Bleaching

Bleaching, in its wider sense, the elimination of colour from a substance, but in a restricted sense the destruction of the colour of organic fibres or fabrics by chemical means, so as to leave them white in appearance. The agent most commonly employed is chlorine, bleaching powder (q.v.) being used as the source of this element. The general mode of operation may be described in the case of cotton fabrics. Before bleaching. the separate pieces are stamped for purposes of identification, then stitched together, and the loose fibres singed. They are thoroughly washed with water, mechanical contrivances being arranged for this as for all other processes. After washing they are subjected to the lime boil, i.e. passed through milk of lime and boiled with water. They are next passed through dilute hydrochloric acid, again washed, boiled with soda, some resin being also added, and subjected to another thorough washing with water. These operations have for their object the removal of mechanical, fatty and other impurities. The fabrics are now ready for treating with the bleaching liquor - chemicking - and are immersed for six or eight hours in a solution of bleaching powder which it is necessary should be perfectly clear. The bleaching powder itself produces no decolorisation, and subsequent treatment with a dilute acid is necessary, which liberates the chlorine contained in the bleaching powder. The fabric is therefore immersed in dilute sulphuric acid and finally thoroughly washed and dried.

In the case of linen, which does not bleach with the ease and rapidity of cotton, the operations of chemicking and washing with acid have to be repeated two or three times. Wool and silk are not bleached with chlorine, but by means of a solution of sulphurous acid, being first, as in the case of cotton, well washed and cleansed from all impurities.