Microfadotesting & Light-Sensitivity Assessment

by Christel Pesme, Private Paper Conservator and Freelance Museum Lighting Consultant based in Basel, Switzerland

Christel’s research interests include the development of museum lighting, best practices for display of sensitive collection artifacts, and further refinement of Microfadotesting (MFT) as a light-sensitivity assessment tool. She has a Master’s Degree in both Art History and Conservation and previously worked at The Getty Conservation Institute and The Getty Research Institute.

Light

Light is essential in order to see and appreciate a collection item or work of art on display. At the same time, light is a very common cause of damage to collections. Institutions frequently struggle with the compromise between caring for and protecting collections while making them accessible and visible to the public. Many materials are sensitive to light, including paper, photographs, textiles, leather, inks, dyes and other colorants. Light exposure can induce chemical reactions resulting in physical and chemical deterioration of the material on display. Light exposure can lead to fading and other color change if not properly controlled. Light induced color change of the exposed surface item can be so extreme that it compromises the aesthetic, informational, and historic value of the object.

In most cases, light induced color change is cumulative and irreversible. Light induced degradation directly depends on the total exposure (i.e. light intensity in lux per duration of exposure in hours) received by the surface and follows the reciprocity principle:  6 months under 50 lux will induce the same color change to a given item as 3 months under 100 lux.

There are three aspects to consider as you develop and implement a responsible institutional lighting policy for vulnerable collection materials:

  • The light-sensitivity of the item on display, which defines its behavior to light exposure: the higher the sensitivity, the larger the risk of change in the item on display;
  • The minimum level of light required to properly see and appreciate the item on display;
  • The balance between the institutional responsibility to give access to its collection items and its responsibility to preserve the collection item for future generations: indeed, displaying collection items will provide access for the audience while at the same time it may lead to light induced degradation.

A compromise has to be foundwhich is expressed by the Preservation Target (PT), a key concept in collection management. In regard to light exposure, the PT is often defined as the maximum color change induced by light exposure that is acceptable for a given timeframe of stewardship, generally set to 50 years.

ISO Blue Wool Standards

Setting a PT is an important part of defining an appropriate lighting policy—it helps guide the development of lighting standards or guidelines that optimize collection management by maintaining or increasing item/collection value while minimizing its damage. Successful development of these guidelines requires the ability to both understand the connection between color change and light exposure and to quantify value loss due to color change. While the value loss equation is fairly difficult to assess and requires close collaboration between curators and preservation specialists, it is possible to assess the relative sensitivity of the surface of a given item to that of ISO BW 1, 2 or 3 (British Standards Institution, Methods of Test for Colour Fastness of Textiles and Leather, BS 1006: 1978. BSI: London) by using a microfader. Fugitive colors are the colors susceptible to change faster than BWS 3 of the ISO Blue Wool light fastness standards (British Standard 1006:1990) under the same lighting conditions (Michalski 2011).

The microfader exposes a sub-millimeter section of the collection item surface to very intense UV free light and uses a spectrophotometer to measure the light reflected by the irradiated spot (%R) in real time of exposure. The monitored %R is then converted into color coordinates, and the color change produced by the illumination is calculated in real time of exposure. The test is terminated before any visual impact on the tested surface takes place.

The color change against duration of the test is plotted and compared to those obtained on BWS 1 to 3 after the same test duration. The light sensitivity of the tested location is then ranked relative to that of ISO BW1, 2 or 3 and expressed in Blue Wool Standard Equivalency (BWSE). The BWSE corresponds to the BWS with the closest fading rate and the overall BWSE of the tested item will be the one of its most fugitive tested location.

Suppose, for example, that it has been determined through microfadotesting that the tested item has a BWSE of 1. The total exposure corresponding to the PT set for the given item can then be estimated based on the well-characterized dose-response function of the ISO BWS.  For instance, after accelerated aging tests carried out under mild conditions of light exposure, it has been found that a color change that is just perceptible can occur on BWS 1 after a minimum of 100,000 lux-hours of a UV free light source. Let us accept that this just perceptible color change corresponds to the Preservation Target set for the next 50 years. It can then be deduced that the collection item could be exposed within the next 50 years to a maximum of 100,000 lux-hours of a UV free light source before a just perceptible color change to the viewer occurs on its surface. By applying the reciprocity principle, various lighting scenarios in terms of light intensity and time of exposure can be selected to accommodate special visual requirements as long as the total exposure corresponding to the set Preservation Target is not reached.

Microfadotesting is rapid and virtually nondestructive since exposure is terminated before its impact becomes visible. One incomparable advantage of microfadotesting is that it can provide results specific to the real tested item surface. This is an important advance in the field of preventive conservation since other approaches to light sensitivity assessment tend to oversimplify the chemical complexity of the items surface and do not take into account its history of light exposure. Use of a microfader to assess the light sensitivity of the collection item to be displayed optimizes the use of PT values for the display of sensitive collection items and allows institutions to budget light exposure with greater flexibility.

The recent development of an inexpensive, easy-to-use, portable microfader, like the one illustrated here, increases the potential for a more systematic use of the technique, which until now has only been available in a limited number of conservation science labs. The portable microfader could be systematically used by a trained conservator as a screening tool to distinguish sensitive items from more robust ones. This could help to rationalize the policy of rotating items on display and focusing on only the sensitive ones (Ford and Smith 2011).

The next step is to offer training for conservators in charge of vulnerable collections, enabling them to master the portable microfader and associated software for collection management and preservation, and to continue offering support for data interpretation.

Contact Christel by email to learn more about training opportunities – christel.pesme@gmail.com

 

FURTHER READINGS:

Ford, B. and J. Druzik. 2013. Microfading: The State of the Art for Natural History Collections. Collection Forum 27 (1): 54-7.

Ford, B. and N. Smith. 2011. Lighting guidelines and the light fastness of Australian indigenous objects at the National Museum of Australia. Proceedings of the 16th Triennial Conference ICOM-CC, Lisbon.

Michalski, S. 2011. Caring for collections: light, ultraviolet and infrared. http://www.cci-icc.gc.ca/caringfor-prendresoindes/articles/10agents/chap08-eng.aspx (12 November. 2013).

Pesme, C., Lerwill, A., Beltran, V., and J. R. Druzik. Development of Contact Portable MFT to Assist Display Decision of Light Sensitive Collection Items (in preparation).

Strlič, M., Thickett, D., Taylor, J. and M. Cassar. 2013. Damage functions in heritage science. Studies in Conservation (58) 2: 80-87.

Tse, S., L. Cipera, and C. Leckie. 2011. Microfading to support exhibit decisions. Collection Forum 25 (1): 92–106.

Whitmore, P. M., Pan, X., Baillie, C. 1999. Predicting the fading of objects: Identification of fugitive colourants through direct non-destructive light fastness measurements, J. Am. Inst. Cons. 38: 395-409