BLEACHING. (FE., Blanchiment ; GER., Bleichen.) By the term " bleaching," is understood all those processes by which certain animal and vegetable products, more especially those used in the manufacture of clothing, are made white. Whatever the processes adopted, the impurities, natural or accidental, should be more readily affected by the chemical or mechanical means employed than the materials to be bleached ; and, being thus either decolorized or removed, without appreciahle injury to the texture of the materials themselves, the latter acquire the desired purity of white, which enhances their beauty and value, or, as in the case of textile materials sometimes, renders them more fitted for being dyed or Finted. Hence the method of bleaching vegetable fibres is not at all applicable to animal fibres, the latter being readily destroyed by the chemical agents used for the former.
Bleaching is a very ancient process, its exact origin being unknown. The earliest methods must have been simple and tedious, and probably consisted in washing with water filtered through wood-ashes, and exposing to air, light, and moisture, after the manner practised by our laundry maids to-day. All the earlier accounts of bleaching processes refer to linen, since the use to any notable extent of cotton goods in Europe only dates from about the middle of the eighteenth century. About this time, the principal seat of linen bleaching was Holland, where the process consisted in steeping the " goods " in a solution of potash for several days, then in buttermilk for about a week, and in spreading them on the grass, repeating the operations till the goods were deemed sufficiently white. One of the first improvements at this period, by Dr. Home, of Edinburgh, was the use of dilute sulphuric acid in place of buttermilk, by which the duration of the process, formerly about eight months, was reduced to four months. Lime, also used at this time by the linen bleachers, seems to have been employed from a very early date ; hut whether as a direct agent or only to render the potash caustic is not perfectly clear. The greatest improvement was the application of chlorine, suggested by Berthollet in 1785, and introduced shortly afterwards into Scotland by James Watt, the eminent engineer. It ie interesting to note that the Clober Bleach Works, Milngavie, where chlorine was first applied as a bleaching agent in Britain, are etill active. Dr. Thomas Henry introduced its use to Lancashire bleachers about the same time. Chlorine was
at first applied in the gaseous form, the goads being exposed to its action while hanging up in a stove. Afterwards its solution in water, and later still, in dilute potash, were tried. The latter was known as Eau de Jayelle, and, being almoet without smell, relieved the workmen from the annoyance they formerly experienced from the gas. In 1799, Mr. Charles Tennant, of Glaegow, introduced the dry chloride of lime or " bleaching powder," now universally employed, especially in the bleaching of cotton. For the bleaching of animal fibres, wool, silk, &c., the use of weak alkalies, eoap and sulphurous acid, seems to have been in vogue for a long period.
Cotton Bleaehing.—Cotton is not bleached in the unspun etate, but alwaye in the form of yarn or thread, or of woven material or calico, The most thorough and perfect kind of calico-bleaching is the 6o-called "madder-bleach," in general use with calico-printers and dyere ; a detailed description of the various processes and machinery in actual use at the present time for this style is therefore given. For the madder bleach, it is not simply necessary that the cloth be of snowy whitenese ; it must in addition be thoroughly cleansed from all thoee foreign impurities which would resist the entranc.e of the dyes to be applied afterwards. For light and delicate plain dyes, the purer tlie white the better, but for dark shades this is not essential. When the fabric is to be printed and dyed so that some parts ehall remain white, the bleaching must combine every excellence, and all those impurities which would attract colouring matter in the " dye-beck " must be entirely cleansed away, otherwise a tinged or bad vvhite. destructive of the beauty of the print, will be the inevitable result. Tbe term " madder bleach " has been applied to this particular style of bleaching, because it is very epecially requisite for those printed goods which have subsequently to be dyed with madder, or its present substitute, artificial alizarine. In practice, the whites do invariably become tinged to a greater or less dcgree during dyeing ; but if the bleaching has been well done, the original purity of the white may be readily restored by a slight washing with soap and water, or by using a very weak solution of bleaching powder, without impairing, to any appreciable extent, the coloured parts of tbe design.