(1) Lyons process. The bleach is an aqua regia, prepared by mixipg 5 parts of muriatic acid with 1 of nitric acid. Before being used, the mix-tare is left for at least 4 or 5 days at a gentle heat, about 77°F.(25°C). When it is to be used, it is to be diluted with about 15 times its measure of water, so as to stand at 3° to 4° Tw. This dilation is effected in large square tanks, cut out of grit-stone. The temperature of the liquid should be between 68° and 85° F. (20° to 29 1/2° C). The skeins being placed upon rods, they are plunged into the bath and worked without stopping, turning them quickly or drawing them from one end of the trough to the other. The process is generally complete in 1/4 hour; but it is often at an end in 10 minutes and even less, according to circumstances. As soon as the bleaching is complete, the silk must be taken out, for a too prolonged stay in the acid would be very injurious. After being partially decolorized, it would next be dyed yellow, and in a permanent manner This treatment, therefore, demands great care. Silks of different kinds should never be treated together, as they do not bleach with the same speed.
As soon as the desired effect is obtained, the silks are withdrawn and Immersed successively in 2 troughs full of water, in order to remove every trace of acid without delay. They are then ready for stoving Some prefer to work in the cold, as safer, though slower. Guinon, Marnar, and Bonnet employ, instead of the aqua regia a bath soured with nitro-sulphuric acid, i. e. sulphuric acid which has been allowed to absorb nitrous vapours (or a solution of chamber crystals). Chlorate of potash is also used with mineral acids.
(2) The method with peroxide of hydrogen is: - The silk is first treated with soap baths, and then boiled with concentrated soap solution, in order to deprive it of its gum. It is then treated with carbonate of ammonia. The process for bleaching is the same as that for bleaching hair (q. v.). After bleaching, a treatment in alcohol, to which some glycerine has been added, is recommended.
(3) Guinon proposes to bleach Tussah silk by steeping in soda-lye at 3° B., and at the heat of 212° F. (100° C). It loses its gam and 12 per cent, of its weight, and is rendered white without loss of lustre. The treatment must not last longer than 1/4 hour. The silk is then washed, and passed through dilute sulphuric acid. The fibre is not injured, but the affinity for colours is reduced.
(4) Palangie' and Bedu are inventors of the following process for depriving raw Tussah silk of its natural colour, and rendering it capable of being dyed in all shades by ordinary methods: - The silk, after being deprived of its skin by the ordinary method, is entered into a bromine solution of a degree of concentration varying with the colour of the silk. In this bath it is left for 1/2 hour. The silk is then entered into a bath containing a dilute solution of an acid, and in this it is also left for 1/2 hour. Several bromine and acid baths may be necessary. Tartaric and citric acids give the best result. They can, however, be substituted by alkaline solutions, of which carbonate of soda is considered the best. Sulphides and acid sulphides and also sulphurous acid can be employed for the second bath.
(5) Lecouteur and Girard's method of bleaching Tussah silk. For 1 lb.: In a cold oxygenated bath (35 pints ammonia, 1/8 volume oxygen) the silk is left for 24 hours. The bath is then heated up to 122° F. (50° C.) and kept for 12 hours at this temperature. The same operation is repeated with a new bath, after which the silk is washed in a soap bath and rinsed with cold water. A bath containing binoxide of barium in suspension, through which carbonic acid is passed, after addition of a little bichromate of ammonia, gives the same results.
(6) The following is a summary of Moyret's remarks on silk bleaching: -
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 prolonged action, especially in the case of fine silks, the fibre itself is attacked. They are used, however, and with success too, for scouring the coarser and fancy kinds of silks. The hanks of silk are hung on sticks, and worked in a tub containing the scouring liquor, as in woollen yarn scouring. For 100 lb. silk, a solution of 3 to 4 lb. solid caustic alkali in about 300 gal. water heated to 140° F. (60° 0.) is used, and the yarn is turned during 1/2 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.
This method, 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 1 to 1 1/2 hour, in a bath heated to 185° F. (85° C.) 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 2 or 3 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.
This is preeminently the best method, since it preserves and even increases the valued properties of silk, such as feel, brilliancy, etc.; 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 has 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 linseed oil wash off best; then follow the soaps made from olive oil, suet, etc. (containing stearic 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, "ungumming " (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 1/2 to 3/4 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 2 to 3 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 the 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. Scouring with Acids. - Moyret finds that an aqueous solution containing 5 per cent, 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 3 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.
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 120° to 140° F. (49° to 60° C.); 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..