A concentrated solution of soap in water becomes partially opaque on cooling, by the formation of crystals ; this, however, is not the case with a similarly concentrated alcoholic solution. This fact is applied to the manufacture of transparent soaps, the preparation of which was formerly kept a profound secret. In preparing soap of this description, ordinary soap is thoroughly dried in a stove, and dissolved in hot alcohol. All foreign matters not consisting of soap will remain un-dissolved, and must be removed in this case, with so much the more care, because they cannot remain concealed by an opaque mass as in ordinary soap. They are removed by deposition, or by a filter supported by a funnel, surrounded on the outside with hot water. The alcohol is then separated from the solution by distillation, until the residue is capable of forming a solid mass, when cooled in the metallic moulds. Transparent soap of this kind is generally too hard, and affords a lather with great difficulty.
This article was originally published with the title "Transparent Soap" in Scientific American 8, 51, 401 (September 1853)