The Probert Encyclopaedia of Science & Technology

BLEACHING

Bleaching is the act or art of freeing textile fibres and fabrics and various other substances (such as materials for paper, ivory, wax, oils) from their natural colour, and rendering them perfectly white, or nearly so.

The ancient method of bleaching by exposing the fabrics, etc, to the action of the sun's rays, and frequently wetting them, had been nearly superseded by the start of the 20th century, at least where the business is carried on on the large scale, more complicated processes in connection with powerful chemical preparations being then employed. Among the latter the chief are chlorine and sulphurous acid, the latter being employed more especially in the case of animal fibres (silk and wool), while cotton, flax, and other vegetable fibres are operated upon with chlorine, the bleaching in both cases being preceded by certain cleansing processes.

The use of chlorine as a bleaching agent was first proposed by Berthollet in 1786, and shortly afterwards introduced into Great Britain, where it was first used simply dissolved in water, afterwards dissolved in alkali, and then in the form of bleaching-powder, commonly called chloride of lime, the manufacture of which was patented by Mr. Tennant of St Rollox. Glasgow, in 1799.

In calico bleaching the preliminary process is singeing by passing the fabric over red-hot plates or through a gas-flame to remove the downy pile and short threads from the surface of the cloth. The goods next pass to the liming process, when they are uniformly and thoroughly impregnated with a supersaturated solution of lime. The next process is the bowking or boiling for several hours, after which they are washed. They are then soured by being passed through a solution of hydrochloric acid for the purpose of dissolving any traces of free lime which may have been left in the washing, and to decompose the calcareous soap formed by the bowking process. After boiling in kiers with a solution of soda-ash and rosin and another washing, the cloth is ready for the processes of chemicking or liquoring with bleaching-powder, and white-souring with a very dilute sulphuric acid. Another thorough washing concludes the operations of bleaching proper, after which the cloth goes through various finishing processes. Modifications of the same processes are adopted in bleaching linen, wool, silk, etc.
Research Bleaching