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bleaching

bleaching, process of whitening by chemicals or by exposure to sun and air, commonly applied to textiles, paper pulp, wheat flour, petroleum products, oils and fats, straw, hair, feathers, and wood. Chemical methods include oxidation, as by hypochlorites, ozone, and the per-compounds; reduction, as with sulfur dioxide; and adsorption, as by bone charcoal used to decolorize sugar solutions. Textiles have long been whitened by grass bleaching, a method virtually monopolized by the Dutch from the time of the Crusades to the 18th cent. They developed a technique in which goods were alternately soaked in alkaline solutions and grassed, or crofted, a procedure in which they are exposed to air and sunlight; the goods were then treated with sour milk to remove excess alkali. Later they substituted dilute sulfuric acid for the milk. In 1785 the French chemist Claude Berthollet suggested the commercial application of chlorine for bleaching, and in 1799 the Scottish chemist Charles Macintosh invented bleaching powder, or chloride of lime, the first of the modern chemical bleaches. Bleaching processes vary for different fibers. Cotton, naturally a grayish yellow, contains waxy and oily impurities that interfere with the action of dyes. It must be scoured and boiled in huge kettles (kiers) before bleaching. Grass bleaching has been combined with or superseded by chemical methods, which are deleterious unless rigidly controlled. Four degrees, ranging from quarter to full bleach, are recognized in the industry. Full bleach is reputed to weaken the fiber as much as 20%. Since chlorine bleaches react with the protein of animal fibers, silk and wool are commonly bleached with hydrogen peroxide. Although sulfurous acid or sulfur dioxide are also used for wool, they do not permanently whiten it. For effective bleaching, wool must first be scoured and silk must be degummed. Common bleaching agents used domestically are Javelle water, which is sodium hypochlorite in water, and other chlorine-based mixtures.

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bleaching

bleaching The removal or destruction of colour. In the context of food it usually refers to the bleaching of flour. It also refers to the bleaching of oils, a stage in the purification by which dispersed impurities and natural colouring materials are removed by activated charcoal or fuller's earth. See also ageing.

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