Investigation of a ‘white cast’ on a mid‐nineteenth‐century watercolour painting

Abstract The appearance of a ‘white cast’ on the blue areas on a nineteenth‐century watercolour is described. Investigation using energy dispersive x‐ray analysis and x‐ray diffraction showed that the white cast consisted mainly of an efflorescence containing calcium sulphate dihydrate and that some excess alumina found in these areas may have also contributed to the whitish appearance. The possibility that the calcium sulphate may have been derived from yellow lake pigments is discussed. The alternative idea that the efflorescence has been derived from the interaction of sulphur dioxide with calcium carbonate, which is present both in the mixture of pigments in the paint layer and also as a filler in the paper, is discussed.