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Updated: May 21, 2025
In the silk fibre there are two distinct parts: first, the central portion, or, as we may regard it, the true fibre, chemically termed fibroïn; and secondly, an envelope composed of a substance or substances, chemically termed sericin, and often "silk-glue" or "silk-gum." Both the latter and fibroïn are composed of carbon, hydrogen, nitrogen, and oxygen.
Dilute nitric acid only slightly attacks silk, and colours it yellow; it would not so colour vegetable fibres, and this forms a good test to distinguish silk from a vegetable fibre. Cold strong acetic acid, so-called glacial acetic acid, removes the yellowish colouring matter from raw silk without dissolving the sericin or silk-gum.
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