On June 26th 2014, Project Conservator Julia Sybalsky and Graduate Intern Gisella Campanelli attended the Microfading and Light Management Study Day, a half-day workshop and demonstration organized by Paul Whitmore at Yale University’s Institute for Preservation of Cultural Heritage (IPCH), and sponsored by IPCH and the Andrew W. Mellon Foundation.
After catching an early morning train from New York to West Haven, Connecticut, we arrived at Yale’s West Campus Conference Center for the Microfading and Light Management Study Day, hosted by the Institute for Preservation of Cultural Heritage. We enjoyed two presentations. The first talk, presented by Paul Whitmore, summarized the basics of microfading: what a microfade tester does, how it is constructed, and how and why one might use one. In the second, Bruce Ford, presented case studies from the National Museum of Australia in which microfading informed the revision of overly restrictive lighting guidelines. The result of his study was a cost-effective policy that protects light sensitive materials and materials of special significance while increasing access to collections in general.
These presentations were followed by an informal tabletop demonstration of the microfade tester. Paul gave us a comprehensive tour of the instrument and explained how to interpret the spectral data, whose stability is the measure of the relative permanence of the color. Tests on several of the inks from a set of “permanent” marking pens made the point that the label is not always an accurate description of stability.
A selection of topics covered in the course of the day’s presentations and demonstration are summarized below.
Light has the potential to cause irreversible damage to objects. It can be especially deleterious to colorants, resulting in visible fading. Conservators understand that light-sensitive objects have a finite display life. Nowadays, collecting institutions often adopt guidelines to indicate the lighting conditions and durations of exhibitions of classes of objects (e.g. oil paintings, watercolors, metal sculpture, etc). These policies assume that objects within a class share the same, known stability. Some of those assumptions have been brought into question.
In his presentation, Bruce discussed the way in which museums have traditionally managed this ‘display-destroy’ dilemma (table 1).
Table 1. Lighting requirements for display across three institutions
|Institution||Light level (lux)||Display/Total (years)|
He argued that these guidelines are essentially based on guesswork and assumptions, and challenged this dogmatic approach by raising questions such as: How efficient are these standards? Are they cost-effective? Do they limit public access to objects of significance? Are they counter-productive? Do they exist in order to protect artifacts or to protect conservators?
Bruce proposed that the Micro-Fading Tester (MFT) can be used as a reliable tool to predict a colorant’s rate of fade over time. This information, along with an appreciation for the object’s significance, public demand, and aesthetic value, will better inform decisions about that object’s exhibition.
Some colorants are more sensitive to light than others. But how do you know the sensitivity of a pigment or dye used on a specific artifact? The conventional ways to assess fading risks to objects come with a price. One way is to exhibit the object and watch to see if its colors fade, either to the naked eye or by tracking the changes with a color measuring device. This approach obviously risks incurring some degree of fading in order to provide the information about the sensitivity to further light exposure. A second approach is to identify the materials present on the object, and to replicate those materials in all their detail in a sample that can then be exposed to light and evaluated. The challenge here is to identify the composition of the colored materials in every important detail, which can be very difficult for natural colorants which may have altered over time. Replicating such a substance is also not usually practical. This approach then becomes difficult and time-consuming, and it is impractical for application to a large number of objects. The last approach is to do spot testing, to shine light on a small area of color and measure whether the color is altered by that light exposure. That is the approach taken in the microfading tester, and the device has the added essential feature that it can be done so sensitively that the tested colors are not changed visibly: it is essentially nondestructive, so the presentation surfaces of objects can be tested safely.
The MFT delivers a focused 0.3mm beam of high-intensity light from a xenon lamp onto a tiny spot on the surface of the object and progressively measures any color changes that take place. In doing so, it allows a quick determination of whether an object contains light sensitive materials without need for their prior identification. The machine has two fiber optics angled at 45° degrees to the test surface. One fiber optic supplies the incoming beam from the xenon lamp. The incoming light is filtered to remove both infrared (IR) and ultraviolet (UV) radiation, which cause damage that is usually minimized in museum and gallery lighting environments. The object absorbs some of this incoming light, while the rest is reflected into a spectrophotometer via the second fiber optic. The spectrophotometer separates the wavelengths in this reflected light and produces a spectrum. As the spectrum is continuously updated during the test period, color measurements are recorded and used to calculate. ∆E, a measure of color change since the initial spectrum was collected. Any non-zero ∆E values indicate that the colorant being assayed is being altered by the light delivered. If the color does exhibit a color change during the test, the rate of that change is compared to the rate of fading of Blue Wool standards, cloths whose fading rates are known and have been adopted as the international yardstick for color lightfastness. There are eight cloths in that scale, with Blue Wool #1 being least stable and Blue Wool #8 the most stable. “Fugitive” colors are considered to be in the Blue Wool #1-#3 range.
This incoming beam delivers light up to fifty times more intense than sunlight, exposing the tiny test area to a light dose equivalent to what the average museum object might receive over 5-10 years. By comparing the rate of increasing ∆E values in the test colorant to rates for Blue Wool standards exposed in the same way, the conservator is able to make informed predictions about the future behavior of the test material over time: will it retain its color for a long time like a Blue Wool 8, or is it more fugitive than a Blue Wool 1? To fully understand an artifact’s light sensitivity, each color on the object should be tested, and the exhibition policy for the object based on the stability of the most sensitive color.
Microfade testing is considered an essentially nondestructive technique, as it does not leave a visible mark on the object. Not only is the test area minute, but more importantly, color change is monitored in real time by the operator and the experiment can be stopped if it looks as if visible change might occur. Meaningful results can be obtained without exceeding a ∆E value of 5, while we are only able to visually detect color differences in these tiny spots when the changes are larger than about 15. Damage can be avoided by observing ∆E values in real time throughout the duration of the test. The test usually runs for five minutes, but should be stopped earlier if ∆E approaches 5. There can be a temperature change of 5-10°C within the test location. This is usually inconsequential for most objects, but may require consideration for waxes, plastics, or other materials with low melting points. Paul explained that such test sites will often re-solidify on cooling without any visible damage, but the results of the test would not be reliable measures of color stability.
Benefits of Micro-fading
The advantage of using the MFT over traditional practices is that it eliminates the need for guesswork because it is measuring the sensitivity to exhibition lighting of colorants that are present on the actual object, rather than a simulation. Objects that can be micro-fade tested include paintings, prints (even through glass), manuscripts, photographs, 3D objects, textiles, and furs/taxidermy – basically any surface that one can make a color measurement on.
Bruce explained that prior to revision of their lighting guidelines, the National Museum of Australia (NMA) typically displayed materials thought to be light sensitive for 2 years per decade. After a subsequent ‘resting’ period, objects would be returned to display, and the cycle repeated. Through his studies using micro-fading, Bruce was able to determine that numerous objects presumed to be light sensitive were in fact more stable than the 2 years exposed in 10 restriction implied (and could therefore be displayed for longer periods), while for a smaller number of items the same restriction was too generous. By measuring fading rates, very fugitive colorants were able to be identified and better protected without the need to restrict access across the board. By being more selective about restrictions using a combination of microfading and a structured significance assessment, the museum has been able to save thousands of dollars in the cost of exhibit changeovers.
The MFT can also be used in an anoxic environment to determine whether display in such conditions will have any appreciable benefits for the object in terms of color preservation. Some colorants may fade more rapidly in the absence of oxygen, for example Prussian blue and iron gall inks. Interestingly both of these examples undergo a reaction with oxygen which opposes the light-driven color loss but which cannot be measured using microfading or any other accelerated light exposure method. Again, this demonstrates how under-informed assumptions about reducing light damage can be unjustifiable and costly.
Data from microfading tests can supplement other information about the artifact to better inform decisions impacting an artifact’s display life. Using an evidence-based approach to manage display policies can prevent wasteful use of precious funds. Such resources can be redirected towards more needy causes.
How to Access a MFT
MFTs are available for purchase as kits at around $20K. Alternatively, one may contact a nearby colleague or institution that owns one to inquire about arranging for access. Currently, there are 24 MFTs in use worldwide. More information is available through Bruce Ford’s website, www.microfading.com.