BLEACHING VEGETABLE AND ANI MAL FIBERS Bleaching.—Bleaching is the proc ess of treating materials in such a way as to whiten them. Bleaching is commonly applied to textile goods, as linen, cotton, wool, and silk; also to paper, pulp, straw, ivory, wax, and animal and vegetable oils. The oper ation of bleaching textile fabrics con sists of two parts: first, removing dirt and other impurities and all for eign substances, and afterwards alter ing the natural coloring matter of the fabric by chemicals having specific bleaching properties. The preliminary operation of cleansing fabrics for bleaching is much the same as or dinary washing in the domestic laun dry. It depends upon the action of alkaline lyes and certain acids to dis solve the resinous and fatty sub stances and other impurities that may either be natural or may be intro duced into the fabrics in the process of manufacture.
The principal actual bleaching agents now employed are chlorine gas, usually combined with lime as chlor ide of lime or bleaching powder; and sulphurous acid, usually as fumes of burning sulphur. Of these the chlor ine compounds are the more power ful. Like free alkali, however, they tend, after decomposing the coloring matter, to attack the fibers of the fab ric itself and to injure them. Hence it is customary at the proper time to treat fabrics bleached by this agent with such substances as hyposulphite of soda to neutralize the excess of chlorine and prevent its further ac tion.
The various vegetable fibers, as cot ton, flax, and hemp, are composed of cellulose, a substance that withstands to a great degree the action both of the acids and alkalies used in prelim inary cleansing and the chlorine used as a bleaching agent. Anima fibers, on the other hand, as silk, wool, feath ers, and the like, contain no cellulose and are readily destroyed by these agents. Hence they are commonly bleached by the action of sulphurous acid gas. Various other chemicals have been recommended for bleach ing, but none of them are commonly employed.
Previous to the application of mod ern chemistry (during the latter part of the eighteenth century), bleaching was done without the use of chlorine or sulphurous acid, by soaking and washing the articles alternately in al kaline and acid liquids, exposing them on the grass to the action of air, light, and moisture, and sprinkling them with water several times a day.
The exact nature of the change which takes place in bleaching is not known, but it is supposed to be brought about by the action of ozone, or oxygen, in its active form. This is set free during the process of bleaching with chlorine, and is also known to be present in small quan tities in the atmosphere. The ancient method of first soaking and washing articles in lye and acids and after wards exposing them to the action of the elements, is still practiced in many localities, but the modern methods of bleaching by chlorides and sulphurous acid can be practiced successfully in any household.
Bleaching Linen. — The fibers of raw or unbleached linen contain a large amount of resinous and other impurities, so that the operation of bleaching reduces their original weight by about two thirds. These foreign substances protect the fiber from be ing injured by the alkali and acids which are used in bleaching. Hence the treatment recommended for un bleached linen is not suitable for the finer qualities of bleached fabrics, but must be modified according to the quality and condition of the goods.
The Dutch at one time had a mo nopoly in certain grades of fine linens, hence known as " hollands," on ac count of the superiority of their bleaching process. This consisted in treating the fabric by turns with al kaline and acid liquids, and exposing it on lawns or bleaching greens from March until September. Hence the origin of the word "lawn" for cer tain fine grades of linen. The Dutch process consisted of four different operations, frequently repeated: (1) Steeping in alkaline lye forty eight hours, or in pure water for sev eral days.