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WHAT IS MICROFADING?
Microfading is an accelerated method for assessing the vulnerability of individual museum objects to light-fading, including those for which the identity of the colourant is unknown. It tells us nothing about colour changes that occur in the absence of light, including reactions initiated by light but which continue in dark storage. It is a fibre optics reflectance spectroscopic setup in which a tiny area (about 0.3-0.40mm) on the surface of an object is faded to an imperceptible degree using a high intensity (megalux) infrared and ultraviolet filtered xenon source 400-700nm. IR filtering and the small illumination area precludes significant temperature rise of the illuminated surface.Colour change is calculated from spectral change in real time using CIE colour difference equations. Exposures equivalent to 5-10 years display at normal museum light levels are achieved within a 10 minute test period and the results are used to set exhibition and loan display conditions for a particular object based on its measured sensitivity to light. While its most important function is as an exhibition management tool, it also has research uses The technique was developed by Dr Paul Whitmore at the Art Conservation Research Center at Carnegie Mellon University two decades ago and is now receiving rapidly growing attention with, for example, a dedicated session at the Milwaukee AIC annual conference in 2010, a microfading symposium at the Tate in London in 2011 and a well attended conference at the National Museum of Sweden in Stockholm in November 2012 and a symposium at Liverpool Museum in March 2014. ICPH convened a symposium on the subject at the Yale campus in Beijing in late May, 2015. A symposium devoted to the method was convened by the Rathgen-Forschungslabor, Staatliche Museen zu Berlin in November 2016. WHAT COLLECTION MANAGEMENT PROBLEM DOES IT SOLVE? Conservators are routinely asked to set "safe" display conditions for objects and yet the fading rate of even a known colourant typically varies significantly with a range of factors associated with its physical and chemical environment (eg mordants), origin, processing, manufacture, application, shade depth and past history. Many of these factors, including the identity of the dye itself, are either unknowable in principle or too difficult and expensive to routinely determine in practice. Fading rates are often compared to the relatively well characterised lightfastness of a set 8 physical swatches of blue dyed wool produced by the International Standards Organisation, of which Blue Wool 8 (BW8) is the least light-sensitive and BW1 the most. Without microfading, because of the unavailability of fading rate data and variability of fading rates described above, colourants in the BW3 to BW1 (or worse) range are usually lumped together for the purposes of exhibition lighting guidelines under the description "fugitive" or "highly responsive". However because each BW standard differs from the next by a factor of three, a colourant equivalent to BW3 could be left on display 10 times longer than BW1 for the same degree of damage. The ability to make distinctions within this problem range for museums is microfading's great strength. WHAT ARE THE BENEFITS?
The major beneifts of a realistic fading rate estimate are:
Last updated 2023
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IS IT SAFE?
Yes, it is a non-contact and essentially non-desctructive test method because although there is some fading - as is the case whenever an object is illuminated - it is usually less than would be perceptible even if the whole object were exposed to the same degree. The faded test area is confined to a 0.3 - 0.4mm spot, or about the width of a pen stroke or textile fibre, and like most museum lighting, the source is IR and UV and filtered and is therefore cold despite the high light flux. It is highly unlikely to have any significant physical effect on polymers like cellulose, protein or paint vehicles. HOW RELIABLE IS MICROFADING DATA? With any accelerated ageing technique the relationship between what is observed at very high test intensities and what is likely to occur in a particular instance on display is uncertain. The correlation is called "reciprocity" and "reciprocity failure" is the term used where the relationship is not one-to-one. The evidence so far indicates that reciprocity generally holds reasonably well, but that there are also colourants where it doesn't. Nearly all fading data, whether microfading or more conventional studies of surrogate materials in test chambers, is accelerated and suffers from the same kind of uncertainties. In spite of this, microfading tells us with much more confidence (and specificity) compared to the rather limited data in the literature whether a dye or pigment is likely to be at serious risk of unacceptable fading within (say) a week, a month, a year, a decade or a century of exhibition. if you like, it is a red, amber or green light rather than a fully quantitative prediction. For a concise discussion of the relationship between reality and microfading data see Paul Whitmore's 1999 paper. To see how the National Museum of Australia uses microading data and the impact the method has had on the museum's operation, click here. HOW ARE RESULTS EXPRESSED? Microfading results may be expressed as a colour change (DeltaE) for a given cumulative exposure or light-dose (megalux hours) calculated from its spectral change over the test period according to a method defined by the International Commission of Illumination (CIE). A set of dyed fabrics developed by the textile industry to rate the lightfastness of dyes - the ISO Blue Wool Fading Standards - are usually employed as internal calibration benchmarks and results may be expressed as a colour change (DeltaE) equivalent to that of the nearest Blue Wool's response under the same conditions. A very fugitive dye or pigment might fade more rapidly than ISO Blue Wool 1 for example, in which case great care would be taken to limit exposure. A different colourant, equivalent to ISO Blue Wool 3, would be ten times more lightfast and consequently at much less risk of fading. For a discussion of ISO Blue Wool Fading Standards and their use in microfade testing see: Ford, B. and Korenberg, C. 2023. Manufacturing Variations in ISO Blue Wool Fading Standards under Microfading Exposure Conditions. Studies in Conservation, DOI: 10.1080/00393630.2023.2184555 pp.1-12 |