Ultraviolet-induced fluorescence and photo-degradation in zinc oxide watercolour paints

Johnson, Vanessa Marie (2020) Ultraviolet-induced fluorescence and photo-degradation in zinc oxide watercolour paints. Doctoral thesis, Northumbria University.

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Abstract

Paper conservators tasked with the care and treatment of collections containing watercolours often encounter paper which has undergone severe discolouration and deterioration around Chinese white pigments, an effect caused by the photocatalytic reaction between surface electrons on the pigment particles and atmospheric moisture, leading to peroxide formation and oxidation of surrounding paper. This discolouration is difficult to treat and best avoided if possible. While conservators use ultraviolet-induced fluorescence to identify zinc oxide pigments by their intense visible fluorescence, they cannot presently relate zinc oxide’s variety of fluorescent colours and intensities to its rate of photocatalysis. This thesis aims to link types of ultraviolet-induced fluorescence with zinc oxide’s photocatalytic behaviour by examining the physical, chemical and optical properties of mock-up zinc oxide pigments in gum medium and historic watercolour painting case studies and determine whether this fluorescence could be quantified using commercially available DSLR cameras for predicting and diagnosing degradation in watercolours.

The investigation first collected and summarising art manuals and literature detailing zinc oxide’s history and use as a watercolour pigment. This uncovered a near-consensus in the nineteenth century about the inert nature of zinc oxide, along with evidence that Winsor and Newton, the only supplier for nearly a century, annealed their pigments to improve their working properties and reduce their photoconductivity. No other suppliers could produce a good pigment and there were lone voices warning that zinc oxide may be damaging to surrounding materials.
Physical properties of pigments produced by the direct and indirect method as well as commercial and historic case study pigments were examined via x-ray diffraction (XRD), x-ray fluorescence (XRF), energy-dispersive x-ray spectroscopy (EDX) and digital image analysis of micrographs and scanning electron microscopy (SEM) images. XRD and EDX verified the purity of all samples, while XRF revealed a much higher impurity content among direct method pigments than all others. Particle and crystallite sizes and morphologies derived from image analysis of micrographs and SEM images found that commercial and case study pigments were produced by the indirect method, that these had much smaller and shorter crystallites than direct method pigments and that these crystallites were more likely to be photoactive than the long crystallites of the direct method pigments.

The photocatalytic properties of mock-ups were studied via light exposure for 50 hours and subsequent measurements of peroxide formation via Russell-grams which imaged peroxides on indirect and commercial pigments. Colour changes were measured via a colorimeter and indicated that bleaching was the dominant effect, with most pigments causing browning on Whatman filter paper. Analysis of absorption spectra indicated that band gaps were narrowest for direct method pigments and wider for all others, a quality which lengthens the time that excited electrons are available for reactions. Visible deterioration was localised to materials in direct contact with pigments, evidenced by the embrittlement of binding media when paint sat on highly-sized paper and the browning of paper fibres when size was not present and paint was more embedded in the paper substrate.

Fluorescent characteristics were studied with fluorimetry and digital image processing with the goal of determining what qualities relate to photocatalysis. Indirect method pigments including commercial and case study pigments had a strong blue contribution, quantified via a green/blue ratio derived from the fluorescent peak areas and sRGB colour channel intensity values. This ratio was consistently low for photocatalytic pigments as evidenced by peroxide formation and visual deterioration. Historic case studies which were photographed using different cameras and lighting scenarios still were grouped according to this ratio, which was low if visible deterioration was present.

Green/blue sRGB colour channel ratios increased after 50 hours of light exposure, indicating that the surface defects responsible for green fluorescence increased over time. These defects are also responsible for reducing photoactivity; however, given the age of case studies and the similarity in fluorescence between historic pigments and modern photoactive pigments, an endpoint for reactions could not be determined and may not exist. The green/blue ratio appears to be a reliable indicator of photoactivity even after periods of a century or more. More work is needed to standardise image processing procedures to make the method more quantitative, though comparisons within data sets in this study are a reliable indicator of peroxide formation on pigment particle surfaces.

Given the difficulty in treating watercolours damaged by zinc oxide and the evidence presented here of its lengthy photoactivity, paper conservators should avoid using the pigment for retouching and focus on stabilisation and prevention. Localised reduction of staining using alkalis below pH8 may be carried out with caution as strong alkalis dissolve zinc oxide pigments. Consolidation of cracked paint, when carried out after the removal of sulphate salts in a wash, both reintroduces the binder and provides a temporary barrier between surface electrons on pigment particles and surrounding paper, slowing the oxidation of paper cellulose by peroxides.

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