Bleach
Although ancient methods of bleaching remain unknown, historians have evidence that early civilizations must have known how to bleach fabrics. White cloth was produced by the ancient Egyptians, Babylonians, Phoenicians, and Hebrews, as well as by the Greeks and Romans. After the Crusades of the 1100s and 1200s, the practice of bleaching fabric spread throughout Europe. In the old days, people simply spread wet cloth on the ground outdoors and left it to dry in the sunlight until it turned white, which could take weeks or even months. This process came to be called crofting, after the Scottish word for a small meadow (croft). As early as 1322, crofting was practiced on bleaching grounds in England near Manchester. In Scotland and Ireland, some people still bleach their cloth on the grass in this way. High-quality linen that was dried on plots of grass became known as lawn.
By the 1700s, Dutch weavers had improved the bleaching process and emerged as the leaders of Europe's bleaching industry. They discovered that linen, which was still the most common type of cloth, could be bleached more efficiently by first soaking it in lye (a concentrated alkaline solution of potassium or sodium hydroxide). After the lye was washed out, the linen was spread on the ground as usual. After repeating this step a few times the Dutch soaked the linen in buttermilk, or soured milk, then washed it and dried it outdoors again. Although major bleaching operations were known outside Holland, the Dutch enjoyed a near-monopoly on bleaching linen through the 1700s. Fabric produced by the Dutch process was called holland cloth. However, this process was problematic in that it could take several months, especially in northern countries with limited sunlight. Furthermore, it used up large amounts of valuable space.
In 1756, scientists found that dilute sulfuric acid would work better than buttermilk and the time required for the bleaching process was greatly reduced. An even more dramatic improvement in bleaching technology resulted from the discovery of chlorine in 1774 by Swedish chemist Karl Wilhelm Scheele (1742-1786). French chemist Claude Louis Berthollet (1748-1822) discovered that this gas is a very effective bleaching agent. Berthollet, who was director of a French tapestry factory, developed a method of using chlorine to bleach textiles. In 1785, he introduced a bleaching liquid called lye de Javelle and publicized his technique without patenting it. When James Watt learned of the method, he passed the information on to Scottish chemist and manufacturer Charles Tennant, who began using the bleaching liquid in Glasgow. But the chlorine gas needed for the liquid bleaching process was not readily available, so Tennant invented a more convenient bleaching powder and introduced it in 1799. The solid powder, which was made by combining chlorine with slaked lime (calcium hydroxide), was much easier to handle and ship to other fabric manufacturers. When added to a little dilute acid, the powder released the chlorine gas which bleached the cloth very quickly. By the 1830s, factories were churning out huge quantities of bleaching powder for textile use. This abundant supply of chlorine bleach helped stimulate the cotton industry.
The fabric industry found hydrogen peroxide to be another useful bleaching chemical. Peroxide is used on some fabrics, such as wool and silk, which can not tolerate exposure to chlorine. About 95 percent of textiles that require whitening are now bleached with hydrogen peroxide which, like chlorine, is a powerful oxidizing agent. Pure hydrogen peroxide was first prepared by Louis Jacques Thenard (1777-1857) in 1818. Even though it costs more than chlorine, hydrogen peroxide is often preferred because its action on fibers is milder, it leaves no undesirable residues, and the bleaching process is faster. Several sulfur compounds, which act as reducing agents rather than oxidants, are also used in bleaching wool and silk fabrics, as well as feathers and straw.
Historically a variety of other industries have used similar bleaching technology. For example, in the paper industry, wood pulp is usually treated with chlorine compounds first followed with hydrogen peroxide as a final bleaching step. This two step process improves the pulp's ability to stay white through subsequent paper manufacturing steps. Other industries which extensively use bleaching technology include the flour industry, which uses chlorine compounds to bleach over 80 percent of all wheat flour produced in the United States, and the petroleum industry which uses chlorine bleach to deodorize gas filters. Chlorine compounds are also known for their antibacterial properties and are used in certain water treatment processes.
Through the 1990's chlorine-based products have remained the preferred method of fabric bleaching for the consumer. However, perborate bleaches are becoming increasingly popular because they can be safely used on a variety of fabrics. Improved bleaching performance is expected to come from other oxygen-containing bleaches such as percarbonates which are expected to play an important role in the future of commercial bleaching.
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