Series editors' preface - Wood Tools

crease at corners of upholstery, or pleats in
curtains may eventually become a crack.
Fibres are difficult to identify when degraded,
for example the scales on hair fibres, an
identifying characteristic, wear away.
Chemical degradation
Light, particularly ultra violet radiation, acts as
a source of energy for photochemical reactions
that occur in the presence of oxygen and
moisture. The cellulose molecules which are
the main constituent of vegetable fibres in
cotton, linen and paper and the protein fibres
in wool and silk are colourless. They absorb
electromagnetic radiation in the ultra violet
range. These fibres are not found in their pure
state in objects but are nearly always
combined with dyes, mordants, various kinds
of dirt, residual chemical treatments and other
impurities which may promote degradation
reactions. When one or other of the component
molecules of these systems absorbs a
photon of light energy, a number of things
may happen. First, the energy absorbed may
be dissipated as heat when the excited
molecule returns to its ground state, without
chemical reaction occurring. Secondly, the
energy may be transferred to another molecule
which itself does not absorb light. The water
vapour and oxygen in the air may be
energized in this way and the activated
oxygen and hydrogen peroxide thus formed
are capable of oxidizing both the fibre and its
dye. Another possibility is that the absorbed
light energy may cause a direct reaction of a
dye with its fibre (the dyes concerned would
contain carbonyl groups, quinoid configurations
or azine groups). Dyes on wool in particular
may fade by oxidizing the fibre, by
removal of hydrogen atoms from reactive
methyl, methylene and methine groups on
keratin, being themselves reduced in the
process. The same principle applies to the
cellulose in cotton.
As a result of reactions of the type outlined
above areas of textile exposed to light become
brittle. Fibres are weakened by the breaking
of the long molecules to which they owe their
strength, with broken fibres leading to tears
and loss of textile. Coloured decomposition
products stain fibres yellow and the dyes fade
or darken. A further complication that may
arise is that the degradation may increase the
Deterioration of other materials and structures 351
danger of damage through conservation
procedures such as vacuuming and washing,
particularly in the case of alkaline washing of
degraded cellulose.
Heat or low levels of relative humidity cause
embrittlement of fibres. Extremely low
moisture content may render fibres permanently
inflexible (Cooke, 1988), for example
due to denaturing of proteins. High moisture
content accelerates the rate of photo-degradation
and in warm conditions may induce
mould growth. Rapid fluctuations in relative
humidity levels will cause temporary stresses
to be set up in the fibres as they swell and
contract within the twisted, interlaced textile
structures. These tensions and stresses may
cause permanent structural damage and breakage
of degraded fibres.
Generally speaking, cellulose materials are
very sensitive to acid attack but more resistant
to alkaline environments whilst proteins are
resistant to acids but more susceptible to attack
by alkalis. However, strong acids can destroy
both cellulose and protein, silk being the most
vulnerable protein fibre (Landi, 1992/1985). In
cellulose, hydroxyl groups become transformed
to acidic groups as a result of photochemical
reactions. Degraded textiles are more sensitive
to alkaline and acidic conditions than new
textiles. One major source of acid is polluted
air. In particular, sulphur dioxide combined
with moisture may generate sulphuric acid.
This reaction occurs in air, on surfaces and
with objects containing a layer of adsorbed
moisture Also, the materials making up the
upholstered unit may themselves be a source
of pollution. For example, most woods
produce acetic acid as a result of gradual
hydrolysis of acetyl groups (esters) in the
hemicellulose fraction. Sugar acids are also
present in wood. Proteinaceous upholstery
materials such as horsehair and wool
(especially felted wool) may produce sulphurcontaining
gases such as hydrogen sulphide as
sulphur-containing amino acids (cysteine and
cystine), present in these materials, breakdown.
These may in turn interact with other materials,
contributing to the overall degradation of
the composite upholstered unit. For example,
sulphuric acid may assist in the corrosion of
iron tacks anchoring under-structure materials.
In turn, these corrosion products may stain the
textile (usually orange brown coloration) and