When a fat or oil enters into chemical combination with one of the caustic hydrates in the presence of water, the process is called "saponification" and the new compounds formed are soap and glycerine, thus:
OR OH C_{ 3}H_{ 5} OR + 3 NaOH = C_{ 3}H_{ 5} OH + 3 NaOR OR OH
Fat or Oil plus 3 Sodium Hydrate equals Glycerine plus 3 Soap.
It is by this reaction almost all of the soap used today is made.
There are also other means of saponification, as, the hydrolysis of an oil or fat by the action of hydrochloric or sulfuric acid, by autoclave and by ferments or enzymes. By these latter processes the fatty acids and glycerine are obtained directly, no soap being formed.
The various and most important oils and fats used in the manufacture of soap are, tallow, cocoanut oil, palm oil, olive oil, poppy oil, sesame oil, soya bean oil, cotton-seed oil, corn oil and the various greases. Besides these the fatty acids, stearic, red oil (oleic acid) are more or less extensively used. These oils, fats and fatty acids, while they vary from time to time and to some extent as to their color, odor and consistency, can readily be distinguished by various physical and chemical constants.
Much can be learned by one, who through continued acquaintance with these oils has thoroughly familiarized himself with the indications of a good or bad oil, by taste, smell, feel and appearance. It is, however, not well for the manufacturer in purchasing to depend entirely upon these simpler tests. Since he is interested in the yield of glycerine, the largest possible yield of soap per pound of soap stock and the general body and appearance of the finished product, the chemical tests upon which these depend should be made. Those especially important are the acid value, percentage unsaponifiable matter and titer test.
A short description of the various oils and fats mentioned is sufficient for their use in the soap industry.
_Tallow_ is the name given to the fat extracted from the solid fat or "suet" of cattle, sheep or horses. The quality varies greatly, depending upon the seasons of the year, the food and age of the animal and the method of rendering. It comes to the market under the distinction of edible and inedible, a further distinction being made in commerce as beef tallow, mutton tallow or horse tallow. The better quality is white and bleaches whiter upon exposure to air and light, though it usually has a yellowish tint, a well defined grain and a clean odor. It consists chiefly of stearin, palmitin and olein. Tallow is by far the most extensively used and important fat in the making of soap.
