The bleaching of linen follows closely the process used for cotton, but the solutions are made weaker, and the operation is prolonged by repetitions of the chemicking, alternating with alkaline baths. For linen yarns, the ozone process is especially adapted. A newly in vented process for linen known as the Peckham process, from its inventor, first boils the fibre in a solution of benzoic in a soap made from cottonseed oil. It is then thoroughly washed and dried, and then bleaches rapidly in a very dilute solution of sodium hypochlorite. Hemn is well boiled with carbonate of soda, or an alkali silicate, or both, and is bleached in a chlorine bath. The operation is repeated sev eral times until the fibre is white. For hemp, the strength of the chlorine bath is 2/10 of 1 per cent. Jute is first boiled in a warm soapy solution and then immersed in a weak alkali silicate, to avoid corroding the fibre. It is then bleached in a weak chlorine solution, not above a strength of 1/10 of 1 per cent.
The electrolytic method of producing chlor ine in the bleaching bath is of special value in handling cotton and hemp and other vegetable fibres in the raw state, also paper stock, cellu lose, etc.
For wool, the bleaching agents used are those which liberate hydrogen in the bath, as sulphur dioxide, gaseous sulphurous acid, sodium bisulphite, etc. A newer process uses hydrosulphurous acid. The oxidation process is in use in some bleacheries, potash permanga nate being employed, but there is danger that the liberated caustic may damage the fibre.
For silk, sulphur dioxide and sodium bisul phite have long been the dependences. The
peroxides of hydrogen, sodium and barium have also been employed. But in the latest practice neither sulphurous acid nor chlorine are permitted. The bleaching bath is made with sodium peroxide, to which is added a little sulphuric acid. Hydrogen peroxide is formed in the bath, and the coloring matter being ren dered soluble is washed out in the subsequent washings. The same bath is used for mixed goods of silk and cotton. Scouring the silk be fore bleaching has been done away with by the addition of glycerine to the bleaching bath. Bluing the fibre is now done before the bleach ing, instead of afterward, as formerly.
Fats and animal oils are bleached with sodium hydrosulphite, and this agent is equally effective with seed oils if a percentage of for maldehyde is added. Wax is bleached by stirring 10 per cent of fuller's earth into the melted wax, and then raising the temperature to about 300° F., not higher. The same process is em ployed for paraffin wax. Soaps are bleached by adding about 1/5 of 1 per cent of pure an hydrous sodium hydrosulphite, stirring it into the boiling soap. As this salt does not keep, it must be freshly prepared when needed.
For feathers, hair, sponges, leather and ivory the bleaching agent employed is hydrogen peroxide.
Bottler, M., 'Modern Bleaching Agents and Detergents' (London 1910) ; Carter, H. C., 'The Bleaching, Dyeing and Finishing of Flax, Hemp and Jute Yarns and Fabrics' (London 1911) ; Huebner, J., 'Bleaching and Dyeing of Vegetable Fibrous Material' (London 1912).