Page:The American Cyclopædia (1879) Volume II.djvu/725

 BLEACHING 705 are immersed by passing them over a roller, and this bath answers for about 2,000 yards of material. 2. They are then washed twice in clean water at 105 F. 3. Passed three times through a soda solution of the strength of the first solution, adding half a pound of carbonate of soda after each passage. 4. Ex- posed for 12 hours to the vapor of burning sul- phur, using of this about 24 Ibs. to 2,000 yards. 5. Passed three times through a bath contain- ing 30 Ibs. of carbonate of soda to 130 gallons of water, at a temperature of 124, adding three fourths of a pound of soda after each immer- sion. 6. The cloth is again subjected to the sulphur vapor, as in the previous operation. 7. A repetition of the fifth process. 8. Washed twice in water at a temperature of 105 F. 9. Subjected to sulphur vapors for 12 hours. 10. Washed in tepid, and then in cold water. 11. Tinged blue by passing through a bath con- taining indigo and carmine. For the bleaching of silk sulphurous acid is also used, but pre- vious to its application the raw silk must, as in the case of wool, be freed of matter which would interfere with the process. Silk contains, ac- cording to its quality, from 25 to 35 per cent, of extraneous matter, which was formerly con- sidered to be a kind of gum, and is still called by that name. The investigations by M. Hoard, however, have shown this substance to consist of albumen, wax, fat, resin, and coloring matter, and to have the properties of a varnish. After numerous experiments it has been found that nothing removes this varnish so well as a hot soap bath kept somewhat below the boiling point. From 30 to 40 Ibs. of very fine soap are used for every 100 Ibs. of silk; but the pro- portions vary according to the uses that are to be made of the articles. After steeping, the silks are well washed, put into linen bags, and boiled for an hour and a half in a weaker solution of soap. Different shades of white are given to the silk, without further bleaching, by the use of very weak dyes of litmus or indigo. A pure white is obtained by the sulphuring process. The Chinese are said not to use soap in cleaning their silks. One Michel de Grub- bens, who lived in Canton a long time and practised the Chinese method, published in the memoirs of the academy of Stockholm an ac- count of it, according to which they use a small white bean, and also wheat flour and common salt. It is probable that the fineness of Chinese silk is owing much to the superi- ority of the raw material. The process of bleaching silk proposed by Baume' would be an important improvement if it were not too expensive. It consists in macerating the raw silk in 32 parts of alcohol and 1 part of muri- atic acid for about 48 hours, when the silk is quite white. Wheat straw is grown in Tuscany without reference to the grain. The seeds are sown broadcast, and the straw is cut when the grain is in the milk. It is thin and short, but of fine texture. On being cut, it is dried for a few days in the sun, then stacked in bundles, and dried in the mow for a month. After this, it is partially bleached by exposure upon the meadows to the dews and sun ; and the process is completed by steaming and sulphuring. In England, a boiling solution of caustic soda is employed to dissolve the hard natural varnish upon the outside of the straw ; after which the usual bleaching process, with sulphurous acid or chlorine, is applied. This hard coating, it is said, may also be removed with economy by several steepings in dilute al- kaline solutions, alternating with others of chlo- ride of lime and the vapor of -sulphurous acid. Chlorine is the most common agent employ- ed for bleaching a variety of other substances besides those already named ; as, for example, wax, and articles of paper, as maps, prints, books, &c. But frequently, colors imparted to cloth by strong dyes require for their re- moval different chemical reagents, as chromic acid, or the combination of this with potassa. Protochloride of tin is also employed for the same purpose. These are called discharges, and are principally made use of in calico print works. The whitening of candles, paraffine, sugar, &c., will be described in treating of those articles. Wax was formerly bleached merely by exposing it to sunlight and moist- ure ; but since the discovery of chlorine that gas has been the agent generally used. The wax is scraped into very fine shreds and put into a tub of water having a tight cover; chlorine gas is then introduced at the bottom of the tub, while an agitator stirs the water. The bleach- ing is effected in about two hours, when the wax is melted into cakes. A process has been introduced in France of bleaching wax, which is also applicable to oils, by melting it in hot steam, and subjecting it to its action in passing through a kind of worm. It is also washed with hot water alternately with the steaming. Hydrate of alumina, prepared by decompos- ing alum by carbonate of soda, has recently been substituted for animal charcoal, for decol- oring liquids. Experiments made by M. Ch. M6ric, chemist of the metallurgical works at Creuzot, show that 15 grammes of alumina may replace 250 grammes of animal charcoal, in decoloring a quart of water colored by 10 grammes of litmus ; or for sirup colored by molasses, 7 grammes of alumina were equiva- lent to 125 of animal charcoal. The alumina is, moreover, restored with less expense than the charcoal. We pass to the consideration of the process for bleaching cotton, which has long been extensively known as the " American bleaching." Before the year 1836 Dr. Samuel L. Dana, acting as consulting chemist to the Merrimack manufacturing company of Lowell, Mass., had completed an investigation on the adhering and coloring matters of the cotton fibres, which led him to devise and carry into practice the application of chemical agents in such order as to insure uniform results in bleaching. The resino-waxy envelopes of the fibres, as well as the accidental starchy, albu-