SOAP

water-dispersible salt of a fatty acid, commonly used for cleaning. Although the terms soap and detergent are sometimes used interchangeably, the latter usually denotes a synthetic substance. The action of soaps can be broken down into four distinct steps. After the surface being cleaned has become wet, it absorbs the soap. This is followed by dispersion of the soil from the surface into the water, where it is held in suspension and prevented from being redeposited onto the clean surface. Soaps made from animal fat and wood ashes seem to have been used only for medical purposes in antiquity; not until the 2nd century AD were they recognized as cleaning agents. Although soap was made at home for laundry purposes in the European Middle Ages and early modern times, cake soap was a luxury product that came into common use only in the 19th century. Early soapmakers dispersed wood or plant ashes in water, added fat to the solution, then boiled it, adding ashes as the water evaporated. During the process the neutral fat underwent a chemical reaction with the ash, a process called saponification. The transformation of soapmaking from a handicraft to an industry was aided by the introduction of the Leblanc process for manufacturing soda ash from brine (about 1790). Several different techniques are employed in modern soapmaking. The fat-boiling process is still used by small producers, but this process takes 4 to 11 days for saponification, the removal of glycerin from the soap, and the elimination of impurities. Soap is produced in quantity by a continuous process that takes only a few hours. Fat is split into fatty acids and glycerin, then the fatty acids are purified by distillation and neutralized with an alkali. Two other methods are used by small factories: in the cold method, a fat-and-oil mixture is agitated with an alkali solution in an open pan until it thickens; in the semiboiled method, the ingredients are heated in a kettle but not boiled. Soap is finished by pouring into frames or cooling presses, then cutting and stamping. For flakes, the soap mass is extruded into ribbons, dried, and cut. Compare detergent.