Chromatic Degradation Processes of Red Lead Pigment
Science Direct Working Paper No S1574-0331(04)70280-5
8 Pages Posted: 5 Jan 2020
Date Written: June 2002
Red lead was frequently employed as a pigment in paintings, especially in medieval wall paintings. In unfavourable environmental conditions a strong blackening of the pictorial layer occurs due to the instability of the pigment. In order to establish the causes and processes of this alteration, two kind of aged samples were studied. Naturally aged samples were taken from en experimental wall painting, executed in 1977 in a sheltered outer environment. Moreover, accelerated ageing tests were led on samples with an analogous preparation, using a climatic chamber. The samples were characterised using optical and electron microscopes (SEM and HRTEM), XRD, FTIR and EDS. The naturally aged sample had a darkened surface, due to the formation of plattnerite (black PbO) and gypsum (CaSO·2HO). However, the artificial ageing test did not induce a mineralogical nor chromatic change of the pigment. A redox reaction process in aqueous environment is proposed to explain the oxidation of the pigment (PbO + PbO) to plattnerite.
Keywords: experimental wall painting, artificial ageing, natural ageing, pigment oxidation, minium, red lead, plattnerite, Inorganic Chemistry > Dyes and Pigments, inorgchem/0206001
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