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Updated: May 19, 2025
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.
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 caustic alkalis, if sufficiently dilute, will dissolve off the sericin and leave the inner fibre of fibroïn; but they are not so good for ungumming silk as soap solutions are, as the fibre after treatment with them is deficient in whiteness and brilliancy.
Here there is thus one element more than in the vegetable fibres previously referred to, namely, nitrogen; and this nitrogen is contained in all the animal fibres. The outer envelope of silk-glue or sericin can be dissolved off the inner fibroïn fibre by means of hot water, or warm water with a little soap.
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