PHYSICAL AND CHEMICAL PROPERTIES OF SILK Common cocoons (Bombyx mori) enclosing chrysalides weigh each from 0.4 to 2 grams, i.e., there are from 30o to 600 to the pound, for small breeds, and 27o to 30o for large breeds. About one-sixth of this weight is silk and of that one-half can be reeled, the remainder which cannot be reeled consisting of surface floss or blaze and the husk of the chrysalis. It is therefore difficult to estimate the total length of thread produced by the silkworm or even that of the portion reeled, which varies in length and thick ness according to the condition and robustness of the cocoon and may be 500 metres in some breeds and in others 900 to 1,200 metres. It has been estimated that one pound of raw silk requires from 2,100 to 3,00o cocoons. Under favourable conditions 11 kilo grams of fresh cocoons may give 1 kilogram of raw silk for com merce and about the same quantity of waste silk for spinning. The thread is usually thicker and stronger towards the middle of the reeled portion than at the extremities. The mean diameter of the cocoon thread or bave which is composed of two filaments of dif ferent specimens of mulberry silk was found at the Lyons labora tory to vary from o•oo18o to o.0033cm. ; the have of wild silks from 0.0030 to 0.0070 centimetre. The denier or weight in grams of 9,00o metres (see SILK MANUFACTURE) of the bave of mul berry silk varies from 1.8 to 3.8, and that of the bave of the Anthers (wild silks) from 3 to 8 deniers.
The raw silk fibre (see FIBRES) consists essentially of two cores of fibroin cemented together and covered with sericin or silk albumen, besides small quantities of waxy and colouring matters. The ultimate filaments of fibroin, which constitute about 7o to 8o% of dry, raw mulberry silk have in bulk, after removal of the gum, the characteristic soft white appearance and pearly lustre of pure silk. Under the microscope the filaments appear smooth and rod-like and, when examined by polarized light, show the colours given by doubly refracting substances; their cross-section is roughly triangular (maximum diameter, 0.0023 to 0.0014cm.; minimum diameter, 0.0018 to o.0009cm.). The filaments have great tensile strength (3 to 5 X dynes per sq.cm.) extending considerably before breaking, and show true elastic effects under limited stresses (Young's modulus, 0.4 to o.8 X Ion and rigidity, 0.2 X IOU dynes per sq.cm. under ordinary conditions). In fact, silk is said to have almost the equivalent tensile strength of iron wire; that of the wire being 90,000lb. per sq.in. and that of silk 64,000. Its elasticity is also a remarkable characteristic. An elasticity of 2o% means that the silk fibre can be stretched one fifth of its original length and when released will return to its natural state. To the two properties of tensile strength and elas ticity silk owes much of its excellence as a textile material, espe cially in the making of women's hosiery. The density of silk fibres is about 1.3, refractive index 1.5, and specific heat 0.3
calories per gram. The low electrical conductivity of fibroin, utilized in the employment of silk for electrical insulation, may cause a troublesome electrical excitation of the fibres during manufacturing processes in a dry atmosphere. Its thermal con ductivity also is low. Under ordinary atmospheric conditions fibroin contains about I 1% of its dry weight of hygroscopic mois ture which may be removed by heating it at 105° C. It is insolu liquids.
Fibroin is an amphoteric colloid and belongs to the chemical class of proteins; the formula C15H23N506 sometimes assigned to it should be taken to be no more than an approximate expression of its elementary composition. The X-ray method of examination indicates the possible presence in fibroin of a crystalline constitu ent, and there is some chemical evidence that fibroin is chemically heterogenous. It dissolves in the cold in concentrated solutions of the mineral acids or of the caustic alkalies, and in an ammo niacal solution of copper oxide ; from all these solutions it may be reprecipitated in a more or less altered form when the solution is neutralized, but, owing to more far-reaching changes, not after long standing. When heated, fibroin melts and burns giving a smell of burned feathers which serves to distinguish it from the vegetable fibres, including artificial silk. It may be distinguished from wool by its microscopical appearance and by chemical tests. The ultimate filaments of the wild silks are thicker and more ribbon-shaped than those of mulberry silk and exhibit longitudi nal striations along which the filaments tend to split into fibrillae under any mechanical or chemical action; the fibroin of the wild silks possesses properties similar to those of mulberry fibroin but is more resistant to chemical action. Sericin, also a protein, is more active chemically than fibroin, from which it may be separated by the solvent action of hot water, best under pressure, or of acid or alkaline solutions. Its hot aqueous solution gelatin izes on cooling. Its elementary composition is similar to that of fibroin but it contains more oxygen ; like fibroin it may be chemi cally heterogenous. The colouring matter of yellow silk is prob ably carotene or a related substance.
Silk has great power of absorption. Its brilliant colours are due to a greater capacity of absorption toward colouring matters in general than perhaps any other fibre. It also absorbs dye stuffs at much lower temperatures than does wool. So great are its powers of absorption that its weight can be increased as much as 3o% in moisture without any change in appearance. In order to prevent fraud by shipping raw silk to which moisture was added by the seller to secure a greater return on the poundage sold, an international standard of I I% was established many years ago as the permissible limit of "regain" in moisture over the dry weight.