Scouring with Acids.—M. Moyret finds that au aqueous solution containing 5 per eent. of phosphoric or arsenic acid, has an action similar to that of the weak alkalies. Silk, previously moistened with dilute tepid hydrochloric acid to free it from lime, is ungummed, after boiling for three hours in the pockets with the above solutions. The process, however, has not been adopted, owing to the fact that the silk is not rendered so white, and is not so capable of being properly weighted afterwards.
Silk Yarn Bleaching and Tinting.—After scouring, the yarn is opened out, to be hung on sticks, and worked in a bath containing 10 lb. soap per 100 lb. silk, at a temperature of 49°-60° (120°-140° F.) ; it is then drained and straightened out, ready for being sulphured. The total amount of good olive-oil soap required to scour silk for white, varies from 50 to 60 per cent. of the weight of the latter in the raw state.
Sulphuring.—For this purpose, the hanks, while still damp and well straightened out, are hung in the sulphur chamber (which is of the same construction as that described under Woollen bleaching), and are there exposed to the fumes of burning sulphur for five to six hours, or even over night. Afterwards, the silk is well rinsed in a weak tepid solution of soda crystals, in order to wash out the sulphurous an'd sulphuric acids absorbed by the fibre. To ensure the thorough expulsion of the former, it is customary to hang the rinsed hanks, after wringing out the water, in a stove heated to 20°-38° (85°400° F.). With reference to the bleaching of silk by sulphurous acid, M. Moyret's opinion seems to be that probably it does not act directly in destroying the colouring matter of the fibre ; but that along with the formation of sulphuric acid there is also a production of ozone, to the bleaching properties of which reference has been made in Cotton bleaching Tinting.—This nperation is necessary to hide the faint yellow hue which the silk still retains.
Unlike the analogous operation in use with cotton and wool, the question here is not always one of simple blueing ; to suit the tastes of the merchants, the silk is actually dyed in various delicate ' shades, o. g. milk white, snow white (pure white), azure white (blueish white), Chinese white (orange, yellowish and purplish whites). To obtain pure white, a very weak neutral bath of ammonia cal cochineal and indigo carmine is used, care being taken that the dye should not too rapidly fix itself on the fibre ; to prevent this, a little ground chalk is added to the bath. A cold or slightly
tepid solution of aniline violet, with addition of a little soap, is also very much used for this shade. To obtain Chinese white, a weak soapy solution of annotto may be employed. After tinting, the silk is rinsed in fresh water and dried in a moderately warm stove, admitting as little light as possible. A passiog reference must here be made to the wild or Tussah silk, which it is not possible to bleach in the ordinary way ; the process of Tessie du Mothay, however, already described, yields very fair results.
Scouring and Bleaching Woven Silh.—Before scouring, the goods are singed with the gas flame (see Cotton bleaching). The scouring machine consists simply of a winch set over a wooden box or tub. As with the silk yarn, so here, there are two operations, " ungumming" and "boiling," both of which can be done with the same machine. For ungumming, the piece is simply winched backwards and forwards, for about an hour, in an old boiling liquor at 100° (212" F.). After winding the piece on to the winch and allowing it to drip, the liquor is run off and the tub is refilled with fresh liquor, containing 30 to 90 per cent. of white soap, and heated to incipient boiling. Tlae piece is then unwound, and again winched backwards and forwards for about two hours ; it is then rewound on to the winch, and allowed to drip for laalf an hour, when it is ready to be rinsed for dyeing in dark shades, or to be bleached for pale or wl3ite shades. Sometimes, in order to save time, the boiling is done in pockets as in the case of silk yarn. For rinsing, the winch w ith the silk wound on it is transferred to another tub containing a weak solution of soda crystals, where it is unwound and winched for a quarter of an hour, after which it is removed to be streamed in running water, and beaten, till thoroughly clean and ready for dyeing. If for sulphuring, a fresh weak soap bath heated to 49° (120° F.) is given, instead of rinsing ; and, after draining, the pieces are hung in the sulphur stove. According to the degree of purity of white required, this soaping and sulyhuring is repeated several times. J. J. H.