| A data logger hidden behind an exhibit |
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| Data Logger hidden behind an exhibit |
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.
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| 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.
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.
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| Blue wool scale card in exhibition case |
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.
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| 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.
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.
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.
References
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.
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This post was written by Catherine Dand, a Conservator at the Borthwick Institute
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