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is then taken out and rinsed, to remove the acid; and afterwards washed with soap, to give a degree of softness.
This mode of bleaching by the combustion of sulphur is also used for other substances, as straw in the manufacture of hats, &c.
A superior method of employing the sulphureous acid in bleaching is the following. Water is impregnated with the sulphureous acid, and tubs are filled with this; then the stuffs are drawn through it upon reels, till they are whitened. The sulphureous acid is made by decomposing the sulphuric acid by the addition of any combustible matter capable of taking away a part of its oxygen. A cheap method of effecting this is by putting chopped straw, or saw-dust, into a mattrass, and then pouring over it sulphuric acid, and afterwards applying heat. The sulphureous acid gas will be formed, but will be combined with the water in the vessel.
The stuffs are then taken out, and left to drain upon a bench covered with cloth; a precaution which is necessary, because the wood might be decomposed by the sulphuric acid, and would stain the goods. They are afterwards washed in clear water. It is generally necessary to sulphur them twice before the white is sufficiently bright. Sometimes Spanish white is put into the water used for washing them; and they are also azured or blued by disolving some Prussian blue in the water. Nothing then remains to be done but drying, stretching, and pressing.
Bleaching of Silk.
Silk is an animal substance, and is prepared by a caterpillar, usually called the silk-worm. This
insect inhabits warm climates, and cannot be reared in this country without difficulty, nor in sufficient quantity for the purpose of procuring silk. The south of Europe and Asia are its proper countries.
The silk is spun by the silk-worm in the form of threads of a semi-transparent matter, which it winds up round itself when it passes into the crysalis state. The threads, when formed, are connected together by a viscous substance, from which they must be separated before they can be wound off, by putting them into hot water.
The silk itself is covered with a yellow varnish, which is soluble in alcaline leys; and as this varnish conceals the lustre of the silk, it is necessary to detach it. Silk is itself soluble in strong alkaline leys;} care must be taken, therefore, not to injure the silk in taking off the varnish. Water at a boiling heat has no action on silk; but steam dissolves its varnish.
In France they proceed as follows. They fill a boiler with a very weak solution of caustic soda, and place in a chamber connected with the boiler, the skeins of raw silk, wound on frames; then they close the door of the chamber, and make the solution in the boiler boil. Having continued the ebullition for twelve hours, they slacken the fire, and open the door of the chamber. The steam, which is always above 250° Fahr., will have dissolved the gum of the silk. The skeins are then washed in warm water, wrung, and boiled a second time; then washed again several times with soap, till they have acquired the necessary whiteness and softness.
It is not possible, however, to give to silk all the necessary splendour by this process alone; to complete it, the silk must be exposed to the action of
the sulphureous acid, either in the form of gas, or combined with water, as directed for wool.
Bleaching Prints and Printed Books.
An application has been made of the new mode of bleaching to the whitening of books and prints that have been soiled by smoke and time.
Simple immersion in oxygenated muriatic acid, letting the article remain in it a longer or shorter space of time, according to the strength of the liquid will be sufficient to whiten an engraving.
If it be required to whiten the paper of a bound book, as it is necessary that all the leaves should be moistened by the acid, care must be taken to open the book well, and to make the boards rest upon the edge of the vessel in such a manner that the paper alone be dipped in the liquid: the leaves must be separated from each other, so that they may be equally moistened on both sides.
The liquor assumes a yellow tint, and the paper becomes white in the same proportion; at the end of two or three hours the book may be taken from the acid liquor and plunged into pure water, with the same care and precaution as recommended in regard to the acid liquor, that the water may exactly touch the two surfaces of each leaf. The water must be renewed every hour, to extract the acid remaining in the paper, and to dissipate the disagreeable smell.
By following this process, there is some danger that the pages will not be all equally whitened, either because the leaves have not been sufficiently separated, or because the liquid has had more action on the front margins than on those near the
binding. On this account, the best way is to destroy the binding entirely, that each leaf may receive an equal and perfect immersion; and this is the second process recommended by M. Chaptal.
"They begin," says he, "by unsewing the book, and separating it into leaves, which they place in cases formed in a leaden tub, with very thin slips of wood or glass; so that the leaves, when laid flat, are separated from each other by intervals scarcely sensible. The acid is then poured in, making it fall on the sides of the tub, in order that the leaves may not be deranged by its motion. When the workman judges, by the whiteness of the paper, that is has been sufficiently acted upon by the acid, it is drawn off by a cock at the bottom of the tub; and its place is supplied by clear fresh water, which weakens and carries off the remains of the acid, as well as the strong smell. The leaves are then to be dried, and, after being pressed, may be again bound up.
"The leaves may be placed also vertically in the tub; and this position seems to possess some advantage, as they will be less liable to be torn.
"With this view, I constructed a wooden frame, which I adjusted to the proper height, according to the size of the leaves I wished to whiten.
"This frame supported very thin slips of wood, leaving only the space of half a line between them. I placed two leaves in each of these intervals, and kept them fixed in their place by two small wooden wedges which I pushed in between the slips.
When the paper was whitened, I lifted up the frame with leaves, and plunged them in cold water, to remove the remains of the acid as well as the smell; this process I prefer to the other.
By this operation, books are not only cleaned, but the paper acquires a degree of whiteness superior to what is possessed when first made.
"The use of this acid is attended also with the valuable advantage of destroying ink-spots. This liquor has no action upon spots of oil or animal grease; but it has been long known that a weak solution of potash will effectually remove stains of that kind.
"When I had to repair prints so torn that they exhibited only scraps pasted upon other paper, I was afraid of losing these fragments in the liquid, because the paste became dissolved. In such cases, I enclosed the prints in a cylindric glass vessel, which I inverted on the water in which I had put the mixture proper for extricating the oxygenated muriatic acid gas. This vapour, by filling the whole inside of the jar, acted upon the print, extracted the grease as well as ink-spots, and the fragments remained pasted to the paper.'
Bleaching of Paper.
The oxygenated muriatic acid has also been applied to the bleaching of paper, which it has rendered considerably more expeditious.
Bleaching of old printed papers to be worked up again. -Boil the paper for an instant in a solution. of soda, rendered caustic by potash. Steep it in soap water, and then wash it, after which the paper may be reduced to a pulp by the paper-mill.
Bleaching of old written papers to be worked again.-Steep the papers in a cold solution of sulphuric acid in water, after which wash them before they are taken to the mill. If the acidulated water be heated, it will be the more effectual.