Large-scale assessment of light-induced color change in air and anoxic environments

Abstract

A wide-ranging sample set consisting of dry pigments, dyed textiles, organic and aniline-based dyes, gouaches and watercolors, fluorescent inks, and natural history specimens was exposed to light in air (20.9% oxygen) and near-anoxic environments. After a light dosage of approximately 17.5 Mlux-hours under controlled temperature and humidity conditions, 113 of 125 samples (90% of the sample set) were shown to exhibit less color change in a low-oxygen environment compared with its behavior in air. Thirtynine percent of this subset displayed color change in anoxia that was between two and four times lower than that observed in air, whereas 47% showed color change in anoxia reduced by a factor of four or more. In contrast, six samples exhibited greater color change in anoxia than in airthese samples included Prussian blue watercolor (three samples), Antwerp blue watercolor, Verdigris dry pigment, and Fluorescent Yellow Winsor & Newton Gouache. Although the results from this small sample subset may cause concern when considering the use of anoxia in the conservation of cultural heritage, particularly for colorant systems whose behavior in anoxia has not yet been identified, this study demonstrates the overwhelming benefits of anoxic light exposure for the vast majority of samples investigated here.

Micro-fading spectrometry: investigating the wavelength specificity of fading

“…6 .n H 2 O, where M I is a potassium (K + ), ammonium (NH 4+ ) or sodium (Na + ) ion (depending on the method of manufacture), and n=14-16 could be considered more correct [14]. It has been observed to lose colour in the absence of oxygen and regain all or most of its colour on re-exposure to air [1,15,16]. The same effect has been observed in strong light, in air [17,18].…”

Photochemical colour change for traditional watercolour pigments in low oxygen levels

“…3, Table 1) and cochineal benefited from anoxia more than from hypoxia. The fading studies of Beltran et al (2012) similarly showed that well-characterized samples of weld, buckthorn, madder, and brazilwood and other samples labelled sepia, yellow lake, and indigo also showed less colour change in anoxia than in air, sometimes much less. The following organic pigments were too stable to show fading in air (this was predictable under the test conditions) and did not show any colour change in anoxia or hypoxia either, suggesting that within the limitations of our measurements they were not harmed by lower oxygen level environments: carmine, Kopp's purpurine, quercitron lake, genuine Indian yellow, sepia, and indigo.…”

“…Results are encouraging for both the application of anoxic framing for display of traditional watercolours, and further research into the benefits of hypoxia using lower levels of illuminance for accelerated ageing. Beltran et al (2012) recently published such a study on a large sample set of dry artists' pigments, and the same dyes in many cases on wool or silk, exposed to moderately accelerated light ageing below 8000 lux, and reported that the vast majority of their samples (including organic dyes on silk, organic pigments applied dry to paper, fluorescent dyes, and contemporary gouache) faded significantly less in anoxia or at the same rate as in air, with half of the small number of exceptions being Prussian blue samples.…”

“…Recent studies on textile dyes in particular have also aimed to simulate usage (Rowe, 2004;Beltran et al, 2012).…”

“…Much of the information gained can also be applied with reasonable confidence to works made in the second half of the nineteenth century. Other studies have examined a limited number of such pigments, generally with a single sample of each (Korenberg, 2008;del Hoyo & Mecklenburg, 2011b;Beltran et al, 2012). A known concern with anoxic framing for artists' pigments and some other artefacts is Prussian blue, which has been observed to lose colour in the absence of oxygen, then regain most but not all of its colour on re-exposure to air (Russell & Abney, 1888;Beltran et al, 2008;Korenberg, 2008;del Hoyo & Mecklenburg, 2011a;Beltran et al, 2012) This phenomenon is known as phototropy, but the time for this reversion in relation to the time to lose colour is rarely discussed.…”