For yellow soaps, containing a low percentage of fatty acids, solutions of silicate of soda of varying strengths are generally used.
It is always advisable to have a test sample made with the soap to ascertain what proportion and what strength of sodium silicate solution is best suited for the grade of soap it is desired to produce. It is important that the soap to be "silicated" should be distinctly alkaline (i.e., have a distinct caustic taste), or the resultant soap is liable to become like stone with age. The alkaline silicate of soda (140° Tw., 59.5° B.) is the quality most convenient for yellow soaps; this may be diluted to the desired gravity by boiling with water. For a reduction of 3-4 per cent. fatty acids content, a solution of 6° Tw. (4° B.) (boiling) is most suitable, and if the reduction desired is greater, the density of the silicate solution should be increased; for example, to effect a reduction of 20 per cent. fatty acids content, a solution of 18° Tw. (12° B.) (boiling) would probably be found to answer.
In some instances 140° Tw. (59.5° B.) silicate may be added; experiment alone will demonstrate the amount which can be satisfactorily incorporated without the soap becoming "open," but 1/10 of the quantity of soap taken is practically a limit, and it will be found that the temperature should be low; the same quantity of silicate at different temperatures does not produce the same result. Various other strengths of sodium silicate are employed, depending upon the composition and body of the soap base—neutral silicate 75° Tw. (39.4° B.) also finds favour with some soap-makers. Mixtures of soda crystals or soda ash solution with silicate of soda solution are used for a certain grade of soap, which is crutched until smooth and stiff. Glauber's salt (sodium sulphate) produces a good smooth surface when added to soap, but, owing to its tendency to effloresce more quickly than soda carbonate, it is not so much used as formerly.
Common salt sometimes forms an ingredient in liquoring mixtures. Potassium chloride and potassium silicate find a limited use for intermixing with soft soaps.
It will be readily understood that hard and fast rules cannot be laid down for "liquoring" soap, and the correct solution to be employed can only be ascertained by experiment and experience, but the above suggestions will prove useful as a guide towards good results. A smooth, firm soap of clear, bright, glossy appearance is what should be aimed at.
Filling.—Some low-grade soaps contain filling, which serves no useful purpose beyond the addition of weight. Talc is the most frequently used article of this description. It consists of hydrated silicate of magnesium and, when finely ground, is white and greasy to the touch. The addition of this substance to the hot soap is made by suspending it in silicate of soda solution.
Whatever filling material is used, it is important that the appearance of the soap should not be materially altered.
Neutralising, Colouring and Perfuming.—The free caustic alkali in soap, intended for toilet or laundry purposes, is usually neutralised during the cleansing, although some soap manufacturers prefer to accomplish this during the milling operation. Various materials have been recommended for the purpose, those in most general use being sodium bicarbonate, boric acid, cocoa-nut oil, stearic acid, and oleic acid.
The best method is the addition of an exact quantity of sodium bicarbonate (acid sodium carbonate), which converts the caustic alkali into carbonate. The reaction may be expressed by the equation:—
| NaOH | + | NaHCO3 | = | Na2>CO3 | + | H2O |
| Caustic soda | Bicarbonate of soda | Carbonate of soda | Water |