An investigation of the photostability of 19th century artists’ cochineal lake pigments using MFT and conventional accelerated ageing tests

Red lake pigments are known for their high light-sensitivity generally when exposed to museum environments. In the cultural heritage field, the colour loss experienced by organic pigments is of special concern as the appearance of objects may be significantly altered after uncontrolled exposure to light. The present work focuses on the use of microfading testing (MFT) to study the light sensitivity of 19th century cochineal paints. MFT is a technique that combines visible reflectance spectroscopy with accelerated light ageing testing and was developed to perform in situ investigations of the light-sensitivity of cultural heritage materials in a relatively short amount of time. Reconstructions of these pigments following the manufacturing processes used by 19th century Winsor & Newton workers, as well as historic samples of cochineal pigments, were prepared using gum arabic and poly(vinylacetate). Purple cochineal paints showed a moderate sensitivity to light, while Carmine, Crimson and Scarlet paints were classified as high-sensitive materials. Overall, the carmine references turned out to be the most lightfast when mixed with gum-arabic while the scarlet pigment with poly(vinyl acetate) showed a slightly smaller colour change when compared with the almost complete loss of colour of the other poly(vinyl acetate) paints. Finally, advantages and drawbacks of the MFT method are addressed.