The quantity of pearl ashrequired for bleaching linen in general, amounts to or of the weight of the goods to be bleached. This quantity, how ever, is not to be used all at once • but is to be di vided into six or eight portions, to be employed each in as many bucking processes. When the goods are boiled in an alkaline ley, the boiling is usually con tinued for four or six hours. In bucking, the process is generally continued till the liquor is so far evaporat ed, that the whole of it it hich remains is retained by the goods themselves. This generally requires from six to eight hours. • Heavy yarns for making ducks and similar fabrics are most advantageously bleached before being woven into cloth. These yarns are usually boiled with from 16 to 20 per cent. of their pearl ash, divided among two, three, and sometimes four boils, in proportion to the nature of the fabric, and the degree of whiteness required. When two boils are required, th of the weight of the goods may be used in the first, and in the second boil. If the goods are difficult to bleach, or if agreater degree of whiteness be required, it is sometimes customary to give them a third, or even a fourth boil, diminish ing the allowance of potash each time. In the last boil, one-third or one-half of the potash is frequently kept out, and its place supplied by an equal quantity of soft soap. When the weather is favourable, the goods are sometimes exposed on the grass after each boiling.
3. The third process is to expose the goods to the action of oxymuriatic acid in some one of the three states described in a preceding part of this article,—namely, dissolved in water, combined with lime in the state of liquid oxymuriate of lime, or in the state of solid oxymuriate of lime. The first of these states is the most economical ; but its very noxious odour renders its application scarcely practicable. on a great scale. The second state consists of two atoms of oxymuriatic acid, combin• ed with one atom of lime, or it is a bichloride of lime ; and the third state, or the dry powder, is a compound of one atom of oxymuriatic acid and one atom of lime, or it is a chloride of lime. Of these two last substances there can be little doubt that the first, or the one made in the liquid way, is the most efficacious. But we shall suppose the dry chloride of lime to be the substance used ; as its employment is likely to increase very much, cially among those bleachers who are the most ly to require instructions. Besides, it is not diffi cult to apply the observations made on chloride of lime to the liquid bichloride.
The quantity of oxymuriate of lime required for bleaching linen varies considerably, according to the nature of the linen operated upon,--according to the season of the year, and the degree of exposure on the grass,—and according to the whiteness re quired. But we may state the average quantity as varying between and of the weight of the goods employed. This quantity may be divided in to three or four processes. If three processes be reckoned sufficient, of the oxymuriate may be expended on the first operation, Aths in the se cond, and -ths in the third. If four processes are
considered as requisite, then the of the oxymu riate should be used in the first operation, Atha in the second, ths in the third, and -Atha in the fourth. Two wine pipes may be employed for the solution of the bleaching powder. They should be placed on one end, the other end being open ; and a plug-hole should be made in each, about 10 or 12 inches above the bottom. From 20 lbs. to 100 lbs. of the bleaching powder is to be put into a small tub or bucket, where it is to be well bruised and mixed with a little water. This mixture is to be thrown into one of the wine pipes, more water is to be added, and the whole carefully stirred toge ther for a few minutes. A cover is then to be put upon the pipe, and the whole is allowed to stand till the insoluble part of the powder, consisting of quicklime, subsides below the plug-hole. The clear solution, called stock-liquor, is now to be drawn of through the plug-hole, and may either be used im mediately, which is most expedient, or it may be kept under a close cover till wanted.
Repeated portions of powder may, in this manner, be dissolved in the same pipe, till the sediment as cumulate to the height of the plug-hole. When this happens, fresh stock-liquor must be prepared in the other pipe. But instead of using pure water, as in the first operation, the sediment in the first pipe should be repeatedly washed, by filling up the pipe with water,—stirring the sediment well,—allow ing it to settle, and then drawing off the clear liquor by the plug-hole. These washings, as long as they , show any bleaching power, are to be used in the second pipe, instead of pure water, for preparing stock-liquor. The sediment, in every future opera tion, should be exhausted in a similar manner.
The method is as follows : One part of the best indigo is dissolved in nine parts of strong sulphuric acid, and the solu tion is mixed with 990 parts of water, making a solution, part of which is indigo. Of this liquid a quantity is to be poured into the test-tube, so as to fill it up to 0, or the commencement of the scale. The bleaching liquor, whose power is to be tried, is then to be dropt gradually in and mixed with the blue liquor, by shaking the tube from time to time, till the blue is changed into a clear brown. As soon as this takes place, the degree of the scale to which the mixture reaches is observed, and the figure.marked at that degree indicates the strength of the steep-liquor. The lowest on the scale is, of course, the strongest in bleaching power, being ca pable of destroying most colour. The liquor, whose strength is thus ascertained, is deno minated Steep-liquor, of 1, 2, 3, 4, 5, and 6 de grees ; the last of which is the weakest ever used for any kind of goods. By adding stock-liquor, when the steep-liquor is too weak, and water when too strong, this liquor may be obtained of any strength which is required.