Shining a light on the degradation of triarylmethane dyes: Multi-analytical study of a faded 1880s printed cotton dress

• Investigation of the photochemistry of blue-green triarylmethane dyes. • In-situ analysis revealed light fading triarylmethane dyes on an 1885 cotton dress. • Evidence of two degradation pathways clarifies the dress's conservation state. • Dye substituents affect fading; findings support light-safe museum display. This study examines the fading of the blue-green background in an 1885 printed cotton dress by Edmund Potter & Co., in the collection of the Victoria and Albert Museum (V&A). Uneven fading patterns indicate differential light exposure, with protected areas retaining greater vibrancy. An in-situ analytical campaign employing portable UV–VIS–NIR spectroscopy, colorimetry, and VIS hyperspectral imaging identified dye constituents and evaluated their conservation state. Complementary micro-destructive Surface Enhanced Raman spectroscopy suggests the presence of early synthetic triarylmethane dyes. To further investigate their photo-fading behaviour, mordanted cotton mock-ups dyed with selected blue-green triarylmethane dyes commercially available at the time of the dress’s production, namely, diamond green B (C.I. 42,000, Basic Green 4, 1877), diamond green G (C.I. 42,040, Basic Green 1, 1879) and yellowish light green SF (C.I. 42,095, Acid Green 5, late 19th Century) were subjected to controlled aging and monitored. Comparative analysis with in situ data provided contextual insight into the photochemical behaviour of the dyes, highlighting differences associated with substituent type and orientation and their potential influence on observed photoproducts, including benzophenones. These findings advance the understanding of triarylmethane dye fading in important historical textiles and inform conservation strategies for museum display of such dye sensitive collections.