Research on the Chemical and Colour Changes of Historically Important Light-Sensitive Dyestuffs during Accelerated Light Ageing with Filtered Visible Light

Lighting is a key factor for the display and preservation of cultural heritage materials. This research pioneers the chemical investigation of colourants aged by visible light of different wavelengths. Silk samples dyed by historically important colourants including magenta, carthamin and curcumin were accelerated light-ageing by visible light of 10 nm and 60 nm width, the former allowed unprecedentedly precise investigations of the effect of light wavelength on the ageing of colourants. The chemical compositions of the aged samples were analysed by Ultra high performance liquid chromatography with photodiode array detection, and the colour changes of the samples during ageing were tracked by Fibre optic spectrometer. As a result, coloured and colourless ageing products are detected, corresponding to the colour changes of the samples during ageing. The orange degradation products of carthamin and the purple degradation products of magenta are detected for the first time. Samples dyed by the same dye but exposed to visible light of different wavelengths share most degradation products. There are also some special components found in samples exposed to light of specific wavelengths. Comparing samples exposed to different visible light of the same amount of irradiance, a correlation between the extent of light ageing and the absorption spectra of the dyestuffs is confirmed. This research greatly promotes the understanding of the ageing effect of visible light of different wavelengths on materials and would be very helpful for developing alternative museum lighting sources for better preservation of light-sensitive historical materials