Table 1 Summary of the observed discolourations on traditional inorganic mediaeval pigments
Colour | Pigment | Climate effect | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
High RH | Fluctuating RH | RH + chlorides (Cl−) | RH + pollutants (SO2, SO3, NOx) | RH + salts | Oxidizing agents | Alkali | Acids | Hydrogen sulphide (H2S) | ||
White | Calcium carbonates CaCO3 | Swelling of paint layers, mechanical stress | Formation of gypsum, mechanical stress (increased volume), lixiviation of soluble material [20, 45, 46] | – [45] | ||||||
White | Lead white 2PbCO3·Pb(OH)2 | Blackening on wall paintings, watercolours and manuscripts [20, 60, 231]. Synergistic effects with the other parameters | Formation of various salts, sometimes not white [56, 123, 218]. Cerussite is the most stable | Blackening | Blackening [213] | Soluble [20, 98, 229]. Formation of oxalates [32, 56] and acetates [109] upon exposure to oxalic and acetic acid respectively | Blackening (especially manuscripts and watercolours, [9, 20, 60, 98, 190, 191, 200, 204, 211–213, 231, 232]). Formed PbS can be convered to PbSO4 [139, 212], and finally cerussite/hydrocerussite again | |||
Yellow | Orpiment As 2S3 | Dissolution. Formation and transport of As5+ ions [180, 183]. Whitening (formation of As2O3) in presence of light [64] and of oxidizing conditions [9, 176] | Soluble | |||||||
Yellow | Massicot PbO | Initial darkening, then degradation to cerussite/hydrocerussite [64] | Initial darkening, then degradation to cerussite/hydrocerussite [123, 218] | Formation of black PbS [124] | ||||||
Yellow | Lead tin yellow Type I: Pb 2SnO4; Type II: PbSn1−xSixO3 | – [64] | – [246] | Formation of PbS [246] | ||||||
Yellow, red, brown | Yellow, red, brown ochres Fe 2O3, FeOOH, Fe 3O4; MnxOy | Hydration of oxides [red to yellow, 53] | Hydration of oxides, formation of hydrated sulphates [73] | – [1] | –, formation of Ca oxalates from the other minerals in the ochre [32, 67] | |||||
Orange–red/yellow | Realgar/pararealgar As 4S4 | Formation and transport of As5+ ions. Whitening (formation of As2O3) [180, 183] | ||||||||
Orange | Litharge PbO | Degradation to cerussite/hydrocerussite, sulphate and phosphates [218] | Degradation to cerussite/hydrocerussite, sulphate and phosphates [218, 236] | |||||||
Red | Vermillion HgS | Unstable. Synergy with photodegradation and oxidation [64, 66] | Involved in discolouration and sulphation [106, 197, 199, 202, 203, 207–209] | Unstable [103, 123, 200], but still used in wall paintings [73, 193, 201–203] | ||||||
Red | Red lead Pb 3O4 | Discolouration, as lead salts are most of the time white/whitish [68, 93, 123, 200, 218, 228, 237, 238, 244] | Unstable [1, 120]. Formation of black PbO2 and PbSO4 in a sulphation process [237, 242] | Soluble [1, 240]. Formation of PbO2 [242], PbCO3 [55, 67, 109] | Sensitive: formation of PbS [1, 60, 93, 124, 238, 241] or PbSO4 [59, 68, 233, 242] | |||||
Green | Green earths glauconite (K,Na)(Fe 3+,Al,Mg)2 (Si,Al)4O10(OH)2) Celadonite (K[(Al,Fe 3+),(Fe 2+,Mg)] (AlSi3,Si4)O10(OH)2 | Discolouration of oil layers [75] | Soluble [74] | Soluble [74] | ||||||
Green | Malachite CuCO3·Cu(OH)2 | Recrystallization [111] | Recrystallization [111] | Reaction without colour change [103, 119, 121, 123, 126–128] | Reaction without colour change [53, 103, 119, 121, 123, 126–129] | Discolouration [black, 126] | Bluish discolouration on pure pigment; selective attack on other pigments in the mixture if H2S is of biological origin [124, 126, 139] | |||
Green | Verdigris xCu(CH3COO2)·yCu(OH)2·zH2O | Browning [peroxide species, 146] | Formation of blue copper hydroxides [20] | Formation of oxalates [32] | ||||||
Green | Copper resinate copper salts of abietic acid | Darkening [132] | ||||||||
Green | Cu chlorides Cu 2Cl(OH)3 | Formation of Cu salts [167] | Formation of (unstable) blue Cu(OH)2 [115] | Oxalates [127] | ||||||
Green | Cu sulphates CuSO4·yCu(OH)2·zH2O | Oxalates [127] | ||||||||
Blue | Ultramarine Na8[Al6Si6O24]Sn | Discolouration [grey, 9] | Stable [20, 30, 31]. Discolouration (grey, [31, 35, 36, 38, 39]) | |||||||
Blue | Vivianite Fe 3(PO4)2·8H2O | Discolouration to green, and finally to yellow [1, 76, 82, 83] | ||||||||
Blue | Smalt CoO·nSiO2 | Glass alteration, ions leaching and discolouration [89, 91, 96] | In tempera: discolouration (dosimetry, 102). No synergy of SO2 and NOx [87] | |||||||
Blue | Azurite 2CuCO3·Cu(OH)2 | – | – | Discolouration:green [91, 114, 119], black [20, 35, 64, 68, 91, 103, 111, 113, 114, 121–123] | Bluish discolouration on pure pigment; selective attack on other pigments in the mixture if H2S is of biological origin [111, 124] | |||||
Blue | Egyptian blue CaCuSi4O10 | – [28] | – [28] | |||||||
Black | Carbon black C | Alteration, especially if impurities are present [12, 15, 16] | Alteration, especially if impurities are present [12, 15, 16] | |||||||
Colour | Pigment | Biological activity, material selection | Light, heat, chemical analyses | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
Biological attack | Binders | Other pigments/additives | Light | Laser/ion beams | High temperature | |||||
White | Calcium carbonates CaCO3 | Formation of oxalates [54], proteinates [51], carboxylates [55–60] | Reacts with verdigris [50]. Cu ions catalyse degradation of binders and formation of oxalates [57, 60] | – [45] | –/discolouration [61] | Decomposition to CaO and CO2 | ||||
White | Lead white 2PbCO3·Pb(OH)2 | Drying properties [230]. Formation of soaps which increase layer transparency [55, 67, 226] | Blackens when mixed with red lead. Raction with S containing pigments not clear [191, 229] | Influence of the binding medium [70, 142, 150, 210, 215, 233–235]. Formation of massicot-like degradation [72] /discolouration [70, 234]. [151] | Discolouration [formation of massicot, litharge, red lead, 229] | |||||
Yellow | Orpiment As 2S3 | Different stability in oil/water based mediums [184]. Unstable in oil [170] | Reacts with verdigris and lead white [1, 30, 170]. As5+ deposit around Fe/Mn particles [30, 50, 176]. Formation of As2O3 and H2S [9, 176] | Sensitive, especially to green [20, 30, 128, 177–179, 182, 185, 186]. Synergy with relative humidity | Darkening, and then whitening (formation of As2O3 and SO2) [9, 20] | |||||
Yellow | Massicot PbO | Discolouration [formation of red lead, 229] | ||||||||
Yellow | Lead tin yellow Type I: Pb 2SnO4; Type II: PbSn1−xSixO3 | Drying properties [246]. Formation of lead soaps [56, 225, 247] | – | |||||||
Yellow, red, brown | Yellow, red, brown ochres Fe 2O3, FeOOH, Fe 3O4; MnxOy | Fe promotes photo-oxidation, Mn curing of the oil [12]. Reaction products witf proteinaceous binder [51, 65] | – | Dehydration of hydroxides/oxihydroxides (yellow to red); modification of the Mn phases (darkening, [18, 19, 29, 62, 63, 69–72]) | Dehydration of hydroxides/oxihydroxides; modification of the Mn phases [darkening, 1, 12, 68] | |||||
Orange-red/yellow | Realgar/pararealgar As 4S4 | Reacts with verdigris and lead white [30, 50, 176]. As5+ deposit around Fe/Mn particles [180, 183]. Formation of As2O3 and H2S [9, 176] | Sensitive, especially to green [30, 106, 177, 180, 181, 188] | Temporary darkening/Whitening (formation of As2O3) [20, 189] | ||||||
Orange | Litharge PbO | Very reactive towards organic binders, commonly used as dryer [1]. Soaps formation [55] | Discolouration (formation of massicot, red lead, 229) | |||||||
Red | Vermillion HgS | – [56] | Protect vermillion from light and from external chloride sources [19, 65, 121, 139, 193, 194, 198, 199, 202, 205, 206, 210]. Watercolour medium offers little to no protection. No oxalates [56] | Mixtures with lead white, minium, and other pigments show increased stability of vermillion [19, 20, 193, 199, 204]. Reactivity with some materials [65, 198, 205] | Blackening, related to halogen impurities [70, 141, 142, 150, 199, 202, 206, 207] | Blackening, related to halogen impurities [70, 141, 142, 150, 206] | ||||
Red | Red lead Pb 3O4 | Protect the pigment from light and humidity [239]. Lead soaps, lead hydroxide, lead acetates and finally lead carbonates are formed [240] | Sensitive to S containing pigment (arsenic sulphides, vermillion and ultramarine, [1, 60, 93, 124, 186, 238, 241]): formation of dark PbS and white PbSO4, and Pb arsenate species [241]. It promotes lead white blackening [204] | Affects lean layers of water soluble paint. Both blackening (PbS) and lightening (PbSO4, PbCO3) occur. Related to pigment’s composition. [1, 5, 59, 64, 93, 120, 238, 240, 243] | Grey to brown discolouration, related to pigment’s composition. Formation of PbO (which can be re-oxidised to minium, [142]) | |||||
Green | Green earths glauconite (K,Na)(Fe 3+,Al,Mg)2 (Si,Al)4O10(OH)2) celadonite (K[(Al,Fe 3+),(Fe 2+,Mg)] (AlSi3,Si4)O10(OH)2 | |||||||||
Green | Malachite CuCO3·Cu(OH)2 | Organometallic compounds (oxalates, carboxylates, resinates, acetates, etc.) [32, 53, 55, 109, 122, 126, 129–135] | – | |||||||
Green | Verdigris xCu(CH3COO2)·yCu(OH)2·zH2O | Promotes drying [20, 133, 134, 146–148]; formation of soaps and metalloproteins [35, 50, 67, 133, 145, 147] | The acidic conditions, presence of Cu, light and pollution are involved in cellulose degradation [20]. Darkening when mixed with orpiment or lead white [50] | Browning, darkening (release of Cu+) [146] | PIXE [151] | |||||
Green | Copper resinate copper salts of abietic acid | Formation of carboxylates, metalloproteins. Photoxidative processes occur [147, 151] | –/–/darkening (reduction of Cu2+ to Cu+, [151]) | X-rays [151] | ||||||
Green | Cu chlorides Cu 2Cl(OH)3 | |||||||||
Green | Cu sulphates CuSO4·yCu(OH)2·zH2O | |||||||||
Blue | Ultramarine Na8[Al6Si6O24]Sn | Catalytic effect of the pigment on binders hydrolysis [41, 42] | Pb neutralizes the acidity of the binder [35] | |||||||
Blue | Vivianite Fe 3(PO4)2·8H2O | Protect the pigment from degradation [47] | ||||||||
Blue | Smalt CoO·nSiO2 | Leaching of ions, changes in Co coordination and formation of soaps (grey, blanching, [34, 35, 47, 89, 90, 93–96, 98–101]) | Ca neutralizes the acidity of the binder [47, 89, 90, 96], Pb more reactive than K to form soaps [34, 90, 96, 97] | Further discolouration of degraded particles [103] | ||||||
Blue | Azurite 2CuCO3·Cu(OH)2 | Cu acts as a biocide [124] | Discolouration of small particles [35]; formation of bluish verdigris (with humidity, [64]); formation of Cu proteinates in tempera [51, 112]; yellowing of binders [111] | Discolouration (black, [20, 68, 69, 91, 103, 112–114, 121–123, 125]) | ||||||
Blue | Egyptian blue CaCuSi4O10 | Darkening in gum [155] | – | |||||||
Black | Carbon black C | Darkening upon infrared laser exposure [18] | Pb+ dryers: white discolouration [17] | Photodegradation of the aromatic structure catalysed by lead [17] | Stable to 248 nm laser [19] | Burning [13] | ||||