Tussur silk

Tussur and other wild silks require special treatment. Tussur silk, in particular, is so difficult to bleach that it is commonly used in its natural fawn colour. Bleaching in a 0-6 per cent w/w solution at 70°, C (158°F) for 6 hours will give a fairly good white without excessive loss of tensile strength. The colour is improved by after-treatment in a liquor containing 2 g per litre of sodium hydrosulphite at 100°C. Silk can be bleached by stoving with sulphur dioxide, but the method has now virtually fallen into disuse. Treatment with sulphur dioxide after a peroxide bleach is sometimes used for exceptionally good whites. 

The best-known wild silk is tussur, produced by Antheraeapernyiwhrch thrives on a diet of oak leaves and is found in China. I he worm is about 5 J in. long when mature and makes a compact cocoon which has a brownish colour. Spinning tends to be intermittent so that the filament is more broken than that of Bombyx mori, and for this reason it is more suitable for spun yarns. 

Tussur is coarser than cultivated silk an outer sheath of gum is present, but this cannot be distinguished under the microscope. The brown discoloration, which seems to depend, to some extent, upon the source of food, is not confined to the gum but is distributed throughout the whole of the fibre. The cross-section is irregular, and longitudinal striations can be distinguished together with periodic constrictions. 

Chemically tussur is essentially the same as silk from other sources, but it is more resistant to the action of acids, alkalis, and solvent salts, as illustrated by the following comparisons (Table 5.5). 

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