (less reactive towards the acidity of the oil), or by adding yellow glazes on top of it [1, 144]. Verdigris is less stable than malachite and azurite to salt solutions, forming Cu2+ salts when the appropriate anions are present, such as nitrates, sulphates, chlorides [119, 123]. It promotes oil curing and it retards its oxidation . Verdigris (neutral and basic), as well as the copper carbonate pigments azurite and malachite, shows the tendency to react with the oil binding media to form copper soaps [35, 67, 145], to brown and darken, as Cu+ is formed by ambient light absorption (photoreduction), and oxygen promotes the formation of brown peroxide species . Cu-salts of organic acidic compounds are formed in verdigris-containing paint layers (or at the interface with the varnish), as fatty and resin acids can extract copper ions from the verdigris pigment [133, 147]. It seems important to mention that such reactions are equilibria, and therefore the addition of overpaint, or the removal of paint or varnish layers during restoration, might affect the chemical structure of the aged layer . Copper acetates and resinates catalyse the oxidation process of oil films and influences the curing of the oil [133, 134, 147, 148]. In tempera, verdigris seems less prone to discolouration, but still reactions of Cu-extraction can take place [133, 146]. Copper acetates are very soluble in acidic conditions, and copper oxalates are formed in presence of oxalic acid from various sources . In alkaline conditions, blue copper hydroxides are formed . When exposed to H2S, the pure pigment turns dark, as covellite is formed; however, this compound has not been identified yet in paint layers [20, 124]. Verdigris should not be mixed with orpiment or leadwhite, as darkening occurs . Verdigris is not sensitive to lasers [141, 142, 149, 150], but it darkens during particle induced X-ray emission (PIXE) analysis . It is involved in cellulosic materials degradation, especially for critic conditions of illumination and SO2 concentrations .
Copper resinate (copper salts of abietic acid, green)
Copper resinate was used as a glaze, prepared by mixing Cu-acetates (verdigris) with oil and/or resins. In this process, some of the acetates remain unreacted . When mixed with oil, the coordination of the copper ion changes [147, 151]. Copper carboxylates are formed from the oil fraction of the binder, while metalloproteins may appear if egg tempera was used . It darkens with light exposure [98, 132] and in alkaline conditions . Also, being an artificial pigment, its production process might affect its stability, if reactive moieties are not carefully removed . The lipidic fraction of the paint layer undergoes photo-oxidation, with Cu acting as a catalyst: local molecular arrangement changes are likely to be involved in colour changes [20, 147]. Upon prolonged exposure to X-rays, Cu2+ is reduced to Cu+ .
Egyptian blue (CaCuSi4O10)
The artificial pigment Egyptian blue (a silica-rich glass, whose colour is related to the presence of the mineral cuprorivaite, CaCuSi4O10 [152, 153]) is a very stable pigment (to acids, alkali, light) . It is related to Egyptian green, whose colour is due to the presence of Cu-doped (max 2%) parawollastonite (CaSiO3), which does not show evident degradation issues (it transforms into the polymorph pseudowollastonite above 1150 °C) . Egyptian blue requires coarse grinding to maintain its colour, showing poor hiding power . Some discolouration can anyway be observed on Egyptian blue, towards greenish or black compounds. It is reported to degrade to a green coloured material as Cu2+ ions are leached and react with Cl− ions from the glass itself or from circulating solutions to form carbonates and chlorides [156–158]. Degraded Egyptian blue was found to contain chlorides (atacamite and eriochalcite CuCl2·2H2O) . Moreover, the yellowing/darkening of the organic materials (varnishes, gums) can alter the appearance of this blue pigment, and pigment-binder interactions could promote selective darkening, thanks to the release of copper ions from the pigment itself, possibly in relation to pH, impurities and gum type . No evidence of black tenorite CuO formation was found [155, 159].
Copper chlorides (atacamite, paratacamite, clinoatacamite, botallackite: Cu2Cl(OH)3, green)
The polymorphs of copper hydroxychlorides (Cu2Cl(OH)3) pose relevant nomenclature issues, especially as regards paratacamite which is often used as a synonym for clinoatacamite, even though the two minerals have different formulas ((Cu,M)2Cl(OH)3) and Cu2Cl(OH)3, respectively, where M = Zn, Ni) [1, 160]. Chlorides of Cu were identified as pigments in objects from different cultures [1, 118, 161, 162], and recently atacamite was found in colonial art , in China  and in Alhambra’s plasterwork . As a pigment, it can be natural or synthetic, and the particles’ morphology and composition can clarify its origin [162, 164]. It is expected to form as a degradation product of Cu containing pigments, or artefacts such as bronzes, in presence of humidity and chloride anions [115, 119, 121, 158, 160, 165, 166]. It is however not stable in acidic conditions (oxalic acid, for example), which promotes the formation of mooloite (CuC2O4·nH2O) . Also, among the polymorphs, different stabilities are observed, botallackite being the first formed and more unstable, while paratacamite seems the favoured species at ambient temperature . The polymorph paratacamite has so far been identified as degradation product on other Cu-containing pigments [64, 117, 119, 123, 129]. However, an interesting example is reported in literature: after the 1966 flood in Florence, green paratacamite was formed on azurite-containing wall paintings . To treat sulphated walls, ammonium carbonate ((NH4)2CO3) and barium hydroxide (Ba(OH)2) were used. The green paratacamite alterations turned blue after the treatment, but not permanently. In fact, in the alkaline conditions given by the treatment, paratacamite is dissolved by the ammonium carbonate, and blue Cu(OH)2 formed. This hydroxide is not stable and it degrades either to black CuO or to hydroxychlorides. This latter reaction is more probable due to the continuous ingress of Cl− ions via capillary rise in walls, thus generating more green products, making this treatment not suitable in presence of degraded azurite .
Copper sulphates (brochantite, antlerite, langite, posnjakite: CuSO4·yCu(OH)2·zH2O, green)
Green copper sulphates are commonly found as degradation products on copper artefacts exposed to polluted environments . Brochantite was identified as a pigment as well , but it can transform into the more stable polymorphs, antlerite, langite or posnjakite, according to the given conditions of relative humidity, inorganic pollutants (SOx) and biological activity (affecting the pH and the type of acids) [127, 129, 168]. Posnjakite was also identified as a pigment [162, 169]. Brochantite, as well as posnjakite and antlerite are also intermediate reaction products between copper carbonates and copper oxalates, and are therefore sensitive to oxalic acid .
Arsenic (Z = 33)
Geologically, orpiment (As2S3) and realgar (As4S4) occur together, and are often associated with antimonates and other sulphides [1, 30]. Orpiment and realgar were highly appreciated especially in Egypt and China for their rich yellow and red–orange shades [1, 170–174], even though the pigments were considered “unpleasant” by many artists, and not recommended to use in combination with copper and lead based pigments, such as verdigris and leadwhite [1, 30, 170]. Orpiment and realgar exist as minerals, as well as synthetic pigments , and of the two arsenic sulphide pigments, the first one is the most stable. Realgar, being unstable, was less often reported in works of art [1, 176–179]. It has a polymorphic photodegradation product, pararealgar As4S4. Permanence of arsenic sulphide pigments is problematic, especially on exposure to the green region of the visible spectrum [174, 180]. Moreover, smaller particles are more readily degraded . The sensitivity to green light has to be taken into account when lasers need to be used, such as during Raman spectroscopic analysis [177, 181, 182]. Arsenates (As5+) are formed from orpiment, realgar and emerald green degradation: these ions are water soluble and migrate throughout the whole painting, accumulating at interfaces between layers, around Fe/Mn rich particles, and according to the local pH conditions in the paint layer. Due to this water-based transport, appropriate cleaning solvents must be selected, and relative humidity controlled [180, 183]. It seems that polysaccharidic media, or egg yolk, negatively affect the stability of the arsenic sulphide pigments .
It needs to be pointed out that pararealgar was recognized as a mineral in 1980 , and that only recently the mechanisms of arsenic sulphides degradation was investigated [174, 183]. Care needs to be taken when considering published data, according to the year of publication and used techniques (compare for example  with ).
Orpiment (As2S3, yellow)
Permanence of orpiment to light was known to be poor, although some intact paint layers could be found on some illuminated manuscripts [176, 178, 179]. Coarser grain size seem to retard the colour change . In water, upon heating, and in oxidizing conditions (ozone, NOx, for example) it decomposes to arsenic trioxide (As2O3), and hydrogen sulphide (H2S) is formed as well in presence of humidity [9, 176]. Relative humidity alone seems to have no effect, while degradation is observed when both humidity and light are present . Light exposure causes discolouration [30, 128, 182, 185, 186]. Orpiment darkens upon heating, it burns in air producing white arsenolite (As2O3) and volatile SO2, and it decomposes in water (the artificial product having smaller particles, dissolves faster). It is soluble in many inorganic acids, and in alkaline conditions as well, which make the pigment unsuitable for use on wet plaster [123, 176]. It is reported to be unstable in oil . When mixed with lead and copper pigments, darkening occurs, for example the case of leadwhite is reported in literature [30, 50, 176].
Realgar and pararealgar (As4S4, reddish orange)
Realgar, exposed to light, degrades . However, the formed yellow compound is not orpiment, as believed until 1980, but pararealgar [171, 187]. Exposed to (green) light, both high and low temperature realgar transform into brittle, bright yellow pararealgar [106, 177, 181], and finally to arsenic trioxide (As2O3) [106, 180, 188]. The phenomenon is complex and involves intermediate phases such as χ-realgar. Pararealgar is also to be considered an equilibrium phase [175, 177, 189]. Heating causes darkening of the colour of realgar, χ-realgar, as well as of pararealgar, which is not permanent: in fact the high temperature polymorph converts to the low temperature realgar with time . As orpiment, it burns in air and shows similar solubility . The role of the binder on realgar’s stability is not completely understood, yet it could protect the pigment from discolouration . Seen that the photodegradation of realgar yields a yellow material, which was long thought to be orpiment, but was finally identified as pararealgar , one might question the identification of orpiment in museum objects , as well as consider the possibility of pararealgar as a pigment [106, 173, 178, 190–192].
Mercury (Z = 80)
Vermillion (HgS, red)
The deep red coloured cinnabar (HgS), as well as its synthetic counterpart vermillion (HgS), produced either by wet or dry processes [193, 194], were extensively used . Roman sources , however, seemed to use the term minium to identify this material, giving room to misinterpretations and nomenclature issues . It was already known to Romans [195, 196] that mercury sulphide darkens when exposed to light, although not systematically [1, 193, 194, 197–199]. Moreover, HgS was not recommended for use in fresco, as humidity and pH affect its stability [103, 123, 200], but still it was used on wall paintings [73, 193, 201–203]. Paint layers of pure vermillion are very vulnerable , while no darkening is observed when it is mixed with leadwhite [19, 20, 193, 204]. An increased resistance to laser radiation is also observed for other mixtures, such as vermillion with chrome yellow, madder, Prussian blue, and bone black, although the single pigments’ behaviour is hardly predictable . Red lead, often mixed with vermillion to improve the curing of the oil binder, seems to have a protective effect on HgS [20, 199]. A series of reactions of vermillion with other pigments, different binders and substrates are reported in literature [65, 198, 205]. The presence of a varnish layer, and of organic binders, moreover, seems to protect the paint layer, by absorbing the incident light (more effectively when the varnish is thick and/or yellowed, and with different efficiency according to the composition, i.e. UV light is strongly absorbed) and by physically protecting the pigment grains from contact with chlorine-containing materials (disinfectants, dirt, etc.) [199, 206]. Sulphation of the red paint layers in Pompeian wall paintings was observed, and seems to be connected with the Cl− induced degradation of vermillion [106, 202]. The blackening of vermillion was often attributed to the formation of a black compound, supposedly metacinnabar (β-HgS) , although no positive identification could be obtained . Binder type and pigment/binder ratio affect the pigment’s stability, and glazes seem to have a role in preserving the red hue of vermillion, by filtering some of the incident light and protecting the pigment from external sources of chlorine . The same metacinnabar is hypothesized in wall paintings [121, 139, 202], but only identified on laser irradiated vermillion [70, 142]. Pure vermillion is sensitive to lasers of various wavelength and irradiation, so that the original structure is lost [141, 142, 206]. Black β-HgS was identified by X-ray diffraction (XRD) on IR-laser damaged red α-HgS , and by X-ray photoelectron spectroscopy (XPS) . Dark Hg2S was detected as well, by XPS by . XRD and XPS studies suggest a photoreduction of HgS to Hg0/Hg2 2+, instead of polymorph transformation . Finally, the photodegradation of vermillion was explained in terms of formation of metallic mercury and HgCl2, due to the halogen impurities present in the pigment, identified thanks to secondary ion mass spectrometry (SIMS) analysis , and to X-ray spectroscopic analysis . The role of Cl-ions, present as an impurity in the pigment, coming from marine aerosol, from the used binder or from pollution, in the degradation of vermillion has been ascertained thanks to the identification of chlorine species [197, 199, 202, 203, 207–209]. Calomel (HgCl)2 was identified by portable and micro-Raman spectroscopy, while HgCl2 and metacinnabar were not [33, 59, 73, 199]. The photosensitivity of vermillion depends on one hand on the presence of halogen impurities [199, 202, 207], and on the other on the particles’ size and relative humidity . The cause of the discolouration is, however, still under debate (elemental mercury or dark Hg–S–Cl species, as well as grey mercury chlorides) [33, 73, 199, 207]. In tempera, vermillion is only slightly sensitive to 248 nm laser pulses, while in oil medium it shows the lowest discolouration threshold for that laser wavelength compared to a selection of pigments [19, 193, 199, 210]. It is also reported that in oil medium the darkening is worse than in watercolour . In this latter case, vermillion is reported to be sensitive to light, with oxygen and humidity accelerating the darkening . No mercury oxalates were ever identified on vermillion paint layers .
Lead (Z = 82)
Lead pigments are not suitable for use in fresco, but they were still used due to their good hiding properties and to the low prices compared to other pigments [5, 123, 200, 211–213]. The degradation processes of the various Pb pigments yield a range of compounds which cause discolouration, strongly affecting the readability of the polychromy . Lead pigments on ceramic artefacts are sensitive to sulphur containing pollutants, to acidic solutions (rain; CO2; microbial activity), to light and air: anglesite PbSO4, cerussite PbCO3, hydrocerussite 2PbCO3·Pb(OH)2 and lead sulphide PbS are formed . Anglesite, attributed to lead pigment degradation was identified on wall paintings as well . To laser irradiation, reduction of the lead compounds (leadwhite, massicot, red lead) occurs on the surface, so that the dark colour is attributed to formation of metallic lead, or Pb2O . The subsequent oxidation of lead explains the reversibility of such a colour change . The temperature increase related to the laser irradiation causes the formation of massicot on both minium and leadwhite . Care needs to be taken when a laser source is used for analytical purposes, such as during Raman spectroscopic investigations, as the lead pigments and their degradation products, such as PbO2 and PbS, might be altered, affecting the interpretation of the degraded areas [215, 216]. All lead pigments turned black after exposure to H2S vapours (formation of galena, PbS), with increasing reactivity for basic moieties, such as Pb(OH)2 units in leadwhite . It is reported that biological colonization on wall paintings can selectively affect lead (II) compounds, causing brown discolourations identified as PbO2 or PbS [211, 217], while copper pigments act as biocides [124, 212]. Lead dioxide formation is favoured in humid alkaline conditions, such as in frescoes, when peroxides are produces, either by microorganisms or by photo-oxidation of organic materials . When exposed to salt solutions, and light, the most stable phase for all the lead pigments is cerussite [123, 200, 218]. In oil, lead compounds were not only used as pigments, but as siccatives as well [219, 220]. Moreover, lead soaps are formed in, or on the surface of, oil paint layers containing lead based pigments (leadwhite, lead tin yellow type I, litharge, red lead) and are investigated as they affect the appearance of paintings [54, 55, 107, 145, 219–228]. Lead oxalates were observed on leadwhite treated with oxalic acid , and on real samples containing red lead and lead tin yellow . Plumbonacrite 3[2PbCO3·Pb(OH)2]·PbO was detected as well .
The term “leadwhite” refers to a variety of white lead based materials, of which the most common, and most important from the artistic point of view is the basic lead carbonate corresponding to the mineral hydrocerussite (2PbCO3·Pb(OH)2) [1, 229], possibly including cerussite as well, according to the fabrication process [1, 20, 229]. Considering it is a carbonate, it is relatively stable, although soluble in acids [20, 98, 229]. Also, it shows catalytic effects . Upon heating, massicot (orthorhombic PbO) is formed, then litharge (tetragonal PbO) and finally red lead (Pb3O4) . It was used in every medium. Black discolouration is observed on frescoes and watercolours , and it is reported for manuscripts as well [60, 231]. The blackening is attributed to the formation of either PbO2, as a result of exposure to oxidizing agents or from microbiological activity, or PbS [9, 20, 98, 200, 211–213, 232]. The role of S-containing pigments on the blackening of leadwhite is not fully understood [191, 229], while atmospheric and biological sources of H2S strongly affect watercolours [20, 191, 231] and manuscripts [60, 190, 191, 204]. If PbS is formed, a reconversion treatment (oxidation) can be applied to restore the white shades of degraded polichromy. However, the formed lead sulphate (PbSO4) is not sufficiently stable to make the treatment recommended [139, 212]. Humidity seems to be involved in many degradation processes, especially in synergy with atmospheric pollutants such as SO2 and NOx [9, 64, 166]. The presence of sulphuric pollutants is responsible for the formation of lead sulphide, lead sulphates, and cerussite/hydrocerussite [124, 166]. The sensitivity of leadwhite to laser light was studied extensively, especially with respect to laser cleaning and laser based analysis of works of art, but the alteration process is not fully understood, the binding medium probably influencing the results [70, 142, 150, 210, 215, 233, 234]. Analytical evidence of formation of metallic lead and lead (II) oxides upon IR laser irradiation is provided in literature [70, 234]. A visible laser (632 nm) caused the appearance of a massicot-like alterations on leadwhite, possibly via a plattnerite (PbO2) intermediate, which is related to impurities in the leadwhite . Low fluence UV-laser does not affect paint layers prepared with different binders . Moreover, the exposure of the PbO degradation to environmental conditions is likely to yield a carbonate-hydroxide lead salt, making the degradation reversible to some extent, especially in absence of a binder [19, 142, 234]. The influence of ion beam based analysis on polychrome objects can negatively affect leadwhite paint layers, as hydrocerussite is sensitive to PIXE analysis . On wall paintings, the combined presence of humidity and salts can cause the formation of degradation products, sometimes coloured as in the case of NaPb2(CO3)2OH and PbO polymorphs, with cerussite being the most stable of them all [123, 218]. Leadwhite tempera and oil layers show the formation of lead soaps, which negatively affect the appearance of the paintings [55, 67, 226]. Leadwhite tempera applied on fresco paintings yields PbO2, as the reaction takes place in alkaline environment . Also, differences in naturally and artificially aged samples are observed, the latter being less degraded . Lead oxalates were observed on leadwhite treated with oxalic acid , and on real samples ; lead acetate was detected upon exposure to acetic acid . Plumbonacrite (3[2PbCO3·Pb(OH)2]·PbO) was detected as well .
Litharge (PbO, orange)
In antique sources, the term litharge denotes a variety of compounds derived from lead oxidation, and other compounds. It is now recognized as the low temperature polymorph of PbO, which is tetragonal . As a pigment, it was identified on manuscripts and mural paintings . It is less stable than massicot to water and to saline solutions commonly found in wall paintings . As a pigment, litharge is stable to infrared (1064 nm) and red lasers (632, 647 nm) irradiation for Raman spectroscopic studies [215, 233]. Using a 632 nm laser with a power above 9 mW (calculated fluence of approximately 45 kW/cm2) degradation is recorded . For shorter wavelengths, and laser fluence at the sample above 2 kW/cm2, litharge degrades to massicot . In oil and tempera binder, lead carboxylates are observed, showing stronger intensities compared to those formed from lead white . This increased reactivity was probably well known to artists and artisans, as PbO was often used as a siccative for binders and varnishes . Litharge was as well used for the preparation of other pigments . It degrades to cerussite and hydrocerussite, as well as to lead sulphate and phosphate, in archaeological sites attributed to the production of painting materials in Thera, Greece .
Massicot (PbO, yellow)
Also in this case, as for litharge, nomenclature is complex and confusion may arise. Massicot corresponds to orthorhombic PbO, the high temperature polymorph of this compound . It is reported as a pigment in Egyptian artefacts, mural paintings and manuscripts . On wall paintings, the presence of water and of bicarbonate ions from atmospheric CO2 dissolution causes the formation of hydrocerussite and cerussite, the latter being the most stable compound of the series, even when intermediate salts are formed according to the present anions in the saline solutions [123, 218]. As for other lead containing paint layers, humidity causes first a darkening of the colour, and finally the formation of leadwhite . Upon exposure to H2S, black galena (PbS) is formed in a thin superficial layer . Massicot is stable to laser irradiation, except to 10 mW of 514 nm laser, where a whitish degradation is observed . The effect of infrared laser irradiation was investigated as well and it was found to cause reduction of the pigments, and discolouration due to the formation of metallic lead. This process is very intense for massicot, as the formed metallic particles are hardly subjected to re-oxidation, due to their bigger size .
Red lead (Pb3O4)
Red lead, or minium, has a long history of use in worldwide artistic and artisanal practices, although nomenclature issues are evident [1, 237, 238]. Its synthesis and relation with the other lead oxides is well described in literature . As a pigment, it is not stable in fresco, or in watercolour. It was anyway used, for its optical properties and cheap price, compared to other red pigments [1, 120]. It is reportedly blackened on manuscripts and in glue tempera paintings exposed to light and humidity [1, 64]. On the other hand, it is reported that medium-rich proteinaceous red lead paints are stable to light, the binder protecting the pigment . Moreover, the pigmented layer has different properties than an unpigmented one . It is soluble in diluted acids (nitric, chloridric, oxalic, acetic), with formation of the corresponding salts, and when heated it transforms to litharge [1, 240]. It is sensitive to sulphur containing compounds, as H2S and SO2 of atmospheric or biological origin, and pigments such as arsenic sulphides (AsxSy), vermillion (HgS) and ultramarine (Na8[Al6Si6O24]Sn), especially in manuscripts, wall paintings, and in case the pigment is not mixed with a binder, which cause the formation of dark grey PbS [1, 60, 93, 124, 238, 241] and of whitish PbSO4 [59, 68, 233, 242]. This was known to artists and artisans, as demonstrated by the presence of a blank space between yellow orpiment and red lead in a 12th century illuminated manuscript, although mixtures of the two pigments do not behave consistently. The reaction between red lead and orpiment produced galena, which could further degrade into sulphates, and lead arsenate species . Mimetite Pb5(AsO4)3Cl was as well identified as a reaction product of minium and orpiment . Moreover, it seems that minium promotes the blackening of leadwhite, when the pigments are mixed . Some cases of red lead lightening are reported as well, and are related to the formation of lead sulphates and/or carbonates, according to the atmospheric pollutants and to the initial amount of PbO in red lead [68, 93, 200, 237, 238], via the formation of plumbonacrite .
The black discolouration of red lead due to light exposure was initially attributed to the formation of black PbO2, but this compound is not stable to light , and it was rarely positively identified [93, 120, 238], while mixed oxides are also hypothesized . PbO2 could be a metabolite of microorganisms such as fungi [217, 239]. The parameters responsible for the blackening of minium are identified as light [5, 240, 243], including laser light (514 and 488 nm, ), the pigment’s composition, and climate. These parameters all contribute to yield a grey discolouration at first, and finally a chocolate brown one . Red lead semiconductor properties are responsible for the reduction of Pb(IV) to Pb(II), and the formation of PbO; the presence of bicarbonate ions (HCO3 −) promotes then the formation of hydrocerussite and/or cerussite [123, 218, 244]. The PbO formed by laser irradiation can be re-oxidized to minium . A role of Cl− ions is also hypothesized [93, 245]. The formation mechanism of black discolouration products [plattnerite (PbO2) and galena (PbS)] was recently investigated: no plattnerite could be detected in artificial ageing tests, while the natural ageing of fresco red lead samples yielded calcite CaCO3, minium Pb3O4, plattnerite PbO2, anglesite PbSO4 and gypsum CaSO4·2H2O, with the minium grains being gradually converted to alteration products (plattnerite and anglesite) via a sulphation step followed by solvolytic disproportionation [237, 242]. Plattnerite might be formed if acidic pollutants attack the paint layer ; or when biological colonization is present . Red lead is readily affected by organic acids (including the oil binder), which causes the final appearance of lead carbonate, via lead acetate Pb(C2H3O2)2  and lead hydroxide Pb(OH)2 , and/or lead soaps .
Lead tin yellow (type I: Pb2SnO4; type II: PbSn1−xSixO3)
Two types of yellow pigments based on lead and tin are available, type I which is an oxide (Pb2SnO4), and type II which is a stannate silicate of lead (PbSn1−xSixO3). On their nomenclature, chronology, and occurrences, more can be found in literature [1, 20, 246], and references therein. As pigments, they show good hiding power and siccative properties when used in oil . Other than for use as pigments in paintings, lead and tin-based coloured opacifiers were used in glass making [1, 20, 246]. Both pigments are stable to alkali, but sensitive to H2S and sulphides, which causes blackening (PbS) . Whereas other lead containing pigment show fire damage, lead tin yellow type I is detected unaltered , but heating above 900 °C causes decomposition of the material [20, 246]. They are lightfast, opposite to the other Pb based yellow pigment, massicot (PbO), and stable in oil medium [20, 246]. Relative humidity has no negative effect on either pigment . On the other hand, acidic components interact with the materials, including carboxylic acids from the ageing oil (lead soaps [56, 225, 247]), or volatile organic compounds, such as acetic acid, of relevance in air quality evaluation in museum environments (lead acetate Pb(C2H3O2)2 ).
The general understanding of the on-going potentially harmful processes regarding inorganic mediaeval pigments is almost completed, and benefitted from the development of new analytical techniques. This work, based on chemical and archaeometrical literature, summarizes both evidences of pigments discolouration and degradation in works of art, as well as investigations based on mock-ups and simulations. Also, results from a variety of analytical techniques are here considered, as the complexity of the topic requires. In fact, tracking and understanding the reactions causing discolouration of the studied pigments requires a different approach according to the problem: salt dissolution and formation (Ca, Cu and Pb-based pigments); reduction or oxidation reactions (Fe, Cu, Hg, Pb-containing pigments); ions leaching (ultramarine, smalt; Ca, Cu, Pb-pigments); polymorphism and structural modifications of the materials (Cu, As, Hg and Pb-pigments). In each case, an adequate selection of analytical and imaging techniques should be made. Moreover, the role of light (illumination, or laser systems) needs to be taken into account, especially for photosensitive pigments (As and Hg-containing). Finally, humidity, pollutants and contaminants (H2S, SO2/SO3, NOx; Cl-compounds) provide reactive moieties.
The collected information regarding the stability of mediaeval inorganic pigments shows how complex the conservation of polychrome objects can be. First, the understanding of the pigments’ stability to specific factors is required. Even though pigments are not often present without binder in works of art, it is important to know their reactivity. Then, the study of pigment-binder interactions, and how they affect the single components and the paint layer itself needs to be considered. Finally, the implementation of air quality control systems and climatic monitoring should take into account the different sensitivities of each and every material in museums and exhibitions, in storage or on display, in order to ensure its preservation for the future generations.
Measurement, Effect Assessment and Mitigation of Pollutant Impact on Movable Cultural Assets. Innovative Research for Market Transfer
Fourier transform infrared spectroscopy
attenuated total reflection micro Fourier transform infrared spectroscopy
micro synchrotron radiation Fourier transform infrared spectroscopy
micro synchrotron radiation X-ray diffraction
- Nd:YAG laser:
neodimium doped yttrium aluminium garnet
X-ray absorption spectroscopy
particle induced X-ray emission
X-ray photoelectron spectroscopy
secondary ions mass spectroscopy
Eastaugh N, Walsh V, Chaplin T, Siddall R. Pigment compendium: a dictionary of historical pigments. Abingdon: Routledge; 2007.
MEMORI project webpage. http://www.memori.fraunhofer.de/. Accessed 10 Feb 2017.
Grøntoft T, Dahlin E. The MEMORI technology-an innovative tool for the protection of movable cultural assets. In: Euro-Mediterranean conference. 2012. Berlin: Springer; p. 756–64.
Grøntoft T, Thickett D, Lankester P, Hackney S, Townsend JH, Ramsholt K, Garrido M. Assessment of indoor air quality and the risk of damage to cultural heritage objects using MEMORI® dosimetry. Stud Conserv. 2016;61(sup1):70–82.
Cennini C. The craftsman’s handbook (Il libro dell’arte). New York: Dover; 1933.
Pastoureau M. Une couleur en mutation: le vert à la fin du Moyen Âge. Comptes rendus des séances de l’Académie des Inscriptions et Belles-Lettres. 2007;151(2):705–31.
Guineau B. Glossaire des matériaux de la couleur et des termes techniques employés dans les recettes de couleurs anciennes. Turnhout: Brepols; 2005.
Alebic-Juretic A, Sekulic-Cikovic D. The impact of air pollution on the paintings in storage at the museum of modern and contemporary art, Rijeka, Croatia. Stud Conserv. 2009;54(1):49–57.
Saunders D. Pollution and the national gallery. Natl Gallery Tech Bull. 2000;21:77–94.
Haneef S, Johnson J, Dickinson C, Thompson G, Wood G. Effect of dry deposition of NOx and SO2 gaseous-pollutants on the degradation of calcareous building stones. Atmos Environ Part A Gen Top. 1992;26(16):2963–74.
Srivastava RK, Miller CA, Erickson C, Jambhekar R. Emissions of sulfur trioxide from coal-fired power plants. J Air Waste Manag Assoc. 2004;54(6):750–62.
Berrie B, editor. Artists’ pigments: a handbook of their history and characteristics. Vol. 4. National Gallery of Art, Washington D.C., USA and Archetype Publications, London, UK; 2007. p. 233.
Winter J. The characterization of pigments based on carbon. Stud Conserv. 1983;28(2):49–66.
Coccato A, Jehlicka J, Moens L, Vandenabeele P. Raman spectroscopy for the investigation of carbon-based black pigments. J Raman Spectrosc. 2015;46(10):1003–15.
van Loon A, Boon J. The whitening of oil paint films containing bone black. In: 14th triennial meeting of the ICOM committee for conservation. London: ICOM; 2005. p. 511–8.
Marzetti B, Scirpa F. L’alterazione del nero d’ossa nella pittura ad affresco: studi delle fonti e sperimentazione tecnico-scientifica. Kermes la rivista del restauro. 2000;13(37):39–45.
van Loon A, Boon JJ. Characterization of the deterioration of bone black in the 17th century Oranjezaal paintings using electron-microscopic and micro-spectroscopic imaging techniques. Spectrochim Acta Part B. 2004;59(10):1601–9.
Sansonetti A, Realini M. Nd: YAG laser effects on inorganic pigments. J Cult Herit. 2000;1:S189–98.
Bordalo R, Morais PJ, Young CRT, Santos LF, Almeida RM. Characterisation of laser-induced physical alterations of pigmented oil layers. e Preserv Sci. 2012;9:47–59.
Roy A, editor. Artists’ pigments: a handbook of their history and characteristics. Vol. 2. National Gallery of Art, Washington D.C., USA and Oxford University Press, Oxford, UK; 1993. p. 230.
Clark RJ, Curri ML, Laganara C. Raman microscopy: the identification of lapis lazuli on medieval pottery fragments from the south of Italy. Spectrochim Acta Part A Mol Biomol Spectrosc. 1997;53(4):597–603.
Clark RJ, Curri L, Henshaw GS, Laganara C. Characterization of brown–black and blue pigments in glazed pottery fragments from Castel Fiorentino (Foggia, Italy) by Raman microscopy, X-ray powder diffractometry and X-ray photoelectron spectroscopy. J Raman Spectrosc. 1997;28(2–3):105–9.
Colomban P. Lapis lazuli as unexpected blue pigment in Iranian Lâjvardina ceramics. J Raman Spectrosc. 2003;34(6):420–3.
Clark RJ, Gibbs PJ. Analysis of 16th century Qazwını manuscripts by Raman microscopy and remote laser Raman microscopy. J Archaeol Sci. 1998;25(7):621–9.
Osticioli I, Mendes NF, Nevin A, Gil FP, Becucci M, Castellucci E. Analysis of natural and artificial ultramarine blue pigments using laser induced breakdown and pulsed Raman spectroscopy, statistical analysis and light microscopy. Spectrochim Acta Part A Mol Biomol Spectrosc. 2009;73(3):525–31.
Coremans P. Les Primitifs Flamands. III. Contribution a l’etude des Primitifs Flamands 2. L’Agneau Mystique au Laboratoire. Antwerp: De Sikkel; 1953.
Plesters J. Ultramarine blue, artificial. Stud Conserv. 1966;11(2):76–91.
Berke H. The invention of blue and purple pigments in ancient times. Chem Soc Rev. 2007;36(1):15–30.
Raimondi V, Cucci C, Cuzman O, Fornacelli C, Galeotti M, Gomoiu I, Lognoli D, Mohanu D, Palombi L, Picollo M, Tiano P. Study of the effects of low-fluence laser irradiation on wall paintings: test measurements on fresco model samples. Appl Surf Sci. 2013;284:184–94.
Church AH. The chemistry of paints and painting. London: Seeley, Service & Co., Limited; 1915.
Del Federico E, Shöfberger W, Schelvis J, Kapetanaki S, Tyne L, Jerschow A. Insight into framework destruction in ultramarine pigments. Inorg Chem. 2006;45(3):1270–6.
Zoppi A, Lofrumento C, Mendes NF, Castellucci EM. Metal oxalates in paints: a Raman investigation on the relative reactivities of different pigments to oxalic acid solutions. Anal Bioanal Chem. 2010;397(2):841–9.
Dominguez-Vidal A, de la Torre-Lopez MJ, Rubio-Domene R. In situ non-invasive Raman microspectroscopic investigation of polychrome plasterworks in the Alhambra. Analyst. 2012;137(24):5763–9.
Plesters J. A preliminary note on the incidence of discolouration of smalt in oil media. Stud Conserv. 1969;14(2):62–74.
Wyld M, Mills J, Plesters J. Some observations on blanching (with special reference to the paintings of Claude). Natl Gallery Tech Bull. 1980;4:49–63.
Leo RF. Minerals and the visual arts. J Geosci Educ. 2000;48(3):317–20.
Nassau K. The fifteen causes of color: the physics and chemistry of color. Color Res Appl. 1987;12(1):4–26.
Miliani C, Daveri A, Brunetti BG, Sgamellotti A. CO2 entrapment in natural ultramarine blue. Chem Phys Lett. 2008;466(4):148–51.
Del Federico E, Newman J, Tyne L, O’Hern C, Isolani L, Jerschouw A. Solid-state NMR studies of ultramarine pigments discloration. In: MRS Proceedings. vol 984. 2007. p. MM07-13.
Cato E, Borca C, Huthwelker T, Ferreira ES. Aluminium X-ray absorption near-edge spectroscopy analysis of discoloured ultramarine blue in 20th century oil paintings. Microchem J. 2016;126:18–24.
Dudzik Z, Preston KF. Paramagnetism and catalytic activity of sulfur-impregnated zeolites. J Colloid Interface Sci. 1968;26(3):374–8.
Keune K, Schnetz K, Gambardella A, Wallert A, Cotte M, van Elsas R. The degradation of ultramarine paints: a catalytic process? In: 4th international congress chemistry for cultural heritage. 2016. p. 20.
Ambers J. Raman analysis of pigments from the Egyptian Old Kingdom. J Raman Spectrosc. 2004;35(8–9):768–73.
Heywood A. The use of huntite as a white pigment in ancient Egypt. In: Colour and painting in ancient Egypt. London: British Museum Press; 2001. p. 5–9.
Gettens RJ, FitzHugh EW, Feller RL. Calcium carbonate whites. Stud Conserv. 1974;19(3):157–84.
Deneckere A, De Reu M, Martens MP, De Coene K, Vekemans B, Vincze L, De Maeyer P, Vandenabeele P, Moens L. The use of a multi-method approach to identify the pigments in the 12th century manuscript Liber Floridus. Spectrochim Acta Part A Mol Biomol Spectrosc. 2011;80(1):125–32.
Noble P, van Loon A. New insights into Rembrandt’s “Susanna”. ArtMatters Neth Tech Stud Art. 2005;2:76–96.
Spring M. Occurrences of the purple pigment fluorite on paintings in the national gallery. Natl Gallery Tech Bull. 2000;21:20–7.
Stege H. Typisch Deutsch? Material studies on German paintings from the 15th and 16th-century. In: 4th international congress chemistry for cultural heritage. Brussels; 2016. p. 33.
Thompson DV. The materials and techniques of medieval painting. New York: Dover Publications; 1956.
Duce C, Bramanti E, Ghezzi L, Bernazzani L, Bonaduce I, Colombini MP, Spepi A, Biagi S, Tine MR. Interactions between inorganic pigments and proteinaceous binders in reference paint reconstructions. Dalton Trans. 2013;42(17):5975–84.
Ford B, MacLeod I, Haydock P. Rock art pigments from Kimberley Region of Western Australia: identification of the minerals and conversion mechanisms. Stud Conserv. 1994;39(1):57–69.
Lepot L, Denoël S, Gilbert B. The technique of the mural paintings of the Tournai Cathedral. J Raman Spectrosc. 2006;37(10):1098–103.
Boon JJ. Processes inside paintings that affect the picture: chemical changes at, near and underneath the paint surface. In: Boon JJ, E Ferreira, editors. Reporting highlights of the De Mayerne Programme. 2006. p. 21–32.
Mazzeo R, Prati S, Quaranta M, Joseph E, Kendix E, Galeotti M. Attenuated total reflection micro FTIR characterisation of pigment–binder interaction in reconstructed paint films. Anal Bioanal Chem. 2008;392(1–2):65–76.
Salvadó N, Butí S, Nicholson J, Emerich H, Labrador A, Pradell T. Identification of reaction compounds in micrometric layers from gothic paintings using combined SR-XRD and SR-FTIR. Talanta. 2009;79(2):419–28.
Sotiropoulou S, Papliaka ZE, Vaccari L. Micro FTIR imaging for the investigation of deteriorated organic binders in wall painting stratigraphies of different techniques and periods. Microchem J. 2016;124:559–67.
Monico L, Rosi F, Miliani C, Daveri A, Brunetti BG. Non-invasive identification of metal-oxalate complexes on polychrome artwork surfaces by reflection mid-infrared spectroscopy. Spectrochim Acta Part A Mol Biomol Spectrosc. 2013;116:270–80.
Dominguez-Vidal A, Torre-López MJ, Campos-Suñol MJ, Rubio-Domene R, Ayora-Cañada MJ. Decorated plasterwork in the Alhambra investigated by Raman spectroscopy: comparative field and laboratory study. J Raman Spectrosc. 2014;45(11–12):1006–12.
Melo MJ, Araújo R, Castro R, Casanova C. Colour degradation in medieval manuscripts. Microchem J. 2016;124:837–44.
Zucchiatti A, Agulló-Lopez F. Potential consequences of ion beam analysis on objects from our cultural heritage: an appraisal. Nucl Instrum Methods Phys Res Sect B. 2012;278:106–14.
De Faria DL, Venâncio Silva S, De Oliveira MT. Raman microspectroscopy of some iron oxides and oxyhydroxides. J Raman Spectrosc. 1997;28(11):873–8.
Shebanova ON, Lazor P. Raman spectroscopic study of magnetite (FeFe2O4): a new assignment for the vibrational spectrum. J Solid State Chem. 2003;174(2):424–30.
Saunders D, Kirby J. The effect of relative humidity on artists’ pigments. Natl Gallery Tech Bull. 2004;25:62–72.
Ghezzi L, Duce C, Bernazzani L, Bramanti E, Colombini MP, Tiné MR, Bonaduce I. Interactions between inorganic pigments and rabbit skin glue in reference paint reconstructions. J Therm Anal Calorim. 2015;122(1):315–22.
Brommelle NS. The Russell and Abney report on the action of light on water colours. Stud Conserv. 1964;9(4):140–52.
Luxan MP, Dorrego F. Reactivity of earth and synthetic pigments with linseed oil. Surf Coat Int. 1999;82(8):390–402.
Sotiropoulou S, Daniilia S, Miliani C, Rosi F, Cartechini L, Papanikola-Bakirtzis D. Microanalytical investigation of degradation issues in Byzantine wall paintings. Appl Phys A Mater Sci Process. 2008;92(1):143–50.
Athanassiou A, Hill AE, Fourrier T, Burgio L, Clark RJ. The effects of UV laser light radiation on artists’ pigments. J Cult Herit. 2000;1:S209–13.
Zafiropulos V, Balas C, Manousaki A, Marakis Y, Maravelaki-Kalaitzaki P, Melesanaki K, Pouli P, Stratoudaki T, Klein S, Hildenhagen J, Dickmann K. Yellowing effect and discoloration of pigments: experimental and theoretical studies. J Cult Herit. 2003;4:249–56.
Walter D, Buxbaum G, Laqua W. The mechanism of the thermal transformation from goethite to hematite. J Therm Anal Calorim. 2001;63(3):733–48.
De Santis A, Mattei E, Pelosi C. Micro-Raman and stratigraphic studies of the paintings on the ‘Cembalo’model musical instrument (AD 1650) and laser-induced degradation of the detected pigments. J Raman Spectrosc. 2007;38(10):1368–78.
Maguregui M, Knuutinen U, Castro K, Madariaga JM. Raman spectroscopy as a tool to diagnose the impact and conservation state of Pompeian second and fourth style wall paintings exposed to diverse environments (House of Marcus Lucretius). J Raman Spectrosc. 2010;41(11):1400–9.
Feller R, editor. Artists’ pigments: a handbook of their history and characteristics. Vol. 1. Cambridge University Press, Cambridge, UK and National Gallery of Art, Washington D.C., USA; 1987. p. 300.
Genty-Vincent A, Eveno M, Nowik W, Bastian G, Ravaud E, Cabillic I, Uziel J, Lubin-Germain N, Menu M. Blanching of paint and varnish layers in easel paintings: contribution to the understanding of the alteration. Appl Phys A. 2015;121(3):779–88.
Čermáková Z, Hradilová J, Jehlička J, Osterrothová K, Massanek A, Bezdička P, Hradil D. Identification of vivianite in painted works of art and its significance for provenance and authorship studies. Archaeometry. 2014;56(S1):148–67.
van Loon A. Color changes and chemical reactivity in seventeenth-century oil paintings. Amsterdam: Universiteit van Amsterdam; 2008.
Wallert A, Hermens E, Peek M. Historical painting techniques, materials, and studio practice: preprints of a symposium, University of Leiden, the Netherlands, 26–29 June 1995. Los Angele: Getty Publications; 1995.
Richter M. The use of vivianite in Baroque and Rococo polychromy and painting. Historical Polychromy Polychrome Sculpture in Germany and Japan. (pp. 204-8). Munich: Hirmer Verlag; 2004.
Spring M. Studying the materials of Netherlandish paintings from the fifteenth and sixteenth centuries: current knowledge, approaches and interpretation. In: 4th international congress chemistry for cultural heritage. Brussels; 2016. p. 32.
Spring M, Keith L. Aelbert Cuyp’s Large Dort: colour change and conservation. Natl Gallery Tech Bull. 2009;30:71–85.
Pratesi G, Cipriani C, Giuli G, Birch WD. Santabarbaraite. Eur J Mineral. 2003;15(1):185–92.
Frost RL, Martens W, Williams PA, Kloprogge JT. Raman and infrared spectroscopic study of the vivianite-group phosphates vivianite, baricite and bobierrite. Mineral Mag. 2002;66(6):1063–74.
Frost RL, Weier ML, Martens W, Kloprogge JT, Ding Z. Dehydration of synthetic and natural vivianite. Thermochim Acta. 2003;401(2):121–30.
Čermáková Z, Švarcová S, Hradilová J, Bezdička P, Lančok A, Vašutová V, Blažek J, Hradil D. Temperature-related degradation and colour changes of historic paintings containing vivianite. Spectrochim Acta Part A Mol Biomol Spectrosc. 2015;140:101–10.
Mühlethaler B, Thissen J. Smalt. Stud Conserv. 1969;14(2):47–61.
Altavilla C, Ciliberto E. Decay characterization of glassy pigments: an XPS investigation of smalt paint layers. Appl Phys A. 2004;79(2):309–14.
Santopadre P, Verità M. A study of smalt and its conservation problems in two sixteenth-century wall paintings in Rome. Stud Conserv. 2006;51(1):29–40.
Boon JJ, Keune K, van der Weerd J, Geldof M, Van Asperen de Boer JR. Imaging microspectroscopic, secondary ion mass spectrometric and electron microscopic studies on discoloured and partially discoloured smalt in cross-sections of 16th century paintings. CHIMIA I J Chem. 2001;55(11):952–60.
Robinet L, Spring M, Pagès-Camagna S, Vantelon D, Trcera N. Investigation of the discoloration of smalt pigment in historic paintings by micro-X-ray absorption spectroscopy at the Co K-edge. Anal Chem. 2011;83(13):5145–52.
Gil M, Carvalho ML, Longelin S, Ribeiro I, Valadas S, Mirao J, Candeias AE. Blue pigment colors from wall painting churches in Danger (Portugal 15th to 18th century): identification, diagnosis, and color evaluation. Appl Spectrosc. 2011;65(7):782–9.
Giovanoli R, Mühlethaler B. Investigation of discoloured smalt. Stud Conserv. 1970;15(1):37–44.
Daniilia S, Minopoulou E. A study of smalt and red lead discolouration in Antiphonitis wall paintings in Cyprus. Appl Phys A Mater Sci Process. 2009;96(3):701–11.
Cianchetta I, Colantoni I, Talarico F, d’Acapito F, Trapananti A, Maurizio C, Fantacci S, Davoli I. Discoloration of the smalt pigment: experimental studies and ab initio calculations. J Anal At Spectrom. 2012;27(11):1941–8.
Robinet L, Spring M, Pagès-Camagna S. Vibrational spectroscopy correlated with elemental analysis for the investigation of smalt pigment and its alteration in paintings. Anal Methods. 2013;5(18):4628–38.
Spring M, Higgitt C, Saunders D. Investigation of pigment-medium interaction processes in oil paint containing degraded smalt. Natl Gallery Tech Bull. 2005;26:56–70.
Pottasch C, van Loon A, Haswell R. Analysing the influence of lead white on smalt degradation in three paintings by Pellegrini, using SEM-EDX quantitative mapping. In: 4th international congress chemistry for cultural heritage. 2016. p. 70.
Franceschi CM, Costa GA, Franceschi E. Aging of the paint palette of Valerio Castello (1624–1659) in different paintings of the same age (1650–1655). J Therm Anal Calorim. 2010;103(1):69–73.
Van de Voorde L, Van Pevenage J, De Langhe K, De Wolf R, Vekemans B, Vincze L, Vandenabeele P, Martens MP. Non-destructive in situ study of “Mad Meg” by Pieter Bruegel the Elder using mobile X-ray fluorescence, X-ray diffraction and Raman spectrometers. Spectrochim Acta Part B. 2014;97:1–6.
Panighello S, Kavčič A, Vogel-Mikuš K, Tennent NH, Wallert A, Hočevar SB, van Elteren JT. Investigation of smalt in cross-sections of 17th century paintings using elemental mapping by laser ablation ICP-MS. Microchem J. 2016;125:105–15.
Dik J, den Leeuw M, Verbakel W, Peschar P, Schillemans R, Schenk H. The digital reconstruction of a smalt discoloured painting by Hendrick Ter Brugghen. Zeitschrift filer Kunsttechnologie und Konservierung. 2002;16:130–46.
Odlyha M, Cohen NS, Foster GM. Dosimetry of paintings: determination of the degree of chemical change in museum exposed test paintings (smalt tempera) by thermal analysis. Thermochim Acta. 2000;365(1):35–44.
Doménech-Carbó MT, Edwards HG, Doménech-Carbó A, Hoyo-Meléndez JM, Cruz-Cañizares J. An authentication case study: antonio Palomino versus Vicente Guillo paintings in the vaulted ceiling of the Sant Joan del Mercat church (Valencia, Spain). J Raman Spectrosc. 2012;43(9):1250–9.
Eremin K, Stenger J, Huang JF, Aspuru-Guzik A, Betley T, Vogt L, Kassal I, Speakman S, Khandekar N. Examination of pigments on Thai manuscripts: the first identification of copper citrate. J Raman Spectrosc. 2008;39(8):1057–65.
Coccato A, Bersani D, Coudray A, Sanyova J, Moens L, Vandenabeele P. Raman spectroscopy of green minerals and reaction products with an application in cultural heritage research. J Raman Spectrosc. 2016;47(12):1429–43.
Bersani D, Lottici PP. Raman spectroscopy of minerals and mineral pigments in archaeometry. J Raman Spectrosc. 2016;47(5):499–530.
Robinet L. Corbeil-a2 MC. The characterization of metal soaps. Stud Conserv. 2003;48(1):23–40.
Tétreault J, Cano E, van Bommel M, Scott D, Dennis M, Barthés-Labrousse MG, Minel L, Robbiola L. Corrosion of copper and lead by formaldehyde, formic and acetic acid vapours. Stud Conserv. 2003;48(4):237–50.
De Laet N, Lycke S, Van Pevenage J, Moens L, Vandenabeele P. Investigation of pigment degradation due to acetic acid vapours: Raman spectroscopic analysis. Eur J Mineral. 2013;25(5):855–62.
Anselmi C, Buti D, Cartechini L, Miliani C. Evidence and characterization of chemical changes in oil paints containing copper-based green pigments by FTIR spectroscopy. In: 4th international congress chemistry for cultural heritage. Brussels; 2016. p. 21.
Gettens RJ, Fitzhugh EWI. Azurite and blue verditer. Stud Conserv. 1966;11(2):54–61.
Odlyha M, Cohen NS, Foster GM, West RH. Dosimetry of paintings: determination of the degree of chemical change in museum exposed test paintings (azurite tempera) by thermal and spectroscopic analysis. Thermochim Acta. 2000;365(1):53–63.
Taft W, Mayer J. The science of paintings. New York: Springer Science & Business Media; 2001. p. 236.
Mattei E, De Vivo G, De Santis A, Gaetani C, Pelosi C, Santamaria U. Raman spectroscopic analysis of azurite blackening. J Raman Spectrosc. 2008;39(2):302–6.
Dei L, Ahle A, Baglioni P, Dini D, Ferroni E. Green degradation products of azurite in wall paintings: identification and conservation treatment. Stud Conserv. 1998;43(2):80–8.
Lluveras A, Boularand S, Andreotti A, Vendrell-Saz M. Degradation of azurite in mural paintings: distribution of copper carbonate, chlorides and oxalates by SRFTIR. Appl Phys A Mater Sci Process. 2010;99(2):363–75.
Cavallo G. Alteration of azurite into paratacamite at the St. Alessandro church (Lasnigo, Italy). Conservar património. 2009;9:5–11.
Gettens RJ, Stout GL. A monument of Byzantine wall painting—the method of construction. Stud Conserv. 1958;3(3):107–19.
Švarcová S, Hradil D, Hradilová J, Kočí E, Bezdička P. Micro-analytical evidence of origin and degradation of copper pigments found in Bohemian Gothic murals. Anal Bioanal Chem. 2009;395(7):2037–50.
Pereira-Pardo L, Gil M, Prieto B, Silva B. Multi-analytical approach to the material characterization of 16th century wall paintings from Ribeira Sacra (Galicia, NW Spain): pictorial palette, technique and alterations. Color Res Appl. 2016;41(3):263–9.
Vandenabeele P, Lambert K, Matthys S, Schudel W, Bergmans A, Moens L. In situ analysis of mediaeval wall paintings: a challenge for mobile Raman spectroscopy. Anal Bioanal Chem. 2005;383(4):707–12.
Špec T, Retko K, Ropret P, Meden A, Bernard J. The influence of UV–Vis radiation, and oscillations of temperature and relative humidity, on malachite alteration in the presence of different organic binders and varnishes. J Raman Spectrosc. 2014;45(11–12):1068–75.
Kotulanová E, Schweigstillova J, Svarcova S, Hradil D, Bezdička P, Grygar T. Wall painting damage by salts: causes and mechanisms. Acta Res Rep. 2009;18:27–31.
Smith GD, Clark RJ. The role of H2S in pigment blackening. J Cult Herit. 2002;3(2):101–5.
Frost RL, Ding Z, Kloprogge JT, Martens WN. Thermal stability of azurite and malachite in relation to the formation of mediaeval glass and glazes. Thermochim Acta. 2002;390(1):133–44.
Gettens RJ, Fitzhugh EW. Malachite and green verditer. Stud Conserv. 1974;19(1):2–3.
Castro K, Sarmiento A, Martínez-Arkarazo I, Madariaga JM, Fernández LA. Green copper pigments biodegradation in cultural heritage: from malachite to moolooite, thermodynamic modeling, X-ray fluorescence, and Raman evidence. Anal Chem. 2008;80(11):4103–10.
Scott DA, Dennis M, Khandekar N, Keeney J, Carson D, Dodd LS. An Egyptian cartonnage of the Graeco-Roman period. Stud Conserv. 2003;48(1):41–56.
Pérez-Alonso M, Castro K, Madariaga JM. Investigation of degradation mechanisms by portable Raman spectroscopy and thermodynamic speciation: the wall painting of Santa María de Lemoniz (Basque Country, North of Spain). Anal Chim Acta. 2006;571(1):121–8.
Bomford D, Kirby J, Giovanni di Paolo’s’ SS. Fabian and Sebastian’. Natl Gallery Tech Bull. 1978;2:56–65.
Wyld M, Plesters J. Some panels from Sassetta’s Sansepolcro altarpiece. Natl Gallery Tech Bull. 1977;1:3–17.
Braham A, Wyld M, Plesters J. Bellini’s the blood of the redeemer. Natl Gallery Tech Bull. 1978;2:11–24.
Gunn M, Chottard G, Rivière E, Girerd JJ, Chottard JC. Chemical reactions between copper pigments and oleoresinous media. Stud Conserv. 2002;47(1):12–23.
Ioakimoglou E, Boyatzis S, Argitis P, Fostiridou A, Papapanagiotou K, Yannovits N. Thin-film study on the oxidation of linseed oil in the presence of selected copper pigments. Chem Mater. 1999;11(8):2013–22.
Nevin A, Melia JL, Osticioli I, Gautier G, Colombini MP. The identification of copper oxalates in a 16th century Cypriot exterior wall painting using micro FTIR, micro Raman spectroscopy and Gas Chromatography-Mass Spectrometry. J Cult Herit. 2008;9(2):154–61.
Gibson LT, Watt CM. Acetic and formic acids emitted from wood samples and their effect on selected materials in museum environments. Corros Sci. 2010;52(1):172–8.
Padfield T, Erhardt D, Hopwood W. Trouble in store. Stud Conserv. 1982;27(sup1):24–7.
Laine C. Structures of hemicelluloses and pectins in wood and pulp. Helsinki University of Technology; 2005.
Weeks C. The ‘Portail de la Mère Dieu’of Amiens Cathedral: its polychromy and conservation. Stud Conserv. 1998;43(2):101–8.
Valadas S, Candeias A, Dias C, Schiavon N, Cotovio M, Pestana J, Gil M, Mirão J. A multi-analytical study of the fifteenth century mural paintings of the Batalha Monastery (Portugal) in view of their conservation. Appl Phys A. 2013;113(4):989–98.
Oujja M, Sanz M, Rebollar E, Marco JF, Domingo C, Pouli P, Kogou S, Fotakis C, Castillejo M. Wavelength and pulse duration effects on laser induced changes on raw pigments used in paintings. Spectrochim Acta Part A Mol Biomol Spectrosc. 2013;102:7–14.
Chappé M, Hildenhagen J, Dickmann K, Bredol M. Laser irradiation of medieval pigments at IR, VIS and UV wavelengths. J Cult Herit. 2003;4:264–70.
Chaplin TD, Clark RJ, Scott DA. Study by Raman microscopy of nine variants of the green–blue pigment verdigris. J Raman Spectrosc. 2006;37(1–3):223–9.
Woudhuysen-Keller R. Aspects of painting technique in the use of verdigris and copper resinate. In: Wallert A, Hermens E, Peek M, editors. Historical painting techniques, materials, and studio practice: preprints of a symposium. University of Leiden, the Netherlands, 26–29 June 1995. Los Angeles: Getty Publications; 1995. p. 9–11.
Dawson TL. Examination, conservation and restoration of painted art. Color Technol. 2007;123(5):281–92.
Santoro C, Zarkout K, Le Hô AS, Mirambet F, Gourier D, Binet L, Pagès-Camagna S, Reguer S, Mirabaud S, Le Du Y, Griesmar P. New highlights on degradation process of verdigris from easel paintings. Appl Phys A. 2014;114(3):637–45.
Cartechini L, Miliani C, Brunetti BG, Sgamellotti A, Altavilla C, Ciliberto E, D’acapito F. X-ray absorption investigations of copper resinate blackening in a XV century Italian painting. Appl Phys A Mater Sci Process. 2008;92(1):243–50.
Boyatzis S, Ioakimoglou E, Argitis P. UV exposure and temperature effects on curing mechanisms in thin linseed oil films: spectroscopic and chromatographic studies. J Appl Polym Sci. 2002;84(5):936–49.
Pouli P, Emmony DC. The effect of Nd: YAG laser radiation on medieval pigments. J Cult Herit. 2000;1(1):S181–8.
Pouli P, Emmony DC, Madden CE, Sutherland I. Analysis of the laser-induced reduction mechanisms of medieval pigments. Appl Surf Sci. 2001;173(3):252–61.
Calligaro T, Gonzalez V, Pichon L. PIXE analysis of historical paintings: is the gain worth the risk? Nucl Instrum Methods Phys Res Sect B. 2015;363:135–43.
Colinart S, Coupry C. Fabrication processes of archaeological Egyptian blue and green pigments enlightened by Raman microscopy and scanning electron microscopy. J Raman Spectrosc. 1999;30(4):313–7.
Bianchetti P, Talarico F, Vigliano MG, Ali MF. Production and characterization of Egyptian blue and Egyptian green frit. J Cult Herit. 2000;1(2):179–88.
Colinart S. The Egyptian green pigment: its manufacturing process and links to Egyptian blue. Archaeometry. 2003;45(4):637–58.
Daniels V, Stacey R, Middleton A. The blackening of paint containing Egyptian blue. Stud Conserv. 2004;49(4):217–30.
Schiegl S, Weiner KL, El Goresy A. Discovery of copper chloride cancer in ancient Egyptian polychromic wall paintings and faience: a developing archaeological disaster. Sci Nat. 1989;76(9):393–400.
Perez-Rodriguez JL, de Haro MD, Siguenza B, Martinez-Blanes JM. Green pigments of Roman mural paintings from Seville Alcazar. Appl Clay Sci. 2015;116:211–9.
Lau D, Kappen P, Strohschnieder M, Brack N, Pigram PJ. Characterization of green copper phase pigments in Egyptian artifacts with X-ray absorption spectroscopy and principal components analysis. Spectrochim Acta Part B. 2008;63(11):1283–9.
Davies WV. Colour and painting in ancient Egypt. London: British Museum Press; 2001.
Bertolotti G, Bersani D, Lottici PP, Alesiani M, Malcherek T, Schlüter J. Micro-Raman study of copper hydroxychlorides and other corrosion products of bronze samples mimicking archaeological coins. Anal Bioanal Chem. 2012;402(4):1451–7.
Chaplin TD, Clark RJ, Martinón-Torres M. A combined Raman microscopy, XRF and SEM–EDX study of three valuable objects—a large painted leather screen and two illuminated title pages in 17th century books of ordinances of the Worshipful Company of Barbers, London. J Mol Struct. 2010;976(1):350–9.
Naumova MM, Pisareva SA. A note on the use of blue and green copper compounds in paintings. Stud Conserv. 1994;39(4):277–83.
Tomasini EP, Landa CR, Siracusano G, Maier MS. Atacamite as a natural pigment in a South American colonial polychrome sculpture from the late XVI century. J Raman Spectrosc. 2013;44(4):637–42.
Yong LE. Copper trihydroxychlorides as pigments in China. Stud Conserv. 2012;57(2):106–11.
Frost RL, Martens W, Kloprogge JT, Williams PA. Raman spectroscopy of the basic copper chloride minerals atacamite and paratacamite: implications for the study of copper, brass and bronze objects of archaeological significance. J Raman Spectrosc. 2002;33(10):801–6.
Pérez-Rodrı́guez JL, Maqueda C, De Haro MJ, Rodrıguez-Rubio P. Effect of pollution on polychromed ceramic statues. Atmos Environ. 1998;32(6):993-8.
Pollard AM, Thomas RG, Williams PA. Synthesis and stabilities of the basic copper (II) chlorides atacamite, paratacamite and botallackite. Mineral Mag. 1989;53(373):557–63.
Castro K, Pérez-Alonso M, Rodríguez-Laso MD, Etxebarria N, Madariaga JM. Non-invasive and non-destructive micro-XRF and micro-Raman analysis of a decorative wallpaper from the beginning of the 19th century. Anal Bioanal Chem. 2007;387(3):847–60.
Naumova MM, Pisareva SA, Nechiporenko GO. Green copper pigments of old Russian frescoes. Stud Conserv. 1990;35(2):81–8.
Schafer EH. Orpiment and realgar in Chinese technology and tradition. J Am Orient Soc. 1955;75(2):73–89.
Corbeil MC, Helwig K. An occurrence of pararealgar as an original or altered artists’ pigment. Stud Conserv. 1995;40(2):133–8.
Burgio L, Clark RJ. Comparative pigment analysis of six modern Egyptian papyri and an authentic one of the 13th century BC by Raman microscopy and other techniques. J Raman Spectrosc. 2000;31(5):395–401.
Rosalie David A, Edwards HG, Farwell DW, De Faria DL. Raman spectroscopic analysis of ancient Egyptian pigments. Archaeometry. 2001;43(4):461–73.
Daniels V, Leach B. The occurrence and alteration of realgar on Ancient Egyptian papyri. Stud Conserv. 2004;49(2):73–84.
Grundmann G, Ivleva N, Richter M, Stege H, Haisch C. The rediscovery of sublimed arsenic sulphide pigments in painting and polychromy: applications of Raman microspectroscopy. In: Studying old master paintings: technology and practice. The National Gallery Technical Bulletin 30th Anniversary Conference Postprints. 2011. p. 269–76.
FitzHugh E, editor. Artists’ Pigments: a handbook of their history and characteristics. Vol 3. National Gallery of Art, Washington D.C., USA and Oxford University Press, Oxford, UK; 1993. p. 364.
Trentelman K, Stodulski L, Pavlosky M. Characterization of pararealgar and other light-induced transformation products from realgar by Raman microspectroscopy. Anal Chem. 1996;68(10):1755–61.
Nastova I, Grupče O, Minčeva-Šukarova B, Ozcatal M, Mojsoska L. Spectroscopic analysis of pigments and inks in manuscripts: I. Byzantine and post-Byzantine manuscripts (10–18th century). Vib Spectrosc. 2013;68:11–9.
Nastova I, Grupče O, Minčeva-Šukarova B, Turan S, Yaygingol M, Ozcatal M, Martinovska V, Jakovlevska-Spirovska Z. Micro-Raman spectroscopic analysis of inks and pigments in illuminated medieval old-Slavonic manuscripts. J Raman Spectrosc. 2012;43(11):1729–36.
Keune K, Mass J, Mehta A, Church J, Meirer F. Analytical imaging studies of the migration of degraded orpiment, realgar, and emerald green pigments in historic paintings and related conservation issues. Herit Sci. 2016;4(1):10.
Muniz-Miranda M, Sbrana G, Bonazzi P, Menchetti S, Pratesi G. Spectroscopic investigation and normal mode analysis of As4S4 polymorphs. Spectrochim Acta Part A Mol Biomol Spectrosc. 1996;52(11):1391–401.
Correia AM, Clark RJ, Ribeiro MI, Duarte ML. Pigment study by Raman microscopy of 23 paintings by the Portuguese artist Henrique Pousão (1859–1884). J Raman Spectrosc. 2007;38(11):1390–405.
Keune K, Mass J, Meirer F, Pottasch C, van Loon A, Hull A, Church J, Pouyet E, Cotte M, Mehta A. Tracking the transformation and transport of arsenic sulfide pigments in paints: synchrotron-based X-ray micro-analyses. J Anal At Spectrom. 2015;30(3):813–27.
Vermeulen M, Sanyova J, Nuyts G, De Wael K, Janssens K. Influence of the binding medium on arsenic sulfide pigments’ degradation investigated by spectroscopic and electrochemical techniques. In: 4th international congress chemistry for cultural heritage. Brussels; 2016. p. 22.
Cutts H, Harrison L, Higgitt C, Cruickshank P. The image revealed: study and conservation of a mid-nineteenth century Ethiopian church painting. Br Mus Tech Res Bull. 2010;4:1–7.
Hradil D, Hradilová J, Bezdička P, Švarcová S, Čermáková Z, Košařová V, Němec I. Crocoite PbCrO4 and mimetite Pb5(AsO4)3Cl: rare minerals in highly degraded mediaeval murals in Northern Bohemia. J Raman Spectrosc. 2014;45(9):848–58.
Roberts AC, Ansell HG, Bonardi M. Pararealgar, a new polymorph of AsS, from British Columbia. Can Mineral. 1980;18(4):525–7.
Street GB, Munir ZA. The structure and thermal properties of synthetic realgar (As4S4). J Inorg Nucl Chem. 1970;32(12):3769–74.
Wmc G. The light-induced alteration of realgar to pararealgar. Am Mineral. 1992;77:1266–74.
Clark RH, Gibbs P. Identification of lead (II) sulfide and pararealgar on a 13th century manuscript by Raman microscopy. Chem Commun. 1997;11:1003–4.
Clark RJ, Gibbs PJ. Peer reviewed: raman microscopy of a 13th-century illuminated text. Anal Chem. 1998;70(3):99A–104A.
Vandenabeele P, Von Bohlen A, Moens L, Klockenkämper R, Joukes F, Dewispelaere G. Spectroscopic examination of two Egyptian masks: a combined method approach. Anal Lett. 2000;33(15):3315–32.
Gettens RJ, Feller RL, Chase WT. Vermilion and cinnabar. Stud Conserv. 1972;17(2):45–69.
Feller RL. Studies on the darkening of vermilion by light. Rep Stud Hist Art. 1967;1:99–111.
Pliny the Elder. Natural history. Loeb classical library. Cambridge: Harvard University Press; 1938.
Rowland I, Howe T, editors. Vitruvius: ten books on architecture. New York: Cambridge University Press; 2001.
McCormack JK. The darkening of cinnabar in sunlight. Miner Deposita. 2000;35(8):796–8.
Nöller R. Cinnabar reviewed: characterization of the red pigment and its reactions. Stud Conserv. 2015;60(2):79–87.
Spring M, Grout R. The blackening of vermilion: an analytical study of the process in paintings. Natl Gallery Tech Bull. 2002;23:50–61.
Gutman M, Lesar-Kikelj M, Mladenovič A, Čobal-Sedmak V, Križnar A, Kramar S. Raman microspectroscopic analysis of pigments of the Gothic wall painting from the Dominican Monastery in Ptuj (Slovenia). J Raman Spectrosc. 2014;45(11–12):1103–9.
Gil M, Rosado T, Ribeiro I, Pestana JA, Caldeira AT, Carvalho ML, Dias L, Mirão J, Candeias A. Are they fresco paintings? Technical and material study of Casas Pintadas of Vasco da Gama house in Évora (Southern Portugal). X Ray Spectrom. 2015;44(3):154–62.
Cotte M, Susini J, Metrich N, Moscato A, Gratziu C, Bertagnini A, Pagano M. Blackening of Pompeian cinnabar paintings: X-ray microspectroscopy analysis. Anal Chem. 2006;78(21):7484–92.
Cotte M, Susini J, Solé VA, Taniguchi Y, Chillida J, Checroun E, Walter P. Applications of synchrotron-based micro-imaging techniques to the chemical analysis of ancient paintings. J Anal At Spectrom. 2008;23(6):820–8.
Burgio L, Clark RJ, Gibbs PJ. Pigment identification studies in situ of Javanese, Thai, Korean, Chinese and Uighur manuscripts by Raman microscopy. J Raman Spectrosc. 1999;30(3):181–4.
Duce C, Ghezzi L, Onor M, Bonaduce I, Colombini MP, Tine MR, Bramanti E. Physico-chemical characterization of protein–pigment interactions in tempera paint reconstructions: casein/cinnabar and albumin/cinnabar. Anal Bioanal Chem. 2012;402(6):2183–93.
Oujja M, Castillejo M, Pouli P, Fotakis C, Domingo C. Spectral analysis of the effects of laser wavelength and pulse duration on tempera paints. In: Radvan et al., editors. Lasers in the conservation of artworks VIII. London: Taylor & Francis Group; 2011. p. 15–22.
Keune K, Boon JJ. Analytical imaging studies clarifying the process of the darkening of vermilion in paintings. Anal Chem. 2005;77(15):4742–50.
Hogan C, Da Pieve F. Colour degradation of artworks: an ab initio approach to X-ray, electronic and optical spectroscopy analyses of vermilion photodarkening. J Anal At Spectrom. 2015;30(3):588–98.
Radepont M, Coquinot Y, Janssens K, Ezrati JJ, de Nolf W, Cotte M. Thermodynamic and experimental study of the degradation of the red pigment mercury sulfide. J Anal At Spectrom. 2015;30(3):599–612.
Castillejo M, Martín M, Oujja M, Santamaría J, Silva D, Torres R, Manousaki A, Zafiropulos V, van den Brink OF, Heeren RM, Teule R. Evaluation of the chemical and physical changes induced by KrF laser irradiation of tempera paints. J Cult Herit. 2003;4:257–63.
Petushkova JP, Lyalikova NN. Microbiological degradation of lead-containing pigments in mural paintings. Stud Conserv. 1986;31(2):65–9.
Koller M, Leitner H, Paschinger H. Reconversion of altered lead pigments in alpine mural paintings. Stud Conserv. 1990;35(1):15–20.
Giovannoni S, Matteini M, Moles A. Studies and developments concerning the problem of altered lead pigments in wall painting. Stud Conserv. 1990;35(1):21–5.
Barone G, Bersani D, Coccato A, Lauwers D, Mazzoleni P, Raneri S, Vandenabeele P, Manzini D, Agostino G, Neri NF. Nondestructive Raman investigation on wall paintings at Sala Vaccarini in Catania (Sicily). Appl Phys A. 2016;122(9):838.
Burgio L, Clark RJ, Firth S. Raman spectroscopy as a means for the identification of plattnerite (PbO2), of lead pigments and of their degradation products. Analyst. 2001;126(2):222–7.
Batonneau Y, Brémard C, Laureyns J, Merlin JC. Microscopic and imaging Raman scattering study of PbS and its photo-oxidation products. J Raman Spectrosc. 2000;31(12):1113–9.
Rosado T, Gil M, Mirão J, Candeias A, Caldeira AT. Darkening on lead-based pigments: microbiological contribution. Color Res Appl. 2015;41(3):294–8.
Kotulanová E, Bezdička P, Hradil D, Hradilová J, Švarcová S, Grygar T. Degradation of lead-based pigments by salt solutions. J Cult Herit. 2009;10(3):367–78.
Tumosa CS, Mecklenburg MF. The influence of lead ions on the drying of oils. Stud Conserv. 2005;50(sup1):39–47.
Cotte M, Checroun E, Susini J, Walter P. Micro-analytical study of interactions between oil and lead compounds in paintings. Appl Phys A Mater Sci Process. 2007;89(4):841–8.
Higgitt C, Spring M, Saunders D. Pigment-medium interactions in oil paint films containing red lead or lead–tin yellow. Natl Gallery Tech Bull. 2003;24:75–95.
Keune K, Boon JJ. Imaging secondary ion mass spectrometry of a paint cross section taken from an early Netherlandish painting by Rogier van der Weyden. Anal Chem. 2004;76(5):1374–85.
Plater MJ, De Silva B, Gelbrich T, Hursthouse MB, Higgitt CL, Saunders DR. The characterisation of lead fatty acid soaps in ‘protrusions’ in aged traditional oil paint. Polyhedron. 2003;22(24):3171–9.
Keune K, Boon JJ. Analytical imaging studies of cross-sections of paintings affected by lead soap aggregate formation. Stud Conserv. 2007;52(3):161–76.
Catalano J, Yao Y, Murphy A, Zumbulyadis N, Centeno SA, Dybowski C. Analysis of lead carboxylates and lead-containing pigments in oil paintings by solid-state nuclear magnetic resonance. In: MRS proceedings, vol. 1656. Cambridge: Cambridge University Press; 2015. p. mrsf13-1656.
Keune K, van Loon A, Boon JJ. SEM backscattered-electron images of paint cross sections as information source for the presence of the lead white pigment and lead-related degradation and migration phenomena in oil paintings. Microsc Microanal. 2011;17(05):696–701.
Erhardt D, Tumosa CS, Mecklenburg MF. Long-term chemical and physical processes in oil paint films. Stud Conserv. 2005;50(2):143–50.
Vanmeert F, Van der Snickt G, Janssens K. Plumbonacrite identified by X-ray powder diffraction tomography as a missing link during degradation of red lead in a Van Gogh painting. Angew Chem Int Ed. 2015;54(12):3607–10.
Gettens RJ, Kühn H, Chase WT. Lead white. Stud Conserv. 1967;12(4):125–39.
Cohen NS, Odlyha M, Campana R, Foster GM. Dosimetry of paintings: determination of the degree of chemical change in museum exposed test paintings (lead white tempera) by thermal analysis and infrared spectroscopy. Thermochim Acta. 2000;365(1):45–52.
Clark RJ. Pigment identification by spectroscopic means: an arts/science interface. C R Chim. 2002;5(1):7–20.
Andalo C, Bicchieri M, Bocchini P, Casu G, Galletti GC, Mando PA, Nardone M, Sodo A, Zappalà MP. The beautiful “Trionfo d’Amore” attributed to Botticelli: a chemical characterisation by proton-induced X-ray emission and micro-Raman spectroscopy. Anal Chim Acta. 2001;429(2):279–86.
Smith GD, Burgio L, Firth S, Clark RJ. Laser-induced degradation of lead pigments with reference to Botticelli’s Trionfo d’Amore. Anal Chim Acta. 2001;440(2):185–8.
Cooper MI, Fowles PS, Tang CC. Analysis of the laser-induced discoloration of lead white pigment. Appl Surf Sci. 2002;201(1):75–84.
Raimondi V, Andreotti A, Colombini MP, Cucci C, Cuzman O, Galeotti M, Lognoli D, Palombi L, Picollo M, Tiano P. Test measurements on a secco white-lead containing model samples to assess the effects of exposure to low-fluence UV laser radiation. Appl Surf Sci. 2015;337:45–57.
Sotiropoulou S, Perdikatsis V, Apostolaki C, Karydas AG, Devetzi A, Birtacha K. Lead pigments and related tools at Akrotiri, Thera, Greece. provenance and application techniques. J Archaeol Sci. 2010;37(8):1830–40.
Aze S, Vallet JM, Baronnet A, Grauby O. The fading of red lead pigment in wall paintings: tracking the physico-chemical transformations by means of complementary micro-analysis techniques. Eur J Mineral. 2006;18(6):835–43.
Aze S, Vallet JM, Detalle V, Grauby O, Baronnet A. Chromatic alterations of red lead pigments in artworks: a review. Phase Transit. 2008;81(2–3):145–54.
Qingping F, Xiaojun Z, Xiaojun M. Effects of microbes on color changes of red lead in murals. J Gen Appl Microbiol. 1999;45(2):85–8.
Gettens R, Stout G. Painting materials: a short encyclopaedia. New York: Dover Publications, Inc.; 1966. p. 500.
Miguel C, Claro A, Gonçalves AP, Muralha VS, Melo MJ. A study on red lead degradation in a medieval manuscript Lorvão Apocalypse (1189). J Raman Spectrosc. 2009;40(12):1966–73.
Aze S, Vallet JM, Pomey M, Baronnet A, Grauby O. Red lead darkening in wall paintings: natural ageing of experimental wall paintings versus artificial ageing tests. Eur J Mineral. 2007;19(6):883–90.
Daniilia S, Sotiropoulou S, Bikiaris D, Salpistis C, Karagiannis G, Chryssoulakis Y, Price BA, Carlson JH. Panselinos’ Byzantine wall paintings in the Protaton Church, Mount Athos, Greece: a technical examination. J Cult Herit. 2000;1(2):91–110.
Ayalew E, Janssens K, De Wael K. Unraveling the reactivity of minium toward bicarbonate and the role of lead oxides therein. Anal Chem. 2016;88(3):1564–9.
Benquerença MJ, Mendes NF, Castellucci E, Gaspar VM, Gil FP. Micro-Raman spectroscopy analysis of 16th century Portuguese Ferreirim Masters oil paintings. J Raman Spectrosc. 2009;40(12):2135–43.
Kühn H. Lead–tin Yellow. Stud Conserv. 1968;13(1):7–33.
Mecklenburg MF, Tumosa CS, Vicenzi EP. The influence of pigment and ion migration on the durability of drying oil and alkyd paints. In: New insight into the cleaning of paintings. Proceedings from the cleaning 2010 international conference Universidad Politécnica de Valencia and Museum Conservation Institute. Washington, DC: Smithsonian Institution Scholarly Press; 2013. p. 59–67.
AC performed the literature review, and compiled the text of the manuscript. LM and PV supervised the work, provided feedback on the manuscript and general guidelines for the redaction of the review. All authors read and approved the final manuscript.
The authors wish to acknowledge the GOA funds of Ghent University and the MEMORI project for their financial support and for the interesting discussions with the colleagues. The MEMORI, ‘Measurement, Effect Assessment and Mitigation of Pollutant Impact on Movable Cultural Assets. Innovative Research for Market Transfer’ project is supported through the 7th Framework Programme of the European Commission (http://www.memori-project.eu/memori.html).
The authors declare that they have no competing interests.
Availability of data and materials
This review is based on published literature, therefore no datasets/databases exist.
The MEMORI project supported the first phases of literature review, in the framework of the understanding of cultural heritage materials’ sensitivity for the development and implementation of the MEMORI-dosimeter for use in museums.
The GOA funds of Ghent University allowed the finalization of the literature review into a manuscript for submission, as well as its presentation at the 4th International Congress Chemistry for Cultural Heritage in Brussels, July 2016.
Rights and permissions
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
About this article
Cite this article
Coccato, A., Moens, L. & Vandenabeele, P. On the stability of mediaeval inorganic pigments: a literature review of the effect of climate, material selection, biological activity, analysis and conservation treatments. Herit Sci 5, 12 (2017). https://doi.org/10.1186/s40494-017-0125-6
- Copper pigments
- Arsenic sulphide pigments
- Lead pigments
- MEMORI project