It would be hard for us to imagine life without soap. Staying clean is such an important need, that most of us would think soap was one of man's first discoveries. Yet soap was absolutely unknown until the beginning of the Christian Era.
So man has had soap for less than 2,000 years!
Soap is made by the action of alkali on fats or oils. In simple terms, when these are boiled together, soap is produced. The alkalies used are usually soda and potash. How does soap clean?
There are several ideas as to how this happens. One is that soap breaks the greasy dirt into particles so small that the water can wash them away. They become emulsified, or like a milky liquid, with the water and are easily rinsed away.
Another idea is that the soap lubricates the dirt particles, making it easier for the water to remove them. In other words, the soap makes the dirt so slippery that it can not hang on the surface to which it has been attached. Water has what is called a surface tension; that is, it behaves as if it were covered with a thin, elastic film. This surface tension keeps the water from getting in and under and around small particles of dirt, soot, and dust on the skin and in soiled fabrics. Soap dissolved in water is supposed to lower this surface tension so that the soapy solution can surround the dirt particles and pry them off, making it easy to flush them away.
Soaps and other cleansing agents are often called detergents. This comes from the Latin word detergere, which means to wipe off. Many people think that a detergent is not a soap, but actually, soaps as well as special cleansing agents can be called detergents.
Modern chemistry has created powerful cleansers that are special "wetting agents." These are sometimes called "soapless soaps". The special ability of these to clean is due to the way they break down the surface tension of water. They are thus able to penetrate especially well under all kinds of dirt. Wetting agents are used in shampoos, washing powders and in toothpastes.
There are many kinds of soap made for special uses. Scouring soaps contain abrasive material. Naphtha soaps contain naphtha for cutting heavy grease. Saddle soaps have a little wax which remains on the leather when dried. Castile soap is made with olive oil. Special soaps made with natural ingredients have been developed for dealing with acne and head lice.