The various methods adopted in soap making may be thus classified:
1. Boiling the fats and oils in open kettles by open steam with indefinite quantities of caustic alkali solutions until the finished soap is obtained; ordinarily named full boiled soaps. These may be sub-divided into (a) hard soaps with sodium hydrate as a base, in which the glycerine is recovered from the spent lyes; (b) hard soaps with soda as a base, in which the glycerine remains in the soap, e. g., marine cocoanut oil soaps; (c) soft potash soaps, in which the glycerine is retained by the soap.
2. Combining the required amount of lye for complete saponification of a fat therewith, heating slightly with dry heat and then allowing the saponification to complete itself. This is known as the cold process.
3. Utilizing the fatty acid, instead of the neutral fat, and combining it directly with caustic alkali or carbonate, which is incorrectly termed carbonate saponification, since it is merely neutralizing the free fatty acid and thus is not a saponification in the true sense of the word. No glycerine is directly obtained by this method, as it is usually previously removed in the clearage of the fat by either the Twitchell or autoclave saponification method.
In the methods thus outlined the one most generally employed is the full boiled process to form a sodium soap. This method of making soap requires close attention and a knowledge which can only be obtained by constant practice. The stock, strength of lyes, heat, amount of salt or brine added, time of settling, etc., are all influencing factors.
The principles involved in this process are briefly these:
The fat is partly saponified with weak lyes (usually those obtained from a previous boiling in the strengthening change are used), and salt is added to grain the soap. The mass is then allowed to settle into two layers. The upper layer is partly saponified fat; the lower layer, or spent lye, is a solution of salt, glycerine, and contains any albuminous matter or any other impurity contained in the fat. This is known as the killing or glycerine change. Strong lyes are now added and the fat entirely saponified, which is termed the strengthening change. The mass is then allowed to settle and the fluid soap run off above the "nigre." This operation is called the finish or finishing change.
The method may be more fully illustrated by a concrete example of the method of manufacture of a tallow base:
| Charge— | |
| Tallow | 88 per cent. |
| Cocoanut oil | 10 per cent. |
| Rosin w. w. | 2 per cent. |
| Amount charge | 10 tons |
About five tons of tallow and one ton of cocoanut oil are pumped or run into the soap kettle and brought to a boil with wet steam until it briskly comes through the hot fat. The caustic soda (strengthening lyes from former boilings may be used here) is gradually added by the distributing pipe, any tendency to thicken being checked by the introduction of small quantities of brine ("salt pickle"). If the lye is added too rapidly the soap assumes a granular appearance, indicating that the addition of same must be discontinued. Water should then be added and the mass boiled through until it again closes. When the addition of the proper amount of caustic soda is nearing its completion the soap gradually thins. The steam is now cut down to about one turn of the valve, and brine is rapidly added or salt shoveled in. In ten to fifteen minutes the steam again breaks through and, from the appearance of the soap, it can be seen whether sufficient brine has been added. A sample taken out by means of a long wooden paddle should show the soap in fine grains with the lyes running from it clear. The steam is then shut off and the soap allowed to settle from one and one-half to two hours. In all settlings the longer time this operation is permitted to continue, the better will the subsequent operations proceed.