Give it a Rest – Thoughts on Exhibiting Light-sensitive Objects

matt2It is common for museums to include in their collections management policies a schedule for the exhibition of light-sensitive objects such as works on paper and textiles. Frequently these policies include recommendations for light levels, and specify the length of time an object can be exhibited before it is returned to storage for a “rest.” The length of the exhibit period and of the rest time are actually purely arbitrary – in reality, all light-sensitive objects have a finite life span. Think of it as each object having a bank account from which you can make withdrawals, but to which you cannot make deposits. Each amount of time on exhibit is a withdrawal. The “rest” period is not a period in which the object recovers from its exhibit time, because – all together now – light damage is cumulative and irreversible. Once the account is gone, it’s gone. You simply have to tell anyone who asks you to shorten the rest time or lengthen the exhibition time that they are really asking you to spend the object’s life faster. Or, to look at it slightly differently, you can display the object frequently now, or you can display it seldom so that your great-great grandchildren might get a chance to see it.

As for how long this lifespan may be, that depends on many factors – your storage environment, the amount of light and other environmental factors in your exhibit space, and the fibers, dyes, inks and what have you that make up the object itself. Along with time spent on exhibit, these factors, both those you control and those you can’t control, will determine how long that bank account will last.

Anne T. Lane
Mountain Heritage Center
Western Carolina University

This post is also available in Italian, translated by Silvia Telmon.

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6 comments

  1. Mark makes an excellent point about needing to evaluate light damage relative to other risks to the collection in order to understand its importance. It would be a shame to keep objects from view only to have them degrade in other ways or be lost to fire, flood, or theft long before their useful display life has been used. Recognizing the importance of considering risks in context is the driving force behind leading institutions now embarking on comprehensive collection risk assessments. If there are one or more risks other than light damage that dominate the risk profile to a collection then there may be little point to rotating which objects are on exhibit.

    It is also important to realize that rotating objects from the collection on and off exhibit does not reduce light damage to a collection as a whole but simply alters its distribution. In some cases distributing light damage more broadly through the collection will result in greater loss of value from the collection. The change in object value seldom has a simple, straight line relation to extent of light damage. Usually the change from a “pristine” state to a “just noticeable change” results in a much greater loss in value than a just noticeable change step somewhere in the mid-range between pristine and completely damaged. This is generally recognized by herbaria who, knowing that dried plant specimens are extremely light fugitive, will choose only one page of a bound herbarium to be opened on exhibit. In this way light damage to the contents of the book is restricted to a single page. The remaining pages remain in the near pristine state for occasional viewing. In contrast, if the book has 100 pages and a different page is displayed each month then it would only be a matter of decades, if not just years, until every plant specimen in the book was severely faded.

    Agnes Brokerhof and colleagues presented this issue well in their 2008 paper:

    Brokerhof, Agnes W.; Reuss, Margrit; MacKinnon, Fiona; Ligterink, Frank; Neevel, Han; Fekrsanati, Farideh; and Scott, Graeme
    Optimum access at minimum risk: the dilemma of displaying Japanese woodblock prints.
    In Book. Conservation and access: contributions to the London Congress 15-19 September 2008. Saunders, David; Townsend, Joyce H.; and Woodcock, Sally (Editors). International Institute for Conservation of Historic and Artistic Works, London, United Kingdom (2008) pp. 82-87 [English w. English summaries]. 6 figs. (4 color), 3 tables, 11 refs. [ISBN 0-9548169-2-7].

  2. I don’t know of any materials that rejuvenate from light induced damage in dark storage. I do know of materials that continue to degrade due to light-induced damage after removal from display and subsequent dark storage. I also can cite materials that can recover from some discoloration by fresh exposure to light, but it’s kind of a catch-22 as some components (e.g., inks on paper) in the artwork are being further degraded while other components (e.g., color of the media) are being improved by further exposure to light.

    The key knowledge to be acquired by curators and conservators is not easy to gain in many instances, but it is how the light fade resistance compares to other degradation pathways. When a material is very light sensitive, it’s pretty much guaranteed that light exposure on display will be a major factor of concern, but with materials of moderate or high light fade resistance, then other weak links like gas fade resistance, thermal and humidity degradation, etc., may prevail in such a way that worrying about amortizing the time in the light on display may be totally irrelevant. Policies for storage and display need to be decided based on better understanding of all the likely variables of decay and not just one variable only since the variable in question may not be anywhere close to being the weak link in the chain.

  3. ed gregory says:

    What research has there been to discover whether ‘rest periods’ actually halt deterioration? Some materials will continue to suffer light damage after the light source has been removed. It’d be good to know what materials benefit from a ‘rest period’ and those that don’t.

  4. Liz Douglas says:

    Very helpful analogy (which I shall use!) I’ve found it to be quite a common misconception that ‘rest periods’ involve recuperation or recovery of an object in some way.

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