The yellowing/bleaching behaviour of oil paint: further investigations into significant colour change in response to dark storage followed by light exposure

Colour measurements are presented for a set of historically accurate paint reconstructions based on linseed oil pre-processed using various historical recipes, modern lead white, and paint mediums. Colour changes in response to dark/light cycling were reported previously when the samples were fresh. Measurements on the now 10-year-old samples are discussed in relation to colour changes measured during light/dark exposure of 19th-century paints, and in fresh linseed versus poppy oil samples. Visually obvious colour changes were measured in all cases when samples were taken from the dark and exposed to ‘typical gallery' or brighter ‘historic house' lighting over a period of months. These samples all bleached (lost yellowness) and tended towards a common colour regardless of their original formulation. This suggests that retouches made to historic paint that has been stored or kept in the dark for an extended period will appear to develop a mismatch as the paint itself bleaches and the retouches remain unchanged. These results also imply that short and/or intermittent gallery display alternated with longer periods of dark storage can leave light-coloured paint in a yellowed state that would require months or years of 'typical' light exposure for full reversal. No irreversible colour changes were noted in the sample set studied.