Bleaching
Bleaching, the process of making a substance white by destroying its natural or artificial coloring matter. Bleaching is of major importance in the textile and paper industries. Bleaching is also used to lighten the color of hair, flour, wood, and other substances.
Textiles are often bleached not only to make them white, but also in preparation for dyeing or printing. Before textiles are bleached, they must be scoured—that is, thoroughly cleaned of all grease, dirt, oils, waxes, and other substances. Textile fibers may be bleached in a raw state, after they are spun into yarn, or after they are woven into a fabric.
Cotton and linen can be bleached through exposure to the sun and air. This process is slow, however, and used only for very fine linens. Most bleaching is done with chemicals, including hydrogen peroxide, sodium hypochlorite, peracetic acid, and sodium perborate. Hydrogen peroxide is the most widely used chemical for the commercial bleaching of wool and cotton fabrics. Sodium hypochlorite is used primarily with cotton fabrics; it damages animal fibers. Sodium perborate and peracetic acid are chiefly used with synthetic fibers.
Most bleaches used today destroy coloring matter through oxidation. One bleaching method, called stoving, destroys coloring matter through reduction. Stoving—which consists of exposing the fibers to sulfur dioxide in an enclosed chamber—was once commonly used for bleaching animal fibers but is now obsolete.
White garments and sheets are bleached in the home to restore their original color. Bleaches are sometimes used with ordinary detergents to help clean colorfast fabrics. Liquid chlorine bleaches are usually a 5 to 6 per cent solution of sodium hypochlorite. Chlorine bleach is suitable for cotton and most types of synthetic fibers, but should not be used with wool or silk. Milder bleaches include those with sodium perborate compounds.