The cocoon loses its value when the moth is allowed to creep out, because then the filament is torn (f) and it can no longer be spun. When this has happened, the silk is treated as waste, and is spun in a manner different from the usual reeling process. For the avoidance of this, therefore, with the cocoons which are chosen for silk production, the pupas in the cocoons are killed by hot air, and in Southern countries, by the heat from the sun. Then, till required for spinning, the cocoons are stored in a dry, cool room (after the bad ones have been cast out) and before spinning, they are again sorted according to quality and size. The more frequently and exactly they are sorted, the better is the silk—careful sorting and first-class quality of the cocoon give the best foundation for high-class silk.
This grading of the silk does have a distinct bearing upon the quality and price of the finished article.
High-grade silk used in the manufacture of sheer stocking types must be passed for:-
Evenness of silk strand.
Colour and lustre.
Freedom from bad casts and large slugs.
Freedom from long knots and long loops.
Percentage of gum.