Thursday 17 July 2014

Conservation and The Science of Light

A data logger hidden behind an exhibit
The Conservation Department has recently been involved in the installation of two new exhibitions: ‘The Architecture of War Memorials’, which can be found on the third floor of the Raymond Burton Library, and ‘The Pity of War’, which is on display along the ground floor main corridor in the Harry Fairhurst. We decided that this would be a good opportunity to set up some environmental monitoring, so that we could investigate the environmental conditions within the display cases for the duration of an exhibition. We positioned data loggers in various cases to record temperature and relative humidity, hiding the loggers under exhibits and behind stands. We also agreed that we would conduct some light monitoring for the exhibition areas.

It is difficult to be discreet about our light monitoring – the blue wool sample cards that we have positioned in the cases need to be in direct light to give an honest indication of light exposure in the cases. You should be able to spot two sample cards in each exhibition.

Blue Wool Sample Card
 The principles behind the blue wool scale were originally developed for the textile industry in the early eighteenth century. French chemist Dufay was appointed to be Inspector of Dyeworks in 1729, and instructed to develop regulations to control the operations of the dyers.[1] He carried out systematic comparative testing on dyes, by exposing test materials alongside standard samples of graded fastness. Although there have been various developments in the materials, this method of using dyes of known light-fastness is still widely used today.

Blue wool scale card in exhibition case
The blue wool scale tests for light fastness. The sample card we use today is made up of eight swatches of blue wool, which are dyed so that each consecutive dyestuff has an increased resistance to fading when they are exposed to light. Standard 2 takes twice as much exposure to fade to the same level as standard 1, and standard 3 takes twice as much exposure to fade as standard 2 and so on up to standard 8. The sample cards are used in a variety of industries that need to test their products for light-fastness. This could include testing the dyes in clothes, the colour in wallpaper or watercolour paints used by artists. A standard test will expose the blue wool card to light alongside a sample of the dye. Half of the blue wool sample is covered (as you can see in the sample in our exhibition cases) and half of the dye sample is also covered. After a pre-determined amount of light exposure both the blue wool sample and the dye sample are uncovered and compared, and the dye will be awarded the standard on the blue wool scale that has faded the closest amount.

Conservators monitor light exposure so that we can limit damage to the materials that we care for. Light is energy, and energy is damaging to organic materials. I read a comparison of light and heat damage recently that I thought very expressive. Garry Thomson suggested that we imagine organic molecules as people on a commuter train.[2] People are jostled, but this causes minimal physical damage, just as a steady, cool temperature causes minimal chemical damage to our archives. If the temperature on the train is raised, this jostling can get out of hand, and this is when chemical reactions to our molecules also become increasingly likely.

The Electromagnetic Spectrum
Now, Thomson associates light energy with projectiles fired at the commuters by a riot control squad. The damage that the projectiles cause is dependent on the type of projectile: a ping-pong ball or a pebble might only cause minimal damage, whereas a rocket or a hand grenade would cause considerably more. Light travels in waves, with the shortest wave-lengths in the visible spectrum at the violet end, and the longest at the red end. Ultraviolet (UV) light lies beyond the visible at the violet end, and infrared (IR) at the red end. The shorter the wavelength, the more energy delivered; and the more energy delivered, the greater the damage. As a result the violet waves of light are more dangerous than the longer red waves; and the UV waves are the most dangerous of them all, equivalent to the hand grenades thrown by the riot control squad. During my research, I was amazed to discover that to get a supply of useable energy from heat comparable to the energy delivered by light in the UV range one would have to heat up to 200°C.[3] This demonstrates how powerful light energy can be.

 In our exhibition cases we are not testing individual specimens, but have chosen to use the samples to give us an overall indication of light exposure over a fixed period of time. We also take light measurements with a handheld monitor in our exhibition areas, which give us ‘lux’ and UV values. These tell us how much visible light and how much ultraviolet light can be detected. To reduce light exposure in our Borthwick exhibition area there are blinds on the windows. A shaft of light that once escaped from between the blinds was measured to be 1767 lux, whereas the next highest reading from the cases next to the windows with the blinds down has been 582 lux. Direct sunlight can be very intense, and the blinds significantly reduce this exposure.

Graph: Light Levels in Harry Fairhurst Exhibition Area
The glass of windows and exhibition cases also reduce the amount of light that reaches our archives. UV light is reduced, and only between 80 and 90% of visible light is transmitted.[4] As UV is the most damaging type of light, we aim to reduce this as much as possible, and so we also use UV filtered glass for our exhibition cases. The graphs shown here relate to the display cases in the Harry Fairhurst, and demonstrate how much visible light is blocked by the glass as well as how much UV light is filtered by the glass.

Graph: UV levels in Harry Fairhurst Exhibition Area
 We calculate light exposure by multiplying the time by the intensity of exposure. We aim to limit the ‘light hours’ that our archives receive by restricting the length of time we have them on exhibition. Some more light sensitive materials, such as photographs or watercolours, are given even shorter exhibition periods, and are frequently substituted with surrogate images. We also alternate between exhibitions of original items with those full of surrogate material, so that we can continue to raise awareness of the collections we hold at the Borthwick without putting any items at risk from regular display.

 Although our exposure calculations are necessary, we are excited to see the results of our blue wool samples at the end of these exhibitions. Our calculations convey some numerical sense of light exposure, but the sample cards will be a significant visual indication of deterioration.


Forrester, Stanley. ‘The fast and the fugitive: light fastness testing of dyed textiles up to the 1870s’, Journal of the Society of Dyers and Colourists, 91 (July, 1975), 217-223.

 Guthrie, J., N. Tayan and L. Wilson, ‘A novel approach to light-fastness testing’, Journal of the Society of Dyers and Colourists, 111 (July/August, 1995), 220-222.

 Pugh, Samantha and James Guthrie. ‘The development of light fastness testing and light fastness standards’, Review of Progress in Coloration and Related Topics, 31 (2001), 42-56.

 Thomson, Garry. The Museum Environment, Second Edition, London: Butterworth-Heinemann, 1986

With thanks to Tracy Wilcockson for the photographs and light monitoring statistics.


This post was written by Catherine Dand, a Conservator at the Borthwick Institute

[1] Forrester, 218
[2] Thomson, 3
[3] Thomson, 190
[4] Thomson, 5

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