Chemistry Reference
In-Depth Information
degradation
(a form of breakdown related to the chemical effects of ultravio-
let radiation) of the keratin that makes up the fibers and, eventually, the total
destruction of the wool. Wool is resistant to bacteria and some fungi. If
buried in the soil, however, wool is eventually destroyed by rot-producing
bacteria. Some insects, beetles, and larvae (e.g., of moths) use wool as food
and, therefore, destroy wool yarns and textiles. Wool burns easily, albeit
slowly and without a flame; the burning process is accompanied by slight
sputtering.
Silk.
Silk
, the only natural fiber that comes in filament form, has been and
still is one of the most appreciated and valued textile fibers. Silk filaments
are secreted by the larvae of several types of silk moths to make their
cocoons. Most silk is derived, however, from the larvae of the
Bombyx mori
moth, which has been widely cultivated in China for over 5000 years.
Fragments of silk fabric dated to the late fourth millennium B.C.E. were found
at Qianshanyang, in the province of Zhejiang, in China. There are, however,
even earlier indications of the use of silk; silk remains were found together
with an eleventh-century B.C.E. mummy in Egypt, probably also providing
evidence of ancient trading routes between the Far and Middle East.
The larvae of the
Bombyx
moth secrete not one but two parallel filaments
of silk bound together into a single strand by sericin, a wax also secreted by
the larvae. After they have secreted the strand of silk, the larvae also wind
it into
cocoons
, protective cases in which the larvae envelop themselves before
passing into the chrysalis state, between larva and fully developed moth.
Processing the silk filaments for making yarn involves three working stages:
(1) unwinding the strand from the cocoons; (2) separating the twin filaments,
which are very thin and slender, to be woven into textiles; and (3) spinning
(twisting) several filaments together so as to obtain a substantial yarn that
can be either woven into a textile fabric or made into cord (Varron 1938).
The silk filaments are made up mostly of the fibrous protein
fibroin
,
which is characterized by reiterations of amino acid units. The properties of
the silk are determined by its amino acid composition, the complexity of the
repetitive units, and the arrangement of these units into higher-order arrays
within the fibroin molecules.
Sericin
, the natural wax that covers and joins
the twin silk filaments secreted by the silk larvae, stiffens the filaments but
also provides some protection during processing. To remove the sericin, the
silk filaments or the yarn made from them are boiled in water to which some
detergent, which combines with the wax and facilitates its removal, is added.
In early ancient times all the detergents were of natural origin, mostly
derived from plants; more modern detergents, such as soap, are human-
made (see Chapter 10).
