Before scouring, the goods are singed with the gas flame (as in cotton bleaching). The scouring machine consists simply of a winch set over a wooden box or tub. As with the silk yarn, so here, there are two operations, " un-gumming" and "boiling," both of which can be done with the same machine. For ungumming, the piece is simply winched backwards and forwards, for about an hour, in an old boiling liquor at 212° F. (100° C). After winding the piece on to the winch and allowing it to drip, the liquor is run off and the tub is refilled with fresh liquor, containing 30 to 40 per cent, of white soap, and heated to incipient boiling. The piece is then unwound, and again winched backwards and forwards for about 2 hours; it is then rewound on to the winch, and allowed to drip for 1/2 hour, when it is ready to be rinsed for dyeing in dark shades, or to be bleached for pale or white shades. Sometimes, in order to save time, the boiling is done in pockets as in the case of silk yarn. For rinsing, the winch with the silk wound on it is transferred to another tub containing a weak solution of soda crystals, where it is unwound and winched for 1/4 hour, after which it is removed to be streamed in running water, and beaten, till thoroughly clean and ready for dyeing.
If for sulphuring, a fresh weak soap bath heated to 120° F. (49° C.) is given, instead of rinsing; and, after draining, the pieces are hung in the sulphur stove. According to the degree of purity of white required, this soaping and sulphuring is repeated several times. (Spons" Encyclopaedia.')
The same remarks apply to the bleaching of wool and woollen fabrics as to cotton and linen. (See Spons"Encyclopaedia,' p. 508.)
(a) Antique works in ivory that have become discoloured may be brought to a pure whiteness by exposing them to the sun under glasses. It is the particular property of ivory to resist the action of the sun's rays, when it is under glass; but when deprived of this protection, to become covered with a multitude of minute cracks. Many antique pieces of sculpture in ivory may be seen, which, although tolerably white, are, at the same time, defaced by numerous cracks; this defect cannot be remedied; but, in order to conceal it, the dust may be removed by brushing the work with warm water and soap, and afterwards placing it under glass. Antique works in ivory that have become discoloured, may be rubbed with pumice and water, and while yet wet placed under glasses. They should be daily exposed to the action of the sun, and be turned from time to time, that they may become equally bleached; if the brown colour be deeper on one side than the other, that, side will, of course, be for the longest time exposed to the sun.
(c) Expose it in the moist state to the fumes of burning sulphur, largely diluted with air.
(d) Ink stains may be removed by repeatedly using a solution of quadroxalate of potash in water. (See also ii. 33.)
Lace may be restored to its original whiteness by first ironing it slightly, then folding it and sewing it into a clean linen bag, which is placed for twenty-four hours in pure olive oil. Afterwards the bag is to be boiled in a solution of soap and water for fifteen minutes, then well rinsed in lukewarm water, and finally dipped into water containing a slight proportion of starch. The lace is then to be taken from the bag and stretched on pins to dry. (See also ii. 149.)
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 liquor, 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 on the edge of the vessel, in such a manner that the paper alone shall be dipped in the liquid; the leaves must be separated from each other in order that they may be equally moistened on both sides. The liquid 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 touch both sides of each leaf. The water must he renewed every hour, to extract the acid remaining in the paper, and to dissipate the disagreeable smell.
Printed paper may also be bleached by sulphuric acid, or by alkaline or soap lyes. (See also ii. 115.)
A lye of white soap is made by boiling in water 30 lb. of soap for every 100 lb. of silk intended to be bleached, and in this the silk is steeped till the gum in the silk is dissolved and separated. The silk is then put into bags of coarse cloth and boiled in a similar lye for an hour. By these processes it loses 25 per cent. of its original weight. • The silk is then thoroughly washed and steeped in a hot lye composed of 1$ lb. of soap, 90 gallons of water, with a small quantity of litmus and indigo diffused. After this, it is carried to the sulphuring room; 2 lb. of sulphur are sufficient for 100 lb. of silk. When these processes are not sufficiently successful, it is washed with clear hard water and sulphured again. (See also ii. 38.)
The wool is first prepared according to the purposes for which it is intended, by treating it with solutions of soap. By this process it is cleared of a great quantity of loose impurity and grease which is always found in wool, often losing no less than 70 per cent. of its weight. The heat of the lye must be carefully attended to, as a high temperature is found to fix the unctuous matter or yolk of the wool. After washing it is taken to a sulphur chamber, where it is exposed to the fumes arising from the slow combustion of sulphur, for from five to twenty hours, according to circumstances. It is again washed, and then immersed in a bath composed of pure whiting and blue. It is then exposed a second time to the fumes of the sulphur, and washed With a solution of soap, which renders it of the proper whiteness. (See also articles on Bleaching in Spons Encyclopaedia )