Practical Magazine 18, 1876
(Chemistry applied to the Arts, Manufactures, &c.
Dyeing, Calico Printing, Bleaching, Tanning, and Allied Subjects.)
Wool, after having been freed from grease, is bleached by the action of sulphurous acid gas as follows. An isolated chamber without chimney is selected, having a trap-door at the top, which can be open or shut at pleasure, and below a door about the height of a man, with two small openings at the corners to receive the sulphur pans. Poles about 3 yards long are placed across the chamber, and on them is laid the wet wool. To prevent the volatilised flour of sulphur from settling on it during the operation, it is wrapped in white cotton cloth.
When the chamber is ready, the trap-door is closed, and its edges are covered over with clay. Then a pan of sulphur on fire is put through each opening, and both are closed. For 100 kilogrammes (220 lbs.) of wool, 2 kilogrammes (4½ lbs.) of sulphur are employed. This substance is changed into sulphurous acid gas by means of the oxygen of the air, and this acid, condensed by the water which moistens the wool, re-acts on the colouring matter, and causes it to disappear.
At the end of twelve to twenty-four hours, according to circumstances, all the apertures are opened, that the outer air may enter the chamber, drive out the excess of sulphurous acid gas, and dry the wool. In winter, as soon as the suffocating smell is dissipated, the door and trap-door are closed, and the wool is dried with fire in chafing-dishes. Usually after the wool has been subjected to the action of sulphur, it is first put into hot water to get rid of the sulphur, and afterwards into a light soap bath to make it more mellow. It is then tinted with a carmine or an acetate of indigo, alone or with alumina.
Wool bleached by means of sulphurous acid gas soon gets yellow by contact with the air. This disadvantage may be obviated by a more or less prolonged immersion of the wool in a solution of sulphite of soda, with the addition of hydrochloric acid. The salt is put into the bath in large crystals, so that its solution in the water and decomposition by hydrochloric acid going on gradually, the wool may remain longer in contact with the sulphuric acid which is set at liberty. The bleaching is thus more complete. The yellowest and commonest wools are made of a beautiful and lasting white colour by this means. Spun wool thus bleached is always of a finer whiteness than what is so treated in the fleece.
The process of bleaching by sulphite of soda is excellent, but has the disadvantage of being rather slow. A skilful chemist has found the means of simplifying and hastening the process by the use of bisulphite of soda. This product, which is liquid, and known in commerce as leucogène, is manufactured by the inventor, M. CLAUDE, at Rouen. Bleaching by bisulphite of soda is as simple as by sulphite of soda. All that is necessary is to put into a wooden trough a certain quantity of water, add about a twentieth part of bisulphite of soda, and 2 or 3 per cent. of hydrochloric acid. The re-action will then take place. The wool plunged in this liquid being in contact with the sulphurous acid as it is forming, the bleaching takes place more quickly. On being afterwards exposed to the air and dried, it becomes perfectly white. This white may be rendered still more beautiful by the addition of a little bisulphite of indigo, so as to give the wool a slightly azure tinge.
- Moniteur de la Teinture.
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