106 DYEING AND CALICO PRINTING.

Bleaching Silk,

Silk as obtained from the cocoon is unfit for use, even when it is desired thatit should be woven into those extremely fine tissues known as silk gauze,which is made of raw silk thread. For this purpose it is necessary that theraw silk from the cocoon should be submitted to some mechanical operationby which a number of the fibres may become united or spun together. Butour province is not to treat of the processes of spinning; we shall thereforeproceed to the means of removing the colouring-matter from the raw silk, as wellas the gummy matter and wax. The oldest process in use for this purpose isboiling with soap, and is the only one which can be advantageously as well assafely employed. Raw silk loses during this process 25 to 30 per cent of itsweight. At the first boiling the quantity of soap taken is 30 per cent of theweight of the silk; a second boiling is effe&ed with 15 per cent of soap.Great care is required in the management of these operations, as experiencehas proved that the silks from Piedmont, the Cevennes, and Bengal cannotwithstand a prolonged adtion even of soap without becoming very seriouslyweakened in strength.

Sometimes silk is treated with a weak alkaline bath composed of 12 per centof the weight of the silk of caustic soda; but although by this operation silkloses only 12 per cent in weight it becomes very dull and the fibre brittle.Another method of treating silk is to immerse it for a few minutes in a bathcomposed of 1 part of nitric acid, 4 parts of hydrochloric acid, and sufficientwater to obtain at a temperature of 30° to 35 0 a liquid of a density of 15 0 to18 0 Baume. The silk is immersed in this liquid after having been steeped ina tepid soap bath containing 10 per cent of soap. As soon as the silk placedin the acid bath assumes a grey tinge it should be removed, washed thoroughlyin a large quantity of cold water, and next submitted to the adtion of thefumes of burning sulphur. In order to augment the whiteness of the bleachedsilk it is sometimes thought necessary to employ a blue colouring-matter.Baume has introduced the following method for bleaching and softeningsilk:—The raw silk is steeped in cold water for a few days to dissolve thegelatine contained. It is next placed for thirty-six hours in alcohol, to whichpure hydrochloric acid has been added (96 kilos, alcohol of o , 84 sp. gr.,and 400 grins, of pure hydrochloric acid). The silk is afterwards washed incold water and dried. This method is, however, too expensive for use on thelarge scale, but the results are excellent. As regards the water to be employedin the bleaching of silk, the purest and the freest from lime is' the best; thequality of the water is, in France, considered of so much importance that thesilk is sent to localities, especially in the Department de l’Ardeche, where thewater, owing to the prevalence of granite rock, is almost entirely free fromlime salts. Silk is very readily injured by the insoluble lime soap which isformed when soap is dissolved in water containing soluble salts of lime, itssoftness, strength of fibre, and colour being impaired.

The loss of 25 per cent in weight of so valuable a substance as silk has hadthe effedt of turning the attention of many to the devising of processes forremoving the colouring-matter without taking away the gummy substanceand the wax which contribute to so large a portion of the loss, but the attemptis futile on the ground that silk cannot be dyed, or at least not uniformly dyed,unless the whole of the gelatine, wax, resinous, and colouring-matters are