BLEACHING, in the arts, is a process that consists of a series of operations, partly chemical and partly mechanical, to which vegetable and animal fibres are sub jected, for the purpose of discharging their natural colour, and thus rendering them white, either before or after they have been manufactured. Now as almost all the articles of clothing are formed of vegetable or animal fibres, and as these are, for the most part, required to be made as white al possible, either to be worn in this state, or as preparatory to being dyed or printed, it is obvious that the art of bleaching is one of great im portance. The substances upon which the bleacher is required to exercise his art are, cotton, flax, hemp, wool, and silk. The three former being of vegetable origin, require a somewhat similar treat ment, which in many particulars differs from that which is applicable to the two latter.
The art of bleaching consists, not mere ly in discharging the colour of the thread, but likewise in temoving the colouring matter itself, as otherwise a sensible shade would be retained. In the old method this was attained by alternate exposure of the thread or cloth to the action of light, humidity, and atmospheric air, and to an alkaline ley, the cloth being macerated in a solution of potash, exposed on the field to the air and sun, and frequently sprin kled with water ; and these alternate practices being continued until the bleaching was complete. In the new me thod, the action of the oxymuriatic acid is substituted for that of the light, air, and water ; and it answers the same purpose by affording oxygen to the colouring Matter, thus impairing the colour, and probably rendering the matter soluble in the alkaline solution.
At first this process was performed by exposing the cloth to the action of the pure acid in the state of gas. It was found, however, to act unequally on the cloth, the texture being injured in one part, while in another it was imperfectly whitened. The solution of it therefore in water • was substituted, and even this requires to be considerably diluted. The bleaching liquor, according to the direc tions given by Berthollet, is prepared by putting six parts of black oxide of man ganese, and sixteen of muriate of soda, into a glass or earthen retort, or a leaden bottle, and pouring upon them twelve parts of sulphuric acid, diluted with nine of water. The retort, or bottle, is con nected by a tube with a receiver, designed to retain any common muriatic acid that may pass over : from this vessel another tube issues, which is inserted in a large wooden cask filled with water. The tube descends nearly to the bottom of the cask, so that the gas has to rise through the whole body of the water, at the same time the absorption of itis promoted by the mo tion of a circular frame placed in the mid dle of the cask, and which can be turned round at the top. The oxygenated acid is thus easily condensed. After the first disengagement of the gas has ceased, heat is applied to the retort, by placing it in a sand bath, or, if a leaden bottle be used, by placing it in a vessel of boiling water. So much water is used, that the oxygenated acid is very weak ; it requires to be stronger for coarse than fine cloth, and for linen than for cotton ; the ave-. rage quantity stated by Berthollet is 100 quarts for every pound of muriate of so da that has been used. The cloth to be bleached is prepared by macerating it in warm water for some hours, to take up what part of the colouring matter may be soluble. It is then boiled in an alkaline ley, prepared from 20 quarts of water and one part of the potash of commerce, ren dered more active by having been mixed with one-third of lime. After sufficient boiling, it is washed with water, and put into close wooden troughs, containing the oxygenated acid, in which it is allow ed to macerate for three or four hours, pressing the cloth frequently, and expos ing its surfaces to the action of the liquor.
It is thus alternately exposed to the ac tion of the alkaline ley and the oxygenated acid, till its colouring matter is complete ly extracted, or it is sufficiently bleached, which requires in general from four to eight immersions, according to the nature and coarseness of the cloth, cotton re quiring fewer immersions in the bleaching liquor than linen. The subsequent steps of the process are, to rub the cloth strong ly with soft soap in warm water. This renders the surface more smooth and uniform, and takes away the smell of the oxygenated acid, which otherwise re mains a considerable time. The cloth is again washed, and is lastly immersed for a short time in water, in which, from one sixtieth to a hundredth part of sulphuric acid has been dissolved. The cloth thus acquires a much finer whiteness from the sulphuric acid dissolving the remaining colouring matter, which has resisted the action of the alkali and oxygenated acid, as well as a small quantity of iron and calcareous earth contained in all vegeta ble matter, or even deposited in the cloth by the alkaline leys. Lastly, the cloth is generally exposed to the air for some days, and watered, to carry off any re mains of either of the acids, and to re move completely the odour of the oxy genated acid. The theory of the action of the oxygenated muriatic acid in bleaching is very simple, as stated by Berthollet. Its analogy to the common process by ex posure to the air and light, he observes, is complete. The end obtained by either is the combination of oxygen with the co louring matter of the vegetable. By this combination the colour is nearly destroy ed, and the matter on which it depends is at the same time rendered soluble in the alkaline solution. Hence the neces sity of the alternate application of these two chemical agents ; the one removing from the cloth what the other has render ed soluble, and which, although whitened, would regain part at least of its colour in time. Hence it is found, that the oxygen ated muriatic acid is, in this operation converted into common muriatic acid, and the alkaline solution is at length so leaded with colouring matter, that it becomes unfit to be used. The only difference be tween the two methods is, that in the one the oxygen is presented in a much more concentrated state than in the other, which facilitates the process, or renders it more rapid without injuring the strength of tbe fibre. At least, the only injury of this kind that can happen must arise from im proper management ; having used too strong an acid, or the not washing the cloth sufficiently after the process is finish ed. The greatest difficulty attending the use of oxymuriatic acid arose from its suffocating odour, which rendered it al most impossible to work with it in an open vessel, and any apparatus contrived to turn the cloth and expose fresh surfaces of it to the action of the liquid in close vessels, has been found imperfect. The addition of an alkali to the liquid removes in a great measure the odour of the acid, or at least prevents its unpleasant effects; and although it at the same time dimi nishes to a certain extent its bleaching power, this is more than compensated for by the advantage. The quantity of alkali added atnounts to about lib. of the pot ash or pearl-ash of commerce to the quan tity of acid prepared from 411a. of muriate of soda. And to avoid the effervescence which would arise from the diSengage ment of the carbonic acid during the combination of the oxymuriatic acid, the potash is deprived of it by the previous addition of lime, the alkaline solution af ter its operation being poured off clear.