Bleaching of red lake paints in encaustic mummy portraits

The present paper reports on the study of the development of whitish opacity in pink paints in encaustic mummy portraits. Non-invasive measurements carried out on two encaustic portraits belonging to the Ny Carlsberg Glyptotek, Copenhagen, by reflectance FTIR and UV-vis fluorescence have shown that the areas prone to the bleaching phenomenon had been painted with melted beeswax and an anthraquinone vegetal lake mixed with calcium sulphate hemihydrate and dihydrate. The hypothesis that the bleaching disease was neither related to a degradation of the dyes nor to an alteration of the wax but rather to a dehydration-hydration reaction of the CaSO(4)-H(2)O system, has been corroborated by the analyses of two microsamples from the bleached areas and ascertained by accelerated ageing experiments on encaustic models.