3. Silk Industry.—Silk is secured to remove the sericin or silk-glue and adhering matter from the raw silk, producing thereby lustre on the softened fibre and thus preparing it for the dyer.
The very best soap for the purpose is an olive-oil soft soap; olive-oil and oleine hard soaps may also be used. The soap is often used in conjunction with carbonate of soda to assist the removal of the sericin, but, whilst carbonates are permissible, it is necessary to avoid an excess of caustic soda.
Tallow soaps are so slowly soluble that they are not applicable to the scouring of silk.
The dyer of silk requires soap, which is neutral and of a pleasant odour. The preference is given to neutral olive-oil soft soap, but hard soaps (made from olive oil, oleine, or palm oil) are used chiefly on account of cheapness. It is essential, however, that the soap should be free from rosin on account of its frequent use and consequent decomposition in the acid dye bath, when any liberated rosin acids would cling to the silk fibres and produce disagreeable results.
Patent Textile Soaps.—Stockhausen (Eng. Pat. 24,868, 1897) makes special claim for a soap, termed Monopole Soap, to be used in place of Turkey-red oils in the dyeing and printing of cotton goods and finishing of textile fabrics. The soap is prepared by heating the sulphonated oil (obtained on treatment of castor oil with sulphuric acid) with alkali, and it is stated that the product is not precipitated when used in the dye-bath as is ordinary soap, nor is it deposited upon the fibres.
Another patent (Eng. Pat. 16,382, 1897), has for its object the obviating of the injurious effects upon wool, of alkali liberated from a solution of soap. It is proposed to accomplish this by sulphonating part of the fat used in making the soap.
Miscellaneous Soaps.—Under this heading may be classed soaps intended for special purposes and consisting essentially of ordinary boiled soap to which additions of various substances have been made.
With additions of naphtha, fractions of petroleum, and turpentine, the detergent power of the soap is increased by the action of these substances in removing grease.
Amongst the many other additions may be mentioned: ox-gall or derivatives therefrom (for carpet-cleaning soap), alkali sulphides (for use of lead-workers), aniline colours (for home-dyeing soaps), pumice and tripoli (motorists' soaps), pine-needle oil, in some instances together with lanoline (for massage soaps), pearl-ash (for soap intended to remove oil and tar stains), magnesia, rouge, ammonium carbonate, chalk (silversmiths' soap), powdered orris, precipitated chalk, magnesium carbonate (tooth soaps).
Soap powders or dry soaps are powdered mixtures of soap, soda ash, or soda crystals, and other chemicals, whilst polishing soaps often contain from 85 to 90 per cent. siliceous matter, and can scarcely be termed soap.