The agitation being complete, chemical action takes place with the generation of heat, and finally results in the saponification of the fats.
At first the contents of the pan are thin, but in a few hours they become a solid mass. As the process advances the edges of the soap become more transparent, and when the transparency has extended to the whole mass, the soap is ready, after perfuming, to be framed and crutched.
The admixture of a little caustic potash with the caustic soda greatly improves the appearance of the resultant product, which is smoother and milder.
The glycerine liberated during the saponification is retained in the soap.
Although it is possible, with care, to produce neutral soaps of good appearance and firm touch by this method, cold-process soaps are very liable to contain both free alkali and unsaponified fat, and have now fallen considerably into disrepute.
Saponification under Increased or Diminished Pressure.—Soaps made by boiling fats and oils, under pressure and in vacuo, with the exact quantity of caustic soda necessary for complete combination, belong also to this class. Amongst the many attempts which have at various times been made to shorten the process of soap-making may be mentioned Haywood's Patent No. 759, 1901, and Jourdan's French Patent No. 339,154, 1903.
In the former, saponification is carried out in a steam-jacketed vacuum chamber provided with an elaborate arrangement of stirrers; in the other process fat is allowed to fall in a thin stream into the amount of lye required for saponification, previously placed in the saponification vessel, which is provided with stirring gear.
When the quantities have been added, steam is admitted and saponification proceeds.
(C) Treatment of Fat with Indefinite Amount of Alkali and no Separation of Waste Lye.—Soft soap is representative of this class. The vegetable fluid oils (linseed, olive, cotton-seed, maize) are for the most part used in making this soap, though occasionally bone fats and tallow are employed. Rosin is sometimes added, the proportion ranging, according to the grade of soap required, from 5 to 15 per cent. of the fatty matter.
The Soft Soap Manufacturers' Convention of Holland stipulate that the materials used in soft-soap making must not contain more than 5 per cent. rosin; it is also interesting to note that a patent has been granted (Eng. Pat. 17,278, 1900) for the manufacture of soft soap from material containing 50 per cent. rosin.