This section is from the book "Elements Of The Theory And Practice Of Cookery", by Mary E. Williams. Also available from Amazon: Elements Of The Theory And Practice Of Cookery; A Textbook Of Domestic Science For Use In Schools.
Soap. How it cleans. - It is often hard to clean and polish by main force, with the aid of water, tools, and abrasives only. What do we depend upon to "start the dirt"? Soap. And why does soap clean so easily and quickly? because it acts chemically. Water decomposes soap, setting alkali free. Alkali decomposes the grease which is usually mingled with dirt, and so loosens the dirt. It forms with grease a compound soluble in water. Soapsuds emulsifies grease; that is, it holds it suspended in particles. Soap may also act chemically in other ways on some kinds of dirt.
Soap is made out of fat or oil and an alkali (pp. 57,108). The alkali used in soap-factories is a soda compound. In a well-made soap, no fat nor alkali is left uncombined. An excess of alkali injures paint, fabrics, and the skin. All soaps contain water. Some cheap soaps contain so much water that it does not pay to buy them. Others are adulterated with material that weighs, but does not clean. White soaps are usually pure. Floating soap is made light by having air beaten into it while it is hot and soft. Such soap dissolves faster than heavier soaps. Fresh soap also dissolves fast, because it is moist. It is well to unwrap it and pile it loosely to dry. (For laundry-soaps, see p. 360.)
Three alkalies, sal-soda, commonly called washing soda, borax, and ammonia, are used in cleaning. Washing soda is the strongest of these. It comes in crystals, but it is better to use a solution. Dissolve one pound of washing soda in one quart of water in a saucepan over the fire. When it is cool, put it in a bottle and label it sal-soda solution. Do not let it touch the hands. It will make the skin sore. When needed, pour a little into the water to be used for washing or cleaning.
Soap powders consist of washing soda and powdered soap. They may contain much water, and in general are not worth their price. Sand-soap is what its name implies, a mixture of soap and fine sand. It is less used than formerly. Modern scouring soaps and powders contain some gritty mineral and soap. Some contain sal-soda also.
Kerosene, gasoline, naphtha, benzine, all products of petroleum, are valuable cleansers. Kerosene is especially useful for cleaning things which alkalies would injure, for example, polished wood. The other three dissolve grease, but are dangerously explosive, and as a rule unsafe to use indoors.
Soap, alkalies, and kerosene are all good disinfectants. A more powerful disinfectant, not a cleanser, is chloride of lime.