BLAST FURNACE. (See IRON.) BLEACIIING. The art of depriving stuffs and goods of the coloring matters contained within their texture, whether natural or artificial.
When calico, muslin, or other cotton fabrics have been spun and woven, they generally pass into one or other of these establishments before being brought to market. If they are to be sold in the white state, they require bleaching ; if in a colored state, they require dyeing ; if in a decorated or ornamented state, they re quire printing; and hence it arises that there are in one establishment or congre gated together, bleach-works, dye-works, and print-works. As, however, a well printed piece of cotton requires to be bleached and dyed as well as printed, the print-works have, in most cases, the means for carrying on the bleaching and dyeing as well as the printing processes; and there are thus facilities for witness ing all three operations in one establish ment. Most of these works are situated in the valleys (when not work?d by steam-power), in order to have a supply of water from the streams which flow through them.
Bleaching is now a very different pro cess from what it was in the last century. At that time it required a period of several months to bleach a piece of cloth, and this, too, only in the summer time. In some cases the cloth was sent in the spring of the year to Holland, to be bleached on the level grassy plains of that country, and returned in the au tumn; while in other cases, when bleach ed in the English fields, there was so much depredation as to lead to an unhap py system of severe laws and general distrust. Chemists were thence led to inquire whether means might not be adopted more expeditious than that of exposure to the open air of a bleach ground. Home, Scheele, Berthollet, and Henry, made successive steps in this direction, and paved the way for the in troduction of the use of bleaching-powder, by Mr. Tennant, about the year 1800. From that date the present most efficient system of bleaching has been followed in the great works of the north, modified occasionally in the minor details.
Most large bleach-works exhibit a con siderable range of buildings, comprising a croft or bleach-house, a dye-house, re servoirs and water-filters, and subsidiary buildings. The supply of water required in bleaching and dyeing is enormous, and extensive arrangements are necessary for the filtering of the water before using, since the success of the process very much depends on the purtiy of the water. Matters are then ready for the bleach ing process. The croft (so named proba bly because it renders the same service as the croft or bleaching-ground under the old system). is generally a large stone floored building, filled with coppers and vessels of various kinds, abundantly sup plied with water, and not often free from clouds of steam. Here successive wash ings, boilings, and steepings bring the cotton to a white state. In the first place the singed cloth, which has ac quired a kind of nankeen colour, is fur ther sewn up, until five hundred pieces are connected together, end to end; that is, there are 500 X 28 = 14,000 yards, or eight miles of cloth in one continuous piece. This enormous piece passes into a washing-engine, to cleanse it from the "dressing" or mucilage which the weaver had introduced into his warp. The en
gine contains an abundant and constantly renewing supply of water ; and the cloth is wound spirally round a kind of beam above it, hanging in the water in a suc cession of bends or curvatures. The cloth travels onwards, and in so doing passes twenty times through the water beneath, every part of it ascending and descending twenty times before it leaves the machine. About two hundred and fifty yards are thus washed per minute ; and the paste which is washed from the cloth is carried away by a pipe.
As the cloth leaves the washing-ma chine, it is taken by one or two men and folded backward and forward till the whole connected piece forms a cube of five or six feet. From this heap it is again removed to undergo the process of " liming." The cloth passes into a kind of boiler called a "keir," where it is ex posed for eight or ten hours to the action of a solution of lime, 40 lbs. of lime being used for the eight miles of cloth. In this keir or vessel the hot liquor is brought up a central tube in such a manner, that, being echoed or reflected from a concave surface above, it falls down on the cloth in a profuse shower, thus acting equally on the whole of the cloth. The cloth is next subjected to a second washing, to re move the lime which may be retained by its fibres. Then ensues the process of " grey souring," in which the cloth passes through a machine similar to the wash ing-machine, but containing very dilute sulphuric acid instead of water ; and af ter this there is a third washing in the machine, to remove all the adherent acid. After this comes the "first ashing." Twenty-four miles of cloth (the real ex tent of these operations, as conducted at the present day in large establishments, will be better appreciated thus than by speaking of 1500 pieces), are put into a keir, or cast-iron boiler, and exposed for sixteen hours to the action of a boiling hot solution of soda : this constitutes the " ashing." Then for a fourth time the cloth is washed, preparatory to the pro cess named " chemicking." A weak so lution of bleaching-powder, or chloride of lime, is put into a machine something like the washing-machine, and the cloth is passed through it. After lying wet in the heap for six or eight hours, to allow the " chemick " to act on the fibres, the cloth goes through the process of " se cond souring" in weak sulphuric acid, somewhat as before. It is then washed for a fifth time in the machine, to which succeeds the "second ashing ;" then a sixth washing, then a "second chemick ing," then a " third souring," and then a seventh washing. It will thus be seen that there is a succession of processes following in a certain order ; the three agents,—sulphuric acid, soda, and bleach ing-powder,—being separately applied, each more than once, and the cloth being washed in clean water after every such application. So powerful is the bleach ing-agent, that 7 lbs. of chloride of lime are said to suffice for the bleaching of 500 pieces of cloth. The machines here described are a late improvement : for until recently the cloth was dipped in tanks to be " soured" and " chemicked," and thence hauled up by poles.