Photochemical colour change for traditional watercolour pigments in low oxygen levels

Abstract

An investigation for light exposure on pigments in low-oxygen environments (in the range 0-5% oxygen) was conducted using a purpose-built automated microfadometer for a large sample set including multiple samples of traditional watercolour pigments from nineteenth-century and twentieth-century sources, selected for concerns over their stability in anoxia. The pigments were prepared for usage in watercolour painting: ground and mixed in gum Arabic and applied to historically accurate gelatine glue-sized cotton and linen-based papers. Anoxia benefited many colorants and no colorant fared worse in anoxia than in air, with the exception of Prussian blue and Prussian green (which contains Prussian blue). A Prussian blue sampled from the studio materials of J.M.W. Turner (1775 − 1851) was microfaded in different environments (normal air (20.9% oxygen) 0, 1, 2, 3.5, or 5% oxygen in nitrogen) and the subsequent dark behaviour was measured. The behaviour of the sample (in normal air, anoxia, and 5% oxygen in nitrogen) proved to be consistent with the 55 separately sourced Prussian blue samples. When exposed to light in 5% oxygen in nitrogen, Prussian blue demonstrated the same light stability as in air (at approximately 21°C and 1 atmosphere). Storage in 5% oxygen is proposed for 'anoxic' display of paper-based artworks that might contain Prussian blue, to protect this material while reducing light-induced damage to other components of a watercolour, including organic colorants and the paper support.

Impact of medium and ambient environment on the photodegradation of carmine in solution and paints

“…A study of the stability of the dry pigment, monitored using vibrational spectroscopy, indicated enhanced photodegradation under anoxic conditions [14]. In contrast, anoxic conditions were found to stabilize or slow the photo-induced degradation of carmine lake in paint samples [6,7,15]. The latter investigation also indicated that the medium plays a role in the lightfastness of the pigment, under anoxic conditions, with the smallest color change observed for the oil color, followed by the watercolor and pure dye on paper [6].…”

Assessing the Impact of LED Lighting on the Stability of Selected Yellow Paint Formulations

“…The output of tungsten bulbs is concentrated at the lower frequency red end of the spectral distribution. Although dyes and photographs have been assessed and are not adversely affected by exposure to blue chip LEDs when compared to tungsten, previous work indicates that certain pigments might be at risk (Lerwill et al 2014(Lerwill et al , 2015. Assessing material stability is generally based on reflectance spectra exhibiting weak absorption around the 450 nm region; however, this does not definitively indicate stability.…”

Change is permanent: thoughts on the fading of cochineal-based watercolor pigments

“…This was perhaps the first concerted effort to investigate the effect of environmental conditions on color change. Since then there have been numerous investigations on the instability of colorants tempered in aqueous binding media and their response to light and oxidants [15,16]. The red colors based on cochineal, that is carminic acid (CA), 7-d-glucopyranosyl-3,5,6,8-tetra-hydroxy-1-methyl-9,10-dioxo-anthracene-2-carboxylic acid (Fig.…”