Pb(OH) 2), red lead (Pb 3O 4) are employed owing to bright colors and high covering power in paintings.Arsenic sulfide pigments are bound to shift color exposed to visible light and realgar undergoes photo-induced polymorphism and becomes friable, assuming hues from bright yellow pararealgar (As 4S 4) to colorless arsenolite (As 2O 3) whereas the photo-oxidation of orpiment results in arsenolite.
Arsenic-based pigments such as orpiment (As 2S 3) and realgar (As 4S 4) are highly appreciated in China for yellow and red to red–orange tones. In the case of artifacts, the published data indicates the transformation of azurite and malachite into green basic Cu chlorides in the presence of chloride and humidity. Azurite (2CuCO 3♼u(OH) 2) and malachite (CuCO 3♼u(OH) 2), as the best representatives of basic copper salts (carbonates, chlorides, sulphates, etc.), are widely used as mineral blue and green pigments. Studies on degradation processes and products of pigments inside museum on oil paintings, manuscripts and in an outdoor environment on polychrome statue and wall paintings are well known and documented. Degradation of pigments can be visually perceived as color changes or confirmed as structural modifications by employing modern analytical or micro-analytical methods. These processes can be initiated by internal factors, for example the characteristics of pigments with organic binders and other materials, or by external factors, which suffer from environmental conditions (relative humidity, illumination, temperature, etc.), biological activity, pollution or human interventions, or several of these together. The formation of lavendulan give rise to a highlight for identification of green copper-arsenic containing pigments and characterization of degradation in complex conditions.ĭegradation phenomena occur in the paintings where original pigments, binding media, ground layer and substrate to undergo chemical and physical transformations, leading to discolouration or deterioration of materials. Based on the pigment and paint layer stratigraphy analysis, the paintings of the statue have been executed at least four times and the most recent may be after 1850s. These secondary products are formed via multistep progresses as degradation of emerald green and cerussite, oxidation of arsenite ((AsO 2) −), migration of arsenate ((AsO 4) 3−) and precipitation reaction of these ions with other necessary ions from environmental conditions. Moreover, lavendulan (NaCaCu 5(AsO 4) 4Cl♵H 2O) and mimetite (Pb 5(AsO 4) 3Cl) are degradation products of emerald green (Cu(C 2H 3O 2) 2♳Cu(AsO 2) 2) and cerussite(PbCO 3) originally used as green pigment with mixture. Ultramarine blue, emerald green, synthetic atacamite, vermilion, red lead, gold foil, orpiment, cerussite, gypsum and barite are identified as pigments. Paint cross sections from Vairocana Statue in Dazu Rock Carvings, one of World Cultural Heritage, Chongqing, China are studied by optical microscopy (OM), Raman spectroscopy (Raman) and mapping, and scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM–EDS). Even some of these are well studied, more investigations are still in need for heritage science. The spontaneous degradation of pigments in painting may occur depending on environment, especially the surrounding condition of high humidity and soluble salts.