(4) Nigres from several boils of the same kind of soap can be collected, boiled, and fitted. The settled portion may be incorporated with a new charging to keep the resultant soap uniform in colour—unless this is done, the difference in colour between boils from new materials alone, and those containing nigre, is very noticeable. The nigre settled from this fitted nigre boil would be utilised in brown soap.
(5) According to its colour, and consistence, a nigre may be suitable for the production of disinfectant, or cold-water soaps.
(6) Nigre may be bleached by treatment with a 20 per cent. solution of stannous chloride—1 cwt. of this solution (previously heated) is sufficient to bleach 20 tons of nigre.
Transparent Soaps.—The production of transparent soaps has recently been fully studied, from a theoretical point of view, by Richardson (J. Amer. Chem. Soc., 1908, pp. 414-20), who concludes that the function of substances inducing transparency, is to produce a jelly and retard crystallisation.
The old-fashioned transparent soap is prepared by dissolving, previously dried, genuine yellow soap in alcohol, and allowing the insoluble saline impurities to be deposited and removed. The alcoholic soap solution is then placed in a distillation apparatus, or the pan containing the solution is attached by means of a still head to a condenser, and the alcohol distilled, condensed and regained. The remaining liquid soap, which may be coloured and perfumed, is run into frames and allowed to solidify.
The resultant mass is somewhat turbid, but after storage in a room at 95° F. (35° C.) for several months, becomes transparent.
The formation of the transparency is sometimes assisted and hastened by the addition of glycerine or a solution of cane-sugar.
A patent has been granted to A. Ruch (Fr. Pat. 327,293, 1902) for the manufacture of transparent glycerine soap by heating in a closed vessel fatty acids together with the requisite quantity of alcoholic caustic soda solution necessary for saponification, and cooling the resultant soap. It is also proposed to add sugar solution.
Cheaper qualities of transparent soaps are made by the cold process with or without the aid of alcohol and castor oil, and with the assistance of glycerine or cane-sugar.
With the continual demand for cheaper production, sugar solution has gradually, in conjunction with castor oil, which produces transparency, superseded the use of alcohol and glycerine.