In the Krebitz process lime soaps are formed, washed free from glycerin, and the fatty acids liberated by treatment with mineral acid. The processes of hydrolysis by steam in the pres ence of catalysts (lime and magnesia in the autoclave process, sulphonated fatty acid compounds in the Twitchell method) are outlined in the article CANDLE. The only other fat-splitting process of importance (principally used in India) is the fermen tation process, in which hydrolysis is obtained by the action of the ferment lipase, contained in considerable amounts in castor seeds. The process is a reproduction of the natural hydrolysis of oils by living organisms. The ferment is added, in the form of an emulsion of freshly crushed castor seeds, to the oil to be saponified, and the whole is maintained at a temperature of about 25° C until the reaction ceases. The fatty acids from the various processes are separated by settling, and washed. In most cases the acids are purified by wet distillation with superheated steam. (2) Saponification of oils and fats by caustic alkalis.—(A) by processes whereby the glycerin is not recovered from the soap: including the production of (i.) cold process soaps, (ii.) hydrated (semi-boiled) soaps and soft soaps. (B) by processes whereby the glycerin is recovered in lyes separated from the soap : exem plified by the boiling process for the production of curd and "fitted" or "settled" soaps, which is the method employed at present on the largest scale. (See below.) (3) Formation of soap by double decomposition.—This method is illustrated by the old est process of hard soap manufacture, namely, the production of soda soap by double decomposition of potash soap by salt. In another process, the soaps of the alkaline earths are made by saponification of fats with lime, magnesia, etc.; these are decom posed by sodium carbonate forming the soda soap and precipi tating the carbonate of the metal. These processes very little employed in modern practice, and are not used in America.
(2) A (i.) Soap-making by the Cold Process.—This de pends on the fact that fats of the coconut oil group are very readily saponified by relatively concentrated solutions (about 30%) of caustic alkali (lyes) at low temperatures. Further, tallow when admixed with these oils may be saponified under the same conditions. The process is simple and consists in melting the fats at about 60° C (or, in some cases, at a lower temperature) and running exactly sufficient lye with constant stirring; the mixture is thoroughly stirred ("crutched") until it thickens (perfume, colouring matter, etc., may be crutched in as required) and then discharged into the cooling "frames" which may be lagged to prevent over-rapid cooling. The process of saponifica tion completes itself in the frame with the evolution of con siderable heat ; the soap is ready for distribution in two or three days. The method has the advantage of rapidity, and the extreme simplicity of the plant renders it valuable for districts where it is difficult to obtain or transport machinery. The disadvantages are that the glycerin (unnecessary except in toilet soaps) is retained in the soap and its value lost, and that it is difficult to ensure complete saponification : excess fat renders the soap liable to rancidity, but, with modern methods of scientific control, the presence of excess alkali, which was formerly regarded as the great drawback of cold process soaps, can be avoided. The process finds considerable application in the manufacture of the cheaper toilet soaps and transparent soaps (q.v.).
by closed steam to the required concentration and the soap framed or packed into tins (soft soap). The use of soda yields hard soaps while potash produces jelly-like translucent soft soaps. No lye is separated and no glycerin recovered. If soft soap is made entirely from linseed oil the transparency is retained at low temperatures; if the stock contains notable proportions of cotton seed or maize oils the soap is liable to become dull ("blind") in winter. The use of a proportion of tallow or of caustic soda gives rise to stellate clusters of crystals of harder soaps (figging). Mar ine soaps and the bulk of soft soaps are manufactured by this process, which is more suitable for fats of high free fatty acid content than the cold process.
Attempts have been made to reduce the time necessary for saponification by hydrolysing the fats under pressure, and a continuous process has been suggested ; these methods, however, have not yet attained commercial practice.
To obtain a high-grade product by these methods of manufac ture it is essential that all the materials employed shall be of good quality; for, necessarily, all that enters the soap-pan appears in the final product.
(2) B. Boiled, Settled or Fitted Soaps.—In this process the stock (fats) is saponified with an indefinite amount of caustic soda; the excess alkali, together with the glycerin of the fats is recovered in the lyes which are separated from the soap by "graining." Soap-pan charges vary from 10-30 tons as a rule, and the process is conducted in the following stages ("changes").
(I) Pasting or Saponification.—The melted fats are intro duced into the soap-pan, a weak caustic soda lye (about o%) is added and the whole boiled with open steam from perforated coils. The injected steam suffices to keep the soap masses in a state of vigorous agitation. As saponification proceeds, stronger lye is added with continued boiling until saponification is almost com plete, as judged by the taste and texture of the emulsion formed. In order to break this emulsion and to separate the lye, the paste is subjected to (2) Graining or salting out. Boiling is continued and salt is sprinkled on the paste, or brine added, until the soap, which is insoluble in salt solution, separates. On allowing the mixture to settle, the soap rises to the surface, floating as a curdy heterogeneous mass of "open grains" enclosing small amounts of lye. The almost neutral brine solution, termed "spent lye," which contains the glycerin derived from the stock, separates below. The spent lye is run off from the bottom of the pan and subsequently treated for glycerin (q.v.) recovery. In the case of coconut and palm-kernel oils, whose soaps are soluble in salt solutions (whence their use at sea as "marine soaps"), the grain ing or "cutting" of the soap is accomplished by the use of strong caustic soda as in the clear boiling change described below. After graining the soap is just "closed" (grainy structure dissolved) by the addition of water, and the next operation (3) of "making the soap" or "boiling on strength" ("clear-boiling," or "strength change"), is performed by boiling up with excess of strong lye which again opens the soap. The mixture is boiled with continued additions of lye until the last traces of fat are saponified, i.e., until permanent "strength" or alkalinity is observed and the soap remains "open." After settling for several hours, the "half-spent lye" settles out underneath the soap. This contains considerable quantities of alkali and a small amount of glycerin; it is with drawn and used for the early stages of the pasting of a fresh batch of material.