SOAP AND SOAP MAKING Properties of Soap.—Garments of linen and other fabrics become soiled principally by the oily exudations of the body, as in perspiration and the natural oil of the hair, and, in the case of table linen, by animal fats, etc. The skin itself, of course, retains a considerable part of the oily sub stances not absorbed by the clothing. These greasy substances by their ad hesive quality attract and hold parti cles of dirt. When soap is dissolved in water, the neutral alkali salts be come in part separated into alkali which dissolves, and free fatty acid which precipitates. This explains why the transparency of clear water is disturbed by the use of soap even of the purest kind.
The detergent or cleansing proper ties of soap are due to the presence of free alkali, either caustic potash or soda liberated in the soapsuds. This attacks and decomposes the grease contained in soiled linen, in perspira tion, and in dishwater, unites with the fatty acids, and in turn saponifies them. The process is precisely simi lar to that of soap making. The union of the alkali set free in soapsuds with the grease of garments or dishwater produces a soapy substance which is readily soluble, and hence is easily removed by rinsing.
Free Alkali. — Since the cleansing properties of soap are due to the presence of free alkali, it may be asked why the alkalies themselves—as potash lye or sal soda, cannot be used without the trouble of uniting them with animal fats by soap making. It is true that lye and other strong alka lies have strong detergent properties. They attack, however, not only the grease, but also the fabrics themselves and rot or weaken them, and also ir ritate the skin. Hence the object of soap making is to form a compound which will release a small definite quantity of alkali at the moment that it is required.
Soap Test.—Alkali has a strong, bit ing taste. Hence the best test of soap is to apply the tongue to it. If it bites, the soap contains an excess of free alkali and is not suitable for the toilet or laundry. If it does not, it is good soap and will probably not injure the most delicate fabrics.
Importance of Soap. — Whether or not cleanliness is next to godliness, historians say that the degree of civ ilization of a nation is Indicated by the quantity of soap it consumes. The kind and quality of soap and , other cleansing articles used by a house hold is a good indication of the re finement of the family. There should be no economy in the use of soap, but since, if very freely used, it becomes unite an important item of expense, a considerable saving may be made by the use of homemade soap and other cleansing compounds.
The domestic art of soap making also has an educational value. Soap is a chemical compound and we per form a real chemical experiment every time we wash our hands or wash clothing in the laundry. Soap was made as early as the second century of the Christian era, before the mod ern science of chemistry was inaugu rated, and good soap may be made by observing the following instruc tions without troubling oneself to un derstand the chemical principles in volved. On the other hand, it is
interesting, as a matter of general in formation, to understand the chemis try of soap making.
There is a group of substances hav ing similar properties, which is known in chemistry as alkalies, and another group, having very different proper ties, which is known as acids. These two kinds of substances have a strong attraction or affinity for each other, and when brought together under suit able conditions they unite to form another class of substances, the com pounds known as salts. Common salt is a good example; it consists of an alkali, sodium, and an acid, chlorine. Most of the salts are freely soluble in water.
Soaps are alkali salts of fatty acids. The alkalies commonly used in mak ing soaps are soda and potash. All of the animal fats, and also the ani mal and vegetable oils, contain fatty acids. When the proper alkalies are brought into contact with animal fats or oils, under proper conditions, the alkalies attack the globules of fat or oil and unite with the fatty acids to form alkali salts of fatty acids—i. c., soap. This process is known as sa ponification.
To make soap it is customary to dissolve an alkali, either potash or soda, in water, forming a liquid known as lye, to dilute the lye, to then mix with it a suitable quantity of fat or oil, and to stir until saponification takes place. If the mixture is cold, the process may require several days or even months, depending upon the strength and purity of the ingredi ents. But if the mixture is raised to 219° F. by boiling, the process of saponification may take place in a few minutes or hours. Hence there are two processes of soap making— in the cold and by boiling.
The cold process, generally speak ing, produces what is known as a soft soap. This is not true or pure soap, but contains, in addition to the actual dry alkali salts of the fatty acids— I. e., real soap—certain other ingredi ents, as water, glycerin (which is con tained in all natural fats and is lib erated in the process of soap making), more or less free alkali, and other impurities. In other words, soft soap is only partially saponified. The pure or hard soap, completing the process of saponification, is obtained by boil ing soft soap until the glycerin and other impurities are absorbed by the hot lye, and by the addition of salt to remove the surplus of water. The salt, having a stronger affinity for water than the soap has, causes the water and the impurities it holds in solution to sink into the oil. The pure hard soap rises to the surface and forms a cake which may be re moved. The lye, containing glycerin and other impurities, can then be dis carded.
A better quality of soap may be obtained by melting the product of the first boiling a second time, and by adding more clean, strong lye and clear melted grease or oil, until com plete saponification takes place.