Fish or marine animal oils—whale, seal, etc., once largely used as raw material for soft soap, have been superseded by vegetable oils.
The materials must be varied according to the season; during hot weather, more body with a less tendency to separate is given by the introduction of oils and fats richer in stearine; these materials also induce "figging".
The most important material, however, is the caustic potash lye which should average 40° Tw. (24° B.), i.e., if a weak solution is used to commence saponification, a stronger lye must be afterwards employed to avoid excess of water in the soap, and these average 40° Tw. (24° B.). The potash lye must contain carbonates, which help to give transparency to the resultant soap. If the lye is somewhat deficient in carbonates, they may be added in the form of a solution of refined pearl ash (potassium carbonate).
Caustic soda lye is sometimes admixed, to the extent of one-fourth, with potash lye to keep the soap firmer during hot weather, but it requires great care, as a slight excess of soda gives soft soap a bad appearance and a tendency to separate.
The process is commenced by running fatty matter and weak potash lyes into the pan or copper, and boiling together, whilst the introduction of oil and potash lye is continued.
The saponification commences when an emulsion forms, and the lye is then run in more quickly to prevent the mass thickening.
Having added sufficient "strength" for complete saponification, the boiling is continued until the soap becomes clear.
The condition of the soap is judged by observing the behaviour of a small sample taken from the pan and dropped on glass or iron. If the soap is satisfactory it will set firm, have a short texture and slightly opaque edge, and be quite clear when held towards the light. If the cooled sample draws out in threads, there is an excess of water present. If an opaque edge appears and vanishes, the soap requires more lye. If the sample is turbid and somewhat white, the soap is too alkaline and needs more oil.
The glycerine liberated during saponification is contained in the soap and no doubt plays a part in the production of transparency.
Hydrated soaps, both smooth and marbled, are included in this classification, but are soda soaps. Soap made from cocoa-nut oil and palm-kernel oil will admit of the incorporation of large quantities of a solution of either salt, carbonate of soda, or silicate of soda, without separation, and will retain its firmness. These materials are, therefore, particularly adapted for the manufacture of marine soaps, which often contain as much as 80 per cent. of water, and, being soluble in brine, are capable of use in sea-water. For the same reason, cocoa-nut oil enters largely into the constitution of hydrated soaps, but the desired yield or grade of soap allows of a variation in the choice of materials. Whilst marine soap, for example, is usually made from cocoa-nut oil or palm-kernel oil only, a charge of 2/3 cocoa-nut oil and 1/3 tallow, or even 2/3 tallow and 1/3 cocoa-nut oil, will produce a paste which can carry the solutions of silicate, carbonate, and salt without separation, and yield a smooth, firm soap.