Sericin, the protein that binds the pairs of fibroin filaments as they emerge
from the silkworm, and which may have a role in dehydrating the fibroin and
encouraging its crystallisation, has a markedly different composition and structure
to that of fibroin. It is largely amorphous and is rich in serine (32%), aspartic
acid (14%) and glycine (13%); there is a much greater proportion of
residues with polar and/or bulky side-chains. The predominance of these polar,
hydrophilic groups means that sericin is readily soluble in hot water.
4.2 Silk Processing for Use as a Textile Fibre
Silk yarns are thrown or spun; the former are of higher quality being produced
from bundles of lightly twisted continuous filaments, while the latter is spun
from shorter, broken sections of fibre. The fibre can be used ‘raw’, with the
sericin intact, or ‘degummed’, when the sericin has been removed to yield indi-
vidual fibroin strands. Degumming is usually achieved by the use of boiling
water or steam, by immersion in an alkali (soap) bath, by fermentation (micro-
bial or enzymatic breakdown of the protein), or, more recently, by the use of
acids. The degummed fibres are smoother and more lustrous, but their physical
strength is often reduced and they will absorb less water (which in turn affects
the affinity for dyes and weighting agents).
Following degumming, silk can be bleached to remove any residual colour.
Historically sulfur bleaching (‘stoving’) was common, until superseded by
modern ‘wet’ chemical methods, generally including hydrogen peroxide,
although sometimes sodium peroxide, various bisulfites and ‘blueing’ agents.
A particularly important aspect of silk processing is ‘weighting’. Historically,
silk was sold on the basis of weight, but degumming can reduce this by as much
as 25%, so methods were sought to restore this lost weight. Subsequently,
weighting was used to fraudulently increase the mass of the fibre beyond its ori-
ginal level, and finally it became an accepted method of preparation to improve
the feel (crisper and stiffer) and drape of the fabric. In extreme cases, silks may
be loaded by up to 400%, though this can lead to rapid deterioration of the fabric.
A variety of different weighting agents and methods have been used since the
middle ages, including gum Arabic, salts of tannic, gallic and formic acids, cat-
echu and logwood (‘vegetable weighting’), protein glues, waxes, sugar and vari-
ous metal salts. It is this last category, the metallic salts, that have found the
most widespread use, and many serve a dual purpose, also acting as mordants
especially for darker dyes. They include compounds of iron, lead, tin, aluminium
and zinc, and more rarely arsenic, barium, bismuth, chromium, copper, magne-
sium and tungsten; the most common of these are salts of lead and tin (in the
form of lead(II) acetate and tin(IV) chloride). These agents are readily absorbed
within the amorphous regions of the fibre where they tend to concentrate. All
weighting agents, and especially the metal salts, can presentserious problems
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