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WHAT IS MICROFADING?
Microfading is an accelerated (megalux) technique 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 0.3-0.5mm) on the surface of an object is faded to an imperceptible degree using LED or infrared/ultraviolet filtered xenon sources, 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 typically equivalent to 5-10 years display at normal museum light levels are achieved within a 5-15 minute test period, and the results are used to set exhibition and loan display conditions for individual objects based on their measured sensitivity to light. While its most important function is as an exhibition management tool, microfade testing also has research applications. 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. In practice many of these variables, including the identity of the colourant itself, are either unknowable in principle or too slow, difficult and expensive to determine routinely. More than any other analytical method used in conservation, microfading directly addresses a central concern of day-to-day museum management. 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. Due to 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". In microfading BWs 1-3 are almost universally used as internal standards (exposed under the same conditions). Because each BW standard differs from the next by a factor of three, a colourant equivalent to BW3 may be left on display 10 times longer than BW1 for the same fading to occur. The ability to make such 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 nondesctructive 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 - 0.5mm 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? As 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, for example the fading resulting from 100 hours exposure to 10,000 lux is not equivalent to 10,000 hours exposure to 100 lux.. The evidence so far indicates that reciprocity generally holds reasonably well, but that there are also colourants and colourant systems where it may not. Nearly all fading data, whether derived from microfading or more conventional accelerated studies of surrogate materials in test chambers, suffer from similar uncertainties. The same applies to the BW standards, which also suffer from batch-to-batch variations under MFT conditions (Ford and Korenberg 2023). One of the intrinsic limitations of the method is its inability to determine the long-term effect of light exposure on the colour of paper and other organic substrates (like natural fibres) because their discolouration is a complex function of photochemical and long-term thermal reactions some of which are initiated and accelerated by light exposure. Microfading only accelerates the photochemical aspect. Already age-yellowed paper will bleach in visible light, however it will subsequently re-yellow in storage to a greater extent than it would have if it had not been exhibited. The first part of this response is the basis of light-bleaching of paper, which is usually followed by washing to remove reactive radical species generated by the exposure. In spite of this, microfading tells us with much more confidence (and specificity) than the limited data in the literature whether a dye or pigment is likely to be at risk of unacceptable fading within (say) a week, a month, a year, a decade or a century of exhibition. This is consistent with collection risk management assessments for other hazards afflicting collections. 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 (e.g. megalux hours) calculated from its spectral change over the test period according to a method defined by the International Commission of Illumination (CIE). Results may also be expressed as equivalent to the response of nearest BW under the same conditions, or as a Blue Wool Equivalent (BWE) rating in which a colourant midway between BWs 1&2 would have a BWE of 1.5. Some quite common colourants, for example in ballpoint and felt pens, fade more rapidly than BW1, literally off the BW scale, For a discussion of ISO Blue Wool Fading Standards and their use in microfade testing see: Ford, B. and C Korenberg, 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 NOTE The Getty Conservation Institute has produced a booklet on MFT titled Microfading Tester: Light Sensitivity Assessment and Role in Lighting Policy Beltran, Vincent Laudato, Christel Pesme, Sarah K. Freeman, and Mark Benson. 2021. Microfading Tester: Light Sensitivity Assessment and Role in Lighting Policy. Guidelines. Los Angeles: Getty Conservation Institute. http://hdl.handle.net/10020/gci_pubs_microfading_tester. |