Soap is made by boiling down oils or fats in a water solution of caustic soda or potash. Through the acid properties of the fats, the oleine, stearine, margarine, etc., which they contain, combine with the alkali to produce the saponified compound.
Hard soap is made with soda; soft soap with potash. The more oleine in the fat, the softer the soap; the more stearine the harder. Rosin is also largely used, sometimes to the extent of one-third the weight of the soap. It increases its hardness, makes it dissolve easier in water and forms a more copious lather.
The Most Economical Soap.
Soap may be two-thirds water and still remain solid. Even dry, hard soap contains 20 or 25 per cent. of moisture. An excess of water causes soap to waste or dissolve too freely in use; hence, as soap is perpetually losing water by evaporation, the most economical to buy is that with some age and moderately dry, yet not so much dried that it will not dissolve readily and make a good lather or suds.
Effects of Strong Soap on Fabrics.
Soap must not be strong enough to injure fabrics or discharge colors, yet sufficiently powerful to render grease and dirt soluble, so that it may be washed away in water. Rosin soap hardens the fibers of wool, and alkalies, if used to excess, shrink woolen fabrics. Hard water, or that containing lime or magnesia, more or less decomposes soap, and it floats on the surface as a greasy scum. But if an oily film rises to the top of soft water, it shows that the fat in the soap is not all saponified. Soft water is better than hard for fabrics.
What Soaps Are Made Of.
Common Yellow Bar Soap contains soda with fat and rosin. White Soap consists of tallow and soda. Castile Soap is made of olive oil and soda. Common Fancy Soaps are mainly ordinary soap colored and scented. Real Brown Windsor Soap is made of goat tallow, olive oil and soda. Transparent Soaps are those which have been dissolved in alcohol. Fine Toilet Soaps are made with as little alkali as possible, of almond, palm or olive oil, suet, lard, etc., colored and perfumed.
Shaving Soaps and Creams are made either with soda or potash, of fine tallow or cocoanut oil, which has the property of making a strong lather. Mottled Soaps owe their variegations of color to the use of iron oxides. It is said that these cannot be effectively applied if the soap contains an excess of water, and that more skill is required to make good blue mottled soap than any other. The more any soap is worked over, or remelted, cooled, etc., the better it becomes.