For this experiment, small fibres were immersed in a concentrated lithium
thiocyanate solution. This disrupts the hydrogen bonds that hold the protein
chains together, allowing the fibroin polymer to dissolve, but without cleav-
ing the chains. The sample solutions were loaded on to a size-exclusion col-
umn, which is able to separate molecules on the basis of their effective length.
Smaller polymer chains are washed through more slowly. The degraded silk
samples had a much greater retention time (the time that the polymer frag-
ments stay on the column) than pristine silk, demonstrating the fragmentation
of the polymer chains on ageing.
4.5 Degradation of Silk Fibres
To generate deteriorated fibres for the above experiment, new silk was either
thermally or photo-degraded. Silk is particularly susceptible to light damage
and it is suspected that this was a significant factor in the case of the
Shackleton ensign, particularly for the tin weighted cream-white threads.
As for linen and other natural fibres, silk is sensitive to a variety of environ-
mentally driven degradative processes, though in most cases the actual dam-
age is caused by hydrolysis and/or oxidation. Attack on the polymer chains is
generally initiated in the amorphous zones as a consequence of their more open
structure and the incidence of reactive amino-acids (specifically histidine,
lysine, phenylalanine, proline, threonine, tryptophan, tyrosine and valine).
Humidity and Heat. Water bonds strongly to silk due to the high proportion
of polar groups in the material, particularly within the amorphous regions. At
65% humidity, bound water can represent nearly 10% of the weight of the
fibre, leading to some swelling. The presence of salts will alter the pattern of
swelling, as will pre-existing degradation – in heavily deteriorated samples
this can result in the catastrophic breakdown of the fibre structure.
Changes in the environmental conditions during its previous display and
storage seemed to have affected the Shackleton ensign. The evident distortion
of the edges of the fabric and the apparent stresses introduced around the
nails used to attach it to the board suggested to us that there had been moisture-
induced dimensional changes after mounting.
Because water acts as a plasticiser and there is a strong affinity between the
fibre and water, silk retains its flexibility at humidities as low as 40%. Below
this value, desiccation occurs resulting in brittleness and rigidity; the dimen-
sions of the unit cell reduce as well. Besides encouraging loss of water, ele-
vated temperatures will also result in free-radical thermal oxidation, occurring
primarily at amino acid residues with side-groups that readily lose hydrogen
(Figure 18). In addition, the fibres will undergo yellowing as additional chro-
mophores are produced.
80 Chapter 4