Textiles 89
As the formation of disulfide bridges between proximate cysteine residues
plays a particularly important rôle in physical properties of wool, any reagents
or conditions that interfere with these bonds will have a significant effect on
the fibres. A particular and related problem associated with the deterioration
of wool fibres is the release of volatile sulfur compounds, which may then
attack adjacent materials: many of the silver-containing metal threads found
on the Tree of Jessetapestry show signs of surface corrosion, in the form of
silver sulfide.
Heat and Humidity. Wool will normally bind about 16–18% moisture, but
on soaking the fibres can bind up to 200% of their dry weight in water; as a
result, degraded fibres are particularly susceptible to damaging swelling, and
are also much more prone to mechanical damage in this state. Swelling and
desiccation may also lead to the rearrangement and formation of new bonds,
leading to the modification of physical properties and dimensional changes in
the bulk fabric. The presence of moisture, particularly in warm conditions,
will also serve to promote other deleterious reactions.
Acid and Alkaline Hydrolysis. The protein hydrolysis of wool, when faced
with acidic or alkaline conditions is similar to that of silk, as noted above. As
with silk, the presence of acidic or alkaline conditions will lead to the hydrol-
ysis of peptide bonds, although of the two fibres, wool will generally degrade
the more rapidly, particularly if the resilient outer layers are damaged, as a
result of its lesser degree of crystallinity. Due to the greater number of cys-
teine residues, wool proteins are also more prone to cross-linking through the
formation of lanthionine and lysinoalanine bonds. A further effect of acid or
alkaline treatment is the degradation and loss of the protective waxes and fatty
acids, which coat the fibres, facilitating further attack.
Photolytic Damage. Exposure to light, particularly below 380 nm and in the
presence of water, will initially cause wool fibre to undergo yellowing, due to
reactions within the amio acid side-chains, and eventually will lead to chemical
modification, mechanical weakness, embrittlement and the loss of flexibility
due to various free radical reactions; photo-oxidation can lead to the scission
of disulfide bonds. The histidine, tryptophan, tyrosine, methionine and cysteine
residues are particularly susceptible. Longer wavelengths may cause bleaching.
However, the fibres do exhibit a greater resistance to photodegradation than
does silk.
Biological Degradation. Wool may be targeted by a variety of keratinophilic
bacteria and fungi, which break down the component proteins via enzymatic
oxidation, reduction or hydrolysis; this type of attack predominantly tends to
occur in the amorphous regions, particularly in the cuticle. Wool fibresare also
susceptible to attack by several species of moths and beetles.