The Effects of Daylight
Rebecca Ellison looks at the effects of UV damage, thermal expansion, and drying on fine furnishings, fittings and finishes
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| A staircase inlaid with marquetry at Claydon House, Buckinghamshire, faded by sunlight and damaged by heels (National Trust Photographic Library / Rob Matheson) |
Natural daylight is considered the best form of illumination for interior fittings and finishes of historic buildings, not least because it reveals their true colours. However, daylight is also responsible for the degradation of objects, particularly those made from organic material. This article explains the mechanisms and effects of this deterioration and draws attention to the objects at greatest risk. It also highlights the importance of controlling exposure in order to prevent damage and it details some of the methods available.
WHAT IS LIGHT?
Light is a form of energy known as electromagnetic radiation. It
ranges from very short wavelength gamma radiation at one end of
the spectrum, to long wavelength radio waves at the other. Visible
light ranges from blue-violet to red and from a wavelength of 400-760nm*.
Ultra violet (UV) radiation has a shorter wavelength than visible
light (0-400nm) and infra-red (IR) radiation has a longer wavelength
(>760nm).
The wavelengths in daylight which cause most damage to interior
fittings and finishes include visible light and ultra violet light.
This is because the energy content of light varies according to
its wavelength: the shorter the wavelength the higher the energy
content. Therefore, UV radiation has more energy than blue visible
light, which in turn has more energy than red light and so on.
Fortunately, short wavelength radiation is not penetrative, and
in practice UV up to 300nm is absorbed by ozone in the earth's atmosphere.
By the time sunlight reaches the surface of the earth the sun's
radiation consists of approximately 50% visible light and just 3%
UV radiation. Window glass prevents the passage of wavelengths up
to 325nm, and the most harmful effects UV and visible light are
confined to the surface of an object for the same reason.
At an atomic level, when sunlight falls on an object, the high energy
provided by this radiation excites electrons, in some cases causing
them to be displaced from bonds between atoms, particularly in organic
compounds, as detailed in the table. This process can cause material
to deteriorate and colours to fade. Objects also heat up causing
their materials to expand and contract, often at different rates
to each other. The differential movement can lead to stress resulting
in damage to the structure of rigid materials. They can also dry
out, again causing differential movement as well as cracking and
crazing of some surfaces.
Infra-red radiation can also provide the necessary energy to displace
electrons, contributing to the deterioration process. IR also raises
temperature, which dries out objects.
THE EFFECT OF UV RADIATION ON MATERIALS AND FITTINGS
Oil paints and varnishes
Drying oils, such as linseed oil, are used as the medium for oil
paints. Like all but the most simple organic compounds, drying oils
are formed of the long carbon-chain molecules known as polymers.
When exposed to UV radiation in daylight, the long molecular chains
become unstable and a variety of reactions take place including
'free radical chain reactions' and 'auto-oxidation' as described
in the Chemistry table below. As a result the drying oils 'polymerise'
to a semi-solid state. They are also bleached by light and tend
to increase in transparency over time. This is because the refractive
index increases, becoming closer to that of the pigment. Where oil
paintings are affected, alterations made by the artist are sometimes
revealed as the oil medium becomes increasingly transparent, a phenomenon
known as 'penti-menti'.
Pigments in an oil film are often totally surrounded by the medium.
Generally this prevents UV from causing deterioration such as fading,
and only thinly applied glazes of lake pigments (dyes struck onto
an inert base, such as madder lake and quercitron) are susceptible.
Both natural and synthetic varnishes also undergo free radical chain
reactions initiated by UV. This leads to increased polarity, resulting
in solubility only in polar solvents. This can risk solubility to
underlying paint layers during varnish removal. In addition, auto-oxidation
leads to cracking, hazing, loss of gloss and yellowing.
Wood
Wood contains cellulose, a high molecular weight polysaccharide.
Cellulose undergoes auto-oxidation in the presence of UV radiation
(1) leading to bleaching of the surface Cellulose itself does not
absorb UV, however, lignin, hemi-celluloses and some dyes and pigments
act as photo-sensitisers. (Photo-sensitisers absorb UV radiation
and transfer the energy to the cellulose, initiating a reaction.)
As a result some of the long molecular chains break up, lowering
the degree of polymerisation, and weakening the material. Nevertheless,
deterioration is unlikely to have a significant effect on the structural
integrity of joinery because joinery is usually solid and auto-oxidation
only occurs at the surface.
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| Curtains at Blickling Hall, Norfolk made brittle by sunlight and damaged by touch (National Trust Photographic Library / Rob Matheson) |
Textiles
Textiles made from natural fibres also contain a high proportion
of cellulose, which deteriorates in the same manner as the cellulose
of wood. As textiles are far more fragile objects than joinery and
the fibres often become brittle, leading to rapid structural deterioration.
Pigments and dyes
The materials and fittings which are most vulnerable to deterioration
when exposed to visible as well as UV radiation include naturally
dyed textiles, tapestries, and costumes, dyed leather, paintings
in distemper media, gouache and watercolours, prints and drawings,
and wallpapers (2). Damage occurs on two fronts; loss of colour
and pattern, and the structural deterioration of the fabric, as
follows.
The pigments and dyes used to colour these objects are highly susceptible
to fading. In addition to absorbing visible radiation, there is
no protective medium surrounding the pigment particles or dyes to
prevent radiation from initiating 'auto-oxidation', a deterioration
mechanism which is explained further in the table. This quickly
leads to a breakdown of the colour centres in the pigments and causes
fading. Natural dyes are particularly susceptible, as by definition
they dissolve in a medium imparting colour by staining and being
absorbed. Fading of pigments affects the comprehension of surface
decoration and is an irreversible form of damage.
Natural textiles and paper are used as supports for the majority
of these sensitive decorative finishes. As described above, textiles
contain a large amount of cellulose, as does paper. They become
brittle and discolour, becoming darker, due to auto-oxidation. Again
damage is not reversible and interventive treatment is the only
option.
PROTECTION
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| Setting Holland blinds at Kingston Lacy, Dorset, using a light meter to reduce the exposure of damaged textiles to no more than 150 lux (National Trust Photographic Library / Ian Shaw) |
To minimise deterioration, controlling the exposure of objects to radiation is of paramount importance. However, the need to set limits for visible light must be balanced against the need to see objects.
Ideally,
the recommended illumination for moderately sensitive material is
of 200±50 lux*, and the proportion of UV in daylight should
not exceed 75µW/lm* for all materials. Sensitive materials
should receive no more than 50 lux. These are ideals and it may
be more practical to display moderately sensitive material at lux
levels in the low hundreds. They should be kept away from direct
sunlight and UV filtered to 75µW/lm. Lux levels and UV content
for sensitive material does need to be tightly controlled to limit
damage. It is recommended to avoid direct sunlight, to prevent an
excessive rise in temperature.
A variety of methods are available to help achieve and maintain
control of daylight. A number of products are discussed below. Some
filter UV, whilst others reduce visible light and solar heat.
Daylight filters
Filters reducing UV, visible light or solar heat often filter some
of the wavelengths in visible light. The effect this may have on
the colour of transmitted light should be considered alongside their
protective value as, in most situations, the colour rendition of
objects in interiors is vitally important. Colourless UV filters
prevent the passage of all UV radiation below 400nm, whilst allowing
the transmittance of all visible radiation. Filters with a yellow
tint absorb all UV radiation and short wavelength visible radiation
which cause some natural dyes to fade, but they also change the
colour rendition.
Laminated glass contains a layer of polyvinyl-butyral (PVB) sandwiched
between two layers of window glass. Grades with UV absorbers dispersed
in the PVB are available. The PVB also renders the glass shatter
resistant. However, laminated glass is expensive and it implies
either replacing existing glazing, or fitting secondary glazing.
This adds further expense and may not be appropriate for historic
buildings.
Self-supporting acrylic sheeting is available in grades containing
UV absorbers. (Acrylic is the generic name given to polymethyl methacrylate,
a synthetic polymer produced from acrylic and methacrylic acids.)
This option is cheaper than laminated glass, but it can have a yellowish
tint, it is easily scratched and it can cause problems with glare.
As with laminated glass, this material implies either replacing
existing glazing or fitting secondary glazing, which may not be
aesthetically appropriate.
Solar films are self-adhesive polyester sheets, usually acetate.
They can reduce transmittance of UV radiation, visible light or
solar heat or a combination of the three. They are cut to size and
are fitted either internally or externally. They do not require
replacement of existing glazing, are relatively cheap and easy to
apply. Unfortunately they are easily scratched and are not durable
if applied externally.
Varnishes containing UV absorbers can be painted onto the inside
of window glass. This option is useful for windows containing very
small or large panes of glass, where it is impractical to apply
films, or to uneven glass. Varnishes are inappropriate for windows
exposed to extreme temperatures, causing cracking and condensation,
where the water causes lifting. Varnishes are also easily abraded.
Sun-blinds and other methods of control
In addition to filters, other methods of reducing light are available
including sun-blinds and curtains.
Sun-blinds are normally manually controlled blinds which should
be lowered in direct sunlight and adjusted in response to the sun's
movement around the building to inhibit direct sunlight, whilst
still allowing a view outside. They should also be adjusted according
to the light levels in the room - a light meter is a useful aid
- and they must be well fitting to prevent light shining through.
The blinds are usually made of a white or cream coloured fabric
which gives a diffuse light whilst inhibiting the passage of direct
sunlight. Cream blinds give a warmer light. Good quality blinds
will last many years. One disadvantage is that they require frequent
and rigorous adjustment to ensure effectiveness.
Sun curtains can be used to reduce light in situations where blinds
are inappropriate for aesthetic reasons. Thicker material obscures
more light. They should be well fitting to prevent shafts of light
entering the building. Unfortunately, curtains must be completely
closed when in use, preventing a view outside. However, good quality
curtains may last many years.
Careful positioning of furniture and other objects in the interior
can also help minimise their deterioration. Placing them out of
the path of direct sunlight is important because direct light causes
far more damage than diffuse light.
Recommended Reading
-
TB Brill, Light: Its Interaction with Art and Antiquities, Plenum Press, London, 1980
- PC Crews, ' A Comparison of Clear Versus Yellow UV Filters in Reducing Fading of Selective Dyes', Studies in Conservation, Vol 33, 1988
- S Hackney, 'Framing for Conservation at the Tate Gallery', The Conservator, No 14, 1990
- JS Mills and R White, The Organic Chemistry of Museum Objects, Second Edition,
Butterworth-Heinemann Ltd, Oxford, 1994
- S Stainton and H Sandwith, The National Trust Manual of Housekeeping,
Second Edition, Penguin Books, London, 1986
- D Saunders,
'UV Absorbing Films', Conservation News, No 47, March 1992
- S Staniforth,
Problems with UV Filters: Lighting in Museums, Galleries and
Historic Houses, Pre-prints of Conference at Bristol University,
1987
- G Thomson, The Museum Environment, Second Edition, Butterworth-Heinemann Ltd, Oxford, 1986
CHEMISTRY
TABLE The photochemical degradation of organic compounds |
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All organic compounds contain carbon. They are affected by light
and other forms of electromagnetic radiation due to their chemical
structure. The degradation process involves the formation of
free radicals and their subsequent reactions. Free radicals
are short lived atoms or groups of atoms which are formed by
homolytic fission of a covalent bond. (They are highly reactive
due to an unpaired valence electron.) These reactions include
auto-oxidation reactions which involve the gain of oxygen, loss
of hydrogen, or loss of electrons. The rate of oxidation increases
with time and is often initiated by photochemical or thermal
energy. The ways that free radicals are produced differ between
photochemical and thermal energy, but the reactions then follow
the same pathways. Auto-oxidation reactions lead to homolytic
bond cleavage, (Figure 1). This
is when covalent bonds (3) absorb radiation of a specific wavelength
corresponding to the amount of energy necessary to displace
an electron, known as the bond dissociation energy. Certain
groups are highly reactive and promote oxidation, such as double
bonds between carbon atoms (represented: C=C), carbonyl groups
(represented: >C=O) and tertiary hydrogen atoms (represented:
>C-). The ruptured bond forms highly reactive free radicals.
The reaction then follows the pathway illustrated below (Figure
2). It involves reactions with oxygen in the air and
is auto-catalytic. The reactions terminate when free radicals
react together to form stable molecules. Carbonyl groups absorb UV radiation and undergo chain scission reactions, resulting in the formation of two free radicals. |
Figure 2 RADICAL REACTIONS (4) INITIATION An initiator radical (Io) removes a hydrogen atom to form an alkyl radical (Ro) R- H + Io => Ro + IH PROPAGATION Alkyl radicals react readily with oxygen, forming peroxy radicals (ROOo). It then removes another hydrogen atom to form hydroperoxides (ROOH) Ro + O2 => ROOo ROOo + RH => ROOH + Ro CHAIN BRANCHING Highly reactive hydroperoxy radicals lead to the formation of other free radicals by an auto-catalytic process ROOH => ROo + OHo ROo + RH => ROH + Ro oOH + RH o => H2O +Ro TERMINATION The radical chain ends when two free radicals react together. Ro + Ro ROOo + ROOo Ro + ROOo Both UV and visible radiation may initiate deterioration of organic material. Some materials require a relatively large input of energy to dissociate bonds and in the regular environmental conditions of a historic building this is provided by high energy UV radiation. Other objects require less energy to dissociate bonds and radiation in the visible region is capable of this. Therefore these objects are far more vulnerable. |
Notes
1. Mills,
JS & White, R, The Organic Chemistry of Museum Objects.
Second Edition,
Butterworth-Heinemann Ltd, Oxford, 1994; p73
2. Thomson,
G, The Museum Environment. Second Edition, Butterworth-Heinemann
Ltd, Oxford, 1986
3.Covalent bonds are formed between two atoms of non-metallic elements,
where the electrons are shared.
4. .
Mills,
JS & White, R, The Organic Chemistry of Museum Objects.
Second Edition,
Butterworth-Heinemann Ltd, Oxford, 1994; p163
| *UNITS
OF MEASURE nm = nanometer = 10-6mm. lux = 1 lumen per metre square. µW/lm = micro watts per lumen. |
