Cadmium Yellow DeteriorationOver the past years a number of studies have described the instability of the pigment cadmium yellow (CdS). In various paintings studied, the degradation process results in fading, chalking, flaking, darkening, the formation of white crystals on the paint surface, protrusions, and in some cases, the formation of a grey-orange crust. The observed degradation often occurs at the surface, and is caused by oxidation of the original CdS pigment to cadmium carbonates (CdCO3), an insoluble photo-degradation product, and other oxalates and sulphates (such as CdSO4·H2O). Experiments have shown that sulphur, originally present in sulfidic form (S2−), is oxidized during the transformation to the sulphate form (S6+). Upon formation, the highly soluble cadmium sulphate is thought to be transported to the surface and re-precipitates there. These processes are thought to be the result of photo-oxidative degradation but further research is required to confirm these hypotheses.