The fatty materials, carefully strained and freed from particles of dirt and fibre, are boiled with weak caustic soda lye until combination has taken place. Saponification being complete, the solution of salt is added, then the carbonate of soda solution, and finally the silicate of soda solution, after which the soap is boiled. When thoroughly mixed, steam is shut off, and the soap is ready for framing.
The marbled hydrated soap is made from cocoa-nut oil or a mixture of palm-kernel oil and cocoa-nut oil with the aid of caustic soda lye 32-1/2° Tw. (20° B.). As soon as saponification is complete, the brine and carbonate of soda solution are added, and the pan allowed to rest.
The soap is then carefully tasted as to its suitability for marbling by taking samples and mixing with the colouring solution (ultramarine mixed with water or silicate of soda solution). If the sample becomes blue throughout, the soap is too alkaline; if the colour is precipitated, the soap is deficient in alkali. The right point has been reached when the marbling is distributed evenly. Having thus ascertained the condition of the pan, and corrected it if necessary, the colour, mixed in water or in silicate of soda solution, is added and the soap framed.
(D) Treatment of Fat with Indefinite Amount of Alkali and Separation of Waste Lye.—This is the most general method of soap-making. The various operations are:—
(a) Pasting or saponification.
(b) Graining out or separation.
(c) Boiling on strength.
And in the case of milling soap base and household soaps,
(d) Fitting.
(a) Pasting or Saponification.—The melted fats and oils are introduced into the soap-pan and boiled by means of open steam with a caustic soda lye 14° to 23.5° Tw. (10° to 15° B.). Whether the fatty matters and alkali are run into the pan simultaneously or separately is immaterial, provided the alkali is not added in sufficient excess to retard the union.
The commencement of the saponification is denoted by the formation of an emulsion. Sometimes it is difficult to start the saponification; the presence of soap will often assist this by emulsifying the fat and thus bringing it into intimate contact with the caustic soda solution.
When the action has started, caustic soda lye of a greater density, 29° to 33° Tw. (18° to 20° B.), is frequently added, with continued boiling, in small quantities as long as it is being absorbed, which is ascertained by taking out samples from time to time and examining them.