APPENDIX 455 



consisting of two very fine strands or " brins," composed of structureless (colloidal) 

 translucent " fibroin " cemented together by a waxy, glue-like material " sericin." 

 The double filament of the raw silk is technically termed " bave." It is customary 

 to unite the bave from 5 to 15 cocoons in the process of reeling the silk. This com- 

 pound fiber is known as " raw silk." Silk fabrics are produced through sub- 

 sequent treatment of the raw silk, or from the waste obtained in reeling which has 

 been spun. Under the microscope silk has the appearance of single fine structure- 

 less filaments, rarely striated longitudinally and in cross-section irregularly oval. 

 Polarize strongly with brilliant colors. 



Tussah or Wild Silk. — A term applied to silk obtained from a variety of species 

 of silk worm other than 5. mori. A few of these have been reared in captivity. 

 Chinese " Tussah " silk is usually obtained from Anthcrcca pernyi and that from 

 India from A. mylilta. Tussah silks are usually brownish in color, producing fabrics 

 of which " Pongee " silks may be considered the type. Microscopically, wild 

 silk consists of flattened, ribbon-like filaments, of much greater diameter than 

 mulberry silk, with many fine longitudinal striations and where the fibers have 

 crossed one another in the cocoons, while soft, an impression is left on each fiber. 

 These impressions are usually oblique to the axis of the fiber and are visible under 

 the microscope as distinct cross-striations, accompanied by a slight increase in 

 the diameter of the fibers at these points, thus producing in the filament a some- 

 what wavy outline and a variable breadth. In cross-section the fibers are seen 

 to be flattened and more or less triangular in outline. Polarize strongly with 

 high colors if thick. 



GROUP E. ARTIFICIAL OR "FIBER" SILKS. 



A number of very different processes have been developed and placed upon a 

 commercial basis within a comparatively few years. In the United States three 

 types of artificial silk are being manufactured upon a commercial scale at this 

 date (1921). 



Viscose Silk. — (Alkali-Cellulose Xanthate or Thiocarbonate.) Said to be 

 obtained by the action of caustic soda and carbon bisulphide upon wood pulp, 

 which has been previously treated with caustic. The jelly-like material thus 

 obtained is clarified and forced through minute orifices into a concentrated ammo- 

 nium sulphate solution or into dilute sulphuric acid and thus coagulated. 



Under the microscope viscose silk filaments are seen to consist of broad thick 

 ribbons, scored with deep longitudinal striations and ridges. Air and solid inclu- 

 sions are apt to be quite numerous. The filaments yield cross-sections varying 

 from irregular ovals, lenticular or crescent-shaped figures to those with parallel 

 sides and rounded ends. The filaments polarize strongly. 



Lustron Silk. — (Cellulose acetate). Obtained by treating hydrocellulose with 

 acetic anhydride and sulphuric acid. The cellulose acetate is dissolved in a sol- 

 vent such as chloroform, acetic acid, ethylacetate-alcohol, etc. The thick viscous 

 material is forced through orifices into water. 



The filaments are finer than viscose, transparent, structureless, with only fine 

 striations, are of quite uniform diameter and in cross-section are somewhat flattened 

 ovals. Only with careful illumination and focusing may the very fine, almost 

 invisible, longitudinal striations be discovered. Filaments polarize very feebly. 



