The quantity of " chemick " used in practice for linen is always greater than that required for cotton ; but its use is nevertheless restricted, and alternated with exposure in the field. This plan is adopted from the fact that linen, although mechanically much stronger than cotton, does uot possess au equal resistance to chemical decomposition. For the same reason, the necessity for washing well between chemicking and souring, and after souring, cannot be too strongly recom mended, in order to avoid the evolution of chlorine gas within the fihre. Prolonged contact, too, with hydrochloric acid even at 2° Tw. has a weakening. action on the fibre.
The object of the rubbing, which is so essential for many qualities of goods, is to remove small specks of brownish matter called " sprits," which may appear here and there throughout the piece. The " turn-banking " referred to in the above processes consists in loosening and refolding the pieces so that every part may he e,xposed to the action of the hammers of the wash stocks, and the operation is introduced at various stages of the bleaching process, according to the appearance of the goods.
In order to see if the bleach has been successful and complete, a good test is to dip a sample of the goods in a weak solution of ammonia ; well bleached linen will not be affected by it, whereas such goods as are white, but still contain pectic matters, will be tinged yellow, aud van always be liable to become so if stored for a lengthened period.
The usual period required for bleaching brown linen is from three to six weeks, though it is quite possible to bleach many classes of goods in much less time.
Silk is bleached either as yarn or as woven material, the operations on the whole being very similar to those of Woollen bleaching. One of the great centres of the silk dyeing and bleaching industry is Lyons. Following is a resume of what M. Maiius Moyret, of Lyons, says on the subject, in his excellent Traitd de la Teinture des Soles.' Silk-yarn Neaching.—The first operation is that of " scouring," which can be performed in different ways and with different substances, e. g. with weak caustic alkalies, alkaline carbonates, soap, or acids. The object of this scouring is to remove, to a certain extent, from the raw silk, some of its envelope of glutinous matters, in order to render it lustrous, soft, and supple, and fit for being dyed.
Scouring with weak Caustic Alkalies.—These, viz. caustic potash and soda (caustic ammonia has no action), are the most active, but, at the same time, the most dangerous, to employ, since with pro longed action, especially in the case of fine silks, the fibre itself is attacked. They are used, how ever, and with success too, for scouring the coarser and fancy kinds of silk. The hanks of silk are hung on sticks, and worked in a tub eontaining the scouring liquor, as described under woollen yarn scouring. For 100 lb. silk, a solution of 3 to 4 lb. solid oaustic alkali in about 300 galls. water heated to 60° (140° F.) is used, and the yarn is turned during half an hour. It is then well washed and beaten. This plan is advantageous for coarse fancy silks, since it dissolves off the fine down of the fibres. For these qualities, the total loss in scouring is 10 to 12 per cent. of the
weight of raw silk.
Scouring with Alkaline Carbonates.—This naethod, still used in China, has, notwithstanding its economy, almost entirely disappeared from European establishments, on account of certain practical difficulties. The silk yarn is worked for from one to one and a half hour, in a bath heated to 85° (185° F.), containing for 100 lb. silk, 10 to 12 lb. soda crystals. At first, the silk swells up and becomes gelatinous, then the outer envelope dissolves off; the fibre thereby becomes finer and more lustrous. It is sufficiently scoured when it produces a rustling noise on being rubbed with the nail; it is then washed two or three times with tepid water. The loss varies from 18 to 28 per cent. Besides the delicacy of this method, it is inconvenient not to be able to boil the silk without great risk, while experience has shown that boiling will give in many respects a better article.
Scouring with Soap.—Tids is pre-eminently the best method, since it preserves and even increases the valued properties of silk, such as feel, brilliancy, &c. ; the soap used, however, should always be of the best quality. In the north of Europe, soft potash soaps, generally made from linseed oil, are used ; in the south, hard soda soaps made from olive and other oils are preferred. Of late years, soap made from oleic acid bas been more and more employed. Those soaps are to be preferred which wash off best and leave an agreeable odour. In general, those made from oleic acid and lin seed oil wash off best ; then follow the soaps made from olive oil, suet, &c. (containing stearin and margaric acids); last, and worst in this respect, comes palm-oil soap, which, on this account, has been almost entirely given up, notwithstanding its agreeable odour. For scouring silks which are to be subsequently dyed, oleic acid soap may be recommended ; but for those destined to remain white, a good olive-oil soap is best. In the latter case, two operations are necessary, " ungum ming " (degommage), and " boiling." For "ungumming," a boiling solution of 33 lb. soap to 100 lb. silk is used, the yarn being worked in this, from a half to three-quarters of an hour. Previous to placing the silk in this bath, however, it should be softened in a weak solution of soda crystals, or better still, of hydrochloric acid, and should be washed. For " boiling," the same bath may be used (if not too strongly charged with silk-glue), except for the purest whites, or when the raw silk is coloured ; in these cases, a fresh bath is imperative. The yarn is lifted from the ungumming bath, and allowed to drain ; the hanks are then wrung, sewn up in coarse hempen bags or " pockets," and boiled, during two to three hours, with a solution of 17 lb. soap per 100 lb. silk. The yarn is then rinsed in a weak, tepid solution of soda crystals, to avoid tbe precipitation of any fatty compounds on the silk, after which it is rinsed in cold water. For Japanese and Chinese silks, the loss may vary from 18 to 22 per cent. ; for European silks, 25 to 27 per cent.