BLEACHING is the whitening of fibrous materials used as clothing. Wool and silk, cotton and flax, the substances most usually submitted to this process, contain certain co louring matters which, though natural, are not essential constituents ; and these colour ing matters are more readily acted upon by chemical agents, and suffer decomposition with greater facility than the animal and vegetable substances with which they are combined ; so that they may be removed with little or no injury to the texture of the articles, thereby increasing their beauty, and fitting them for the processes of the dyer and calico-printer.
Bleaching is a very ancient process, practised especially in Egypt, but probably in a very simple and tedious way; the process, perhaps, consisting of little more than expo sure to air, light, and moisture. The art was scarcely known in Great Britain until about a century since, it having formerly been usual to send brown Scotch linen to Holland to be bleached, where it was done by steeping seve ral days in a solution of potash, and subse quently for nearly a week in butter-milk, and then spreading it out upon grass for some months. One of the first improvements on this tedious process was the introduction, about the middle of the last century, by Dr. Home, of Edinburgh, of dilute sulphuric acid in lieu of sour milk, by which the process, which formerly occupied from six to eight months, was reduced to four months, the acid being as effectual in one day's application as the milk in six or eight weeks. This improve ment was eclipsed by the application of chlo rine, formerly called depblogisticated marine acid, or oxymuriatie acid, which was discovered by Scheele about 1774. Berthollet suggested its application to bleaching in 1785, in a paper read before the Academy of Sciences at Paris ; and from him the process was shortly afterwards introduced into Scotland by Watt. About the same time Mr. Thomas Henry, of Manchester, introduced the process in Lancashire ; and to these two gentlemen belongs the credit of perfecting and applying in this country a process whereby as much bleaching is as well performed in a few hours, within a space a few hundred yards square, as on the old process would have required weeks of exposure upon a hundred acres of land. The chlorine was first used in a state
of simple solution in water ; but chloride of lime, commonly called bleaching powder, for the manufacture of which Mr. Tennant, of Glasgow, obtained a patent in 1799, is now almost universally employed, especially in the bleaching of cotton. Sulphurous acid gas, or the fumes of burning sulphur, is also often employed in bleaching wool and silk, as well as straw and feathers. Wax is usually bleached simply by exposure'to air, light, and moisture.
Scarcely a year elapses without developing some new processes or apparatus for bleaching, which are made the subjects of patents, though not always with advantage to the patentees. M. David, of Paris, took out a patent in 1810 for a peculiar mode of applying chlorine to the goods to bo bleached. Tho chemical ma terials for making the gas are to be provided by the bleacher ; and as the gas is generated, it is conveyed by pipes into a close chamber. The woven goods are to be laid on perforated shelves in this chamber, and thus be acted on by the chlorine. A fan or blowing machine is used to clear the chamber of gas before the goods are removed from it ; and glass windows are provided to the chamber, through which the process may be watched. Messrs. Cock sey and Nightingale, in the same year patented an apparatus to be used in bleaching, dyeing, or sizing ; it consists in a peculiar way of con ductins the cloth into and through a vessel of liquid,.and beating it on both surfaces imme diately after its emersion ; so that the fibres become impregnated, and the superfluous liquid driven off. A third patent in the same year, by Mr. Thom, relates to a mode of caus ing the woven goods to pass over two rollers, one above another, in a close chamber filled with chlorine or sulphurous acid gas.
The bleach-works of Lancashire and Glas gow are among the largest and finest of our manufacturing establishments ; exhibiting mechanical contrivance, chemical knowledge, and those powers of combination and classifi cation so remarkably developed where many hundred persons are employed.