ABSTRACT
Chapter 4 is a collection of vignettes which describe the chemistry of discoloration affecting ten pigments. The list includes red lead, cadmium yellow, α-realgar, vermilion/cinnabar, chrome yellow, hematite, smalt, ultramarine and Prussian blue. Silverpoint and metallic silver discoloration are also discussed. The PSC group includes red lead, cadmium yellow, α-realgar, vermilion/cinnabar and chrome yellow. Discoloration of PSC pigments is described in the context of band theory. Red lead discolors by several routes which lead to lightening and darkening. Smalt changes from blue to gray by an ion-exchange process. Chrome yellow is often produced as a solid solution containing PbCrO4 and PbSO4. The residual sulfate content is critical in the browning of this pigment. α-Realgar rearranges in the solid state to an isomer, para-realgar. In most cases, discoloration is caused by chromophore degradation. However, fading in the case of “Ultramarine Disease” in wall paintings seems to be the result of chemical destruction of the aluminosilicate framework cages of sodalite which surround the chromophore. Chemical mechanisms are supported by spectroscopic evidence. In summary, the instability of inorganic artists’ pigments is the result of interaction of the pigments with environmental factors which include light and atmospheric gases (O2, H2O, CO2, H2S and SO2).
