This allows the process to be more easily controlled and boiling over is avoided.
It is essential that the boiling by steam should be well maintained throughout the process until all carbonic acid gas has been thoroughly expelled; when that point is reached, the steam may be lessened and the contents of the vat or pan gently boiled "on strength" with a little caustic lye until it ceases to absorb caustic alkali, the soap being finished in the manner described under (D).
It is extremely difficult to prevent discoloration of fatty acids, hence the products of saponification in this way do not compare favourably in appearance with those produced from the original neutral oil or fat.
(B) Treatment of Fat with Definite Amount of Alkali and no Separation of Waste Lye.—Cold-process soap is a type of this class, and its method of production is based upon the characteristic property which the glycerides of the lower fatty acids (members of the cocoa-nut-oil class) possess of readily combining with a strong caustic soda solution at a low temperature, and evolving sufficient heat to complete the saponification.
Sometimes tallow, lard, cotton-seed oil, palm oil and even castor oil are used in admixture with cocoa-nut oil. The process for such soap is the same as when cocoa-nut oil is employed alone, with the slight alteration in temperature necessary to render the fats liquid, and the amount of caustic lye required will be less. Soaps made of these blends closely resemble, in appearance, milled toilet soaps. In such mixtures the glycerides of the lower fatty acids commence the saponification, and by means of the heat generated induce the other materials, which alone would saponify with difficulty or only with the application of heat, to follow suit.
It is necessary to use high grade materials; the oils and fats should be free from excess of acidity, to which many of the defects of cold-process soaps may be traced. Owing to the rapidity with which free acidity is neutralised by caustic soda, granules of soap are formed, which in the presence of strong caustic lye are "grained out" and difficult to remove without increasing the heat; the soap will thus tend to become thick and gritty and sometimes discoloured.
The caustic lye should be made from the purest caustic soda, containing as little carbonate as possible; the water used for dissolving or diluting the caustic soda should be soft (i.e., free from calcium and magnesium salts), and all the materials carefully freed from particles of dirt and fibre by straining.
The temperature, which, of course, must vary with the season, should be as low as is consistent with fluidity, and for cocoa-nut oil alone may be 75° F. (24° C.), but in mixtures containing tallow 100° to 120° F. (38° to 49° C.).
The process is generally carried out as follows:—
The fluid cocoa-nut oil is stirred in a suitable vessel with half its weight of 71.4° Tw. (38° B.) caustic soda lye at the same temperature, and, when thoroughly mixed, the pan is covered and allowed to rest. It is imperative that the oils and fats and caustic lye should be intimately incorporated or emulsified. The agitating may be done mechanically, there being several machines specially constructed for the purpose. In one of the latest designs the caustic lye is delivered through a pipe which rotates with the stirring gear, and the whole is driven by means of a motor.