Mr. Tennant's patent for the liquid chloride of lime was afterwards set aside; but he subsequently obtained a patent for combining chlorine with lime in the dry state. This is a most valuable improvement, the dry chloride being less liable to decomposition than the liquid, and being so much more portable, the smaller manufacturers find it more advantageous to purchase the article of those whose business it is to prepare it, than to establish works for the preparation of the article themselves. The following is the process as practised in bleaching linen, or cotton cloth, or yarn. The same methods are followed as far as the fourth or fifth bucking, as described in the process of grass bleaching, only good washing is substituted for crofting. The goods are then immersed in a solution of chloride of lime, or of the chlorides of potash or soda, and are then well washed, by machinery, in pure water. They are then taken to the souring vessels, containing a portion of very dilute sulphuric acid, and when taken out of these vessels, are again well washed in water; and, lastly, they are submitted once more to the alkaline process already described.
Linen goods require, at least, three immersions in the solution of chloride of lime, followed by an equal number of alternate immersions in the sours and in the alkaline solutions, carefully and thoroughly washing them in pure water between each of these processes. Cottons, however, require fewer immersions in the bleaching liquor, which may likewise he more diluted with water for cottons than for linens. By this method of bleaching, linen goods constantly acquire a yellowish tinge; this, however, is so superficial, that mere exposure to the air for a few days generally removes it. The goods are then finished by boiling them for a short time in a diluted solution of pearlash and white soap, which removes the disagreeable odour which would otherwise be attached to articles bleached by this process. Cotton goods do not require crofting, as the yellow tinge, just mentioned, does not appear in them when finished, being removed by the sulphuric acid, although this acid will not remove it from linen goods. But the routine just described, Mr. Parkes (from whose essay on bleaching much of the preceding account is taken,) observes, is not sufficient for bleaching calicoes intended for the best madder work.
The following outline of a process adopted by a scientific printer in Scotland, for bleaching calicoes for madder work, or resist work, or for pale blue dipping, was communicated by him to Mr. Parkes, with an assurance that it may be relied on. The goods, after being singed, steeped, and squeezed, by passing between rollers, are boiled four times, ten or twelve hours each time, in a solution of caustic potash of a spec. grav. from 1.0127 to 2.0156, washing them carefully and thoroughly between each boiling. They are then immersed in a solution of the chloride of potash, originally of the strength of 1.00625, and afterwards reduced, with twenty-four times its weight of water, but as the specific gravity alone is not a perfect guide, the bleaching power of the liquid is tested by a solution of indigo of a known strength. In the above preparation, the goods are allowed to remain 12 hours, and are then, by some printers, laid, whilst wet, on the grass, and exposed to the sun and weather for two or three days. From thence they are removed to the sours, made of the spec. grav. of about 1.0254, at the temperature of 110° Fahr.; and after lying five or six hours, are taken to the wheel, and washed.
The four boilings in caustic potash, with the washings between each, are then repeated, and the goods, after being again immersed in the diluted chloride of potash, are well washed in pure water, and then rinced in common sours for half an hour. The last process is that of careful washing in clean water, after which they are immediately hung up in the airing sheds to dry gradually. Various articles, besides cloth, such as wax, paper which has become mildewed, etc. are now bleached by means of chlorine, either in the state of gas, or combined with alkalies or alkaline earths. The annexed engraving represents the apparatus employed for the preparation of chloride of potash, or soda, for the use of calico printers, a is the furnace for heating the materials which furnish the chlorine; b a cast iron vessel containing water, and forming a water bath for the reception of the still; c the body of the still made of lead, and the upper part surrounded by a deep cup d d formed in one piece with it, and containing a portion of water into which the head of the still descends to the depth of about six inches, thus forming what is termed a water-joint, which prevents the escape of the gas; e is the still head descending, as we have just stated, into the water; in the cup f is a bent funnel, through which the acid is poured into the still; g an agitator for stirring the materials in the still, working through an air-tight aperture; and h the eduction pipe, by which the gas passes into i, an intermediate vessel, partly filled with water, and designed to arrest any uncombined muriatic acid which may occasionally rise from the still during the process; k a safety tube; and l the pipe which conveys the gas into m, the large receiver made of lead, and charged with the alkaline solution; n the agitator for constantly stirring the alkaline solution; this is necessary to promote the absorption of the gas; and, in large works, the agitator is moved by power from a steam-engine; o an opening for filling the receiver, occasionally cleaning it out, etc.; p discharge cock for drawing off the saturated solution.
The junction of all the various pipes and openings are rendered air-tight by water-joints. At the first introduction of the new process, the chlorine was obtained by distilling muriatic acid upon the black oxide of manganese; but it is now procured in a simpler and more economical manner, by mixing together black oxide of manganese, common salt, and diluted sulphuric acid; various proportions are used by different manufacturers: Mr. Tennant recommends equal weights of salt, oxide, and acid, and a quantity of water equal to the measure of acid. Silk anil woollen goods being animal productions, different processes are employed in bleaching them. The colouring principle of silk being resinous, M. Baume has proposed a process for extracting it by digesting the silk in alcohol acidulated by muriatic acid, but the ordinary method of bleaching silk is the following. The silk being still raw, is inclosed in a linen bag, and boiled in a solution of soap for two or three hours, the bag being frequently turned. It is then taken out and beaten, and next washed in cold water; and, after being slightly wrung, it is a second time put into the boiler filled with cold water, mixed with soap and a little indigo, which gives it that bluish cast commonly observed in white silk.