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The organ destined to the production of the silk is made up of two long glands positioned symmetrically on either side of its body. At the ends of the two small channels where these come together to form the spinneret, are the two small salivary glands, known as the De Filippi glands, which produce a sort of lubricant, a clear, viscous fluid secreted in the form of a thread. The caterpillar, by continually moving its head in a figure of eight motion, facilitates the ejection of the liquid silk from the spinneret; this solidifies on contact with the air, thus giving rise to two joined baves of fibroin glued together by the seracin.
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Raw Silk and Boiled Silk
The raw filament is normally made up of 74% fibroin and 23% seracin, the remainder being composed of colorant substances. The seracin, which makes the silk opaque and rough, is completely dissolved in hot, soapy solutions. Silk in which the seracin has not been removed takes the name of raw or crude silk; this is opaque and rough and fairly stiff, white or yellowish gold in colour. When the raw silk is subjected to degumming by washing, then boiled-off or degummed silk is produced, which is soft, smooth and brilliant white in colour.
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Weighted Silk
Since the boiling-off or degumming produces a considerable reduction in the weight of the silk, due to the loss of part of the sericin (approximately 25%), in order to compensate or reduce the economic damage deriving from this, the silk is subjected to treatment in solutions of mineral salts, which adhere to the fibres and increase their weight. This treatment, which is known as weighting (or loading) can be repeated a number of times until the original weight is recovered (equal weighting) or even exceeded. In the latter case, the resistance of the fibre to wear is considerably reduced.
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