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Removal of Spots and Stains: Hence
The principal actual bleaching agents now employed are chlorine gas, usually combined with lime as chloride of lime or bleaching powder; and sulphurous acid, usually as fumes of burning sulphur. Of these the chlorine compounds are the more powerful. Like free alkali, however, they tend, after decomposing the coloring matter, to attack the fibers of the fabric 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 action.
The various vegetable fibers, as cotton, 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 preliminary cleansing and the chlorine used as a bleaching agent. Animal fibers, on the other hand, as silk, wool, feathers, and the like, contain no cellulose and are readily destroyed by these agents. Hence they are commonly bleached by the action of sulphurousacid gas. Various other chemicals have been recommended for bleaching, but none of them are commonly employed.
Previous to the application of modern 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 alkaline 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 quantities in the atmosphere. The ancient method of first soaking and washing articles in lye and acids and afterwards exposing them to the action of REMOVAL OF SPOTS AND STAINS 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 being injured by the alkali and acids which are used in bleaching. Hence the treatment recommended for unbleached 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 monopoly in certain grades of fine linens, hence known as "hollands," on account of the superiority of their bleaching process. This consisted in treating the fabric by turns with alkaline and acid liquids, and exposing it on lawns or bleaching greens from March until September. Hence the origin of the word "lawn" for certain fine grades of linen. The Dutch process consisted of four different operations, frequently repeated: (1) Steeping in alkaline lye fortyeight hours, or in pure water for several days. (2) Boiling in alkaline lye, also called "bucking " or "bawking." (3) Exposing on the grass for weeks at a time and sprinkling frequently with water. (4) Souring with buttermilk.
After each operation the cloth was washed in soapsuds and rinsed with water. This method is still employed and is suitable for either raw linen or cotton. It may be shortened by the employment of dilute sulphuric acid in place of buttermilk, and also by the use, under proper conditions, of chlorine in the form of chloride of lime or bleaching powder.
Soaking in water or lye, washing, boiling in lye, and exposure on the grass, are still required, and the series of operations must be often repeated.