Soft Soaps are really impure solutions of potash-soaps, with glycerine, in caustic lye. They form transparent jellies, and often exhibit, in cold weather, a white graining, or 'figging,' due to alkaline stearates. The oil (e.g. linseed, or any other of those named above) is run into the 'copper,' potash lye added, and the steam turned on. The boiling is continued, latterly by close steam or fire heat, and lye added, until a small sample appears clear on cooling, and no liquid separates from it. When finished the soap is run into barrels or tins.
Toilet Soaps.—The basis or 'stock' of the better qualities is generally good curd or yellow soap, special precautions being taken to ensure absence of free alkali. The finest toilet soaps are now 'milled.' For this purpose the soap is cut into shavings, dried partially, the colouring material and perfumes added, and passed several times between granite rollers, to make it perfectly homogeneous. It is then subjected to great pressure, or 'plotted,' to form it into bars, which are afterwards cut, and stamped into tablets. The lower qualities of toilet soaps are generally made by the 'cold process.'
'Transparent' soaps are prepared by dissolving good dry soap in alcohol, pouring off the clear solution, and removing the bulk of the spirit by distillation. The remaining soap is transferred to moulds, allowed to cool, and preserved in warm chambers for several months, until it becomes quite transparent. Many transparent soaps, however, are made by the 'cold process,' their transparency being obtained by the addition of sugar. Glycerine is often added to both opaque and transparent soaps, imparting to them its characteristic emollient properties, while such substances as carbolic acid, coal-tar, eucalyptus-oil, &c. are added to soaps intended for disinfecting purposes.
The following table gives the average composition of some genuine soaps of English make:
| Fatty Anhydr. | Alkali (Na2O). | Water. | Salts. | Glycerine and Salts. | |
|---|---|---|---|---|---|
| Curd..... | 63.21 | 8.44 | 26.76 | 1.59 | .. |
| Mottled... | 64.17 | 7.80 | 25.66 | 2.37 | .. |
| Yellow (best).... | 61.95 | 7.07 | 30.45 | 0.53 | .. |
| Cocoa-nut... | 52.65 | 8.55 | 30.03 | .. | 8.77 |
| Toilet ('milled') | 80.06 | 9.56 | 9.11 | 1.27 | .. |
| (K2O). | |||||
| Soft..... | 40.36 | 9.18 | 44.78 | .. | 5.68 |
The commercial value of a soap depends upon its percentage of fatty anhydride. Soap is used otherwise than as a detergent—as a handy ointment for skin irritated by friction, as a laxative medicine, in making pills, liniments, and plasters, and as a test for the hardness of Water (q.v.).
See R. S. Cristiani, Technical Treatise on Soap and Candles (1881); W. L. Carpenter, Soap, Candles, Lubricants, and Glycerine (1885); A. Watt, The Art of Soap-making (1885; 4th ed. 1890); J. Cameron, Soap and Candles (Churchill's 'Technological' series, 1888); and C. F. Cross's 'Health Exhibition Lecture on Soap' (1884).