Manufacture.— Soap is made generally by one of three processes: (1) that in which the glycerine of the fats is set free and separated out; (2) that in which the glycerine is set free, but not separated; (3) that in which there is di rect saponification and no glycerine is produced. More than 90 per cent of the soap produced is made by the first plan, the by-product glycerine being very valuable. The oils and fats in pre determined proportions are measured into the "kettle," a large cylindrical tank which may be as much as 25 feet in diameter and 30 feet deep. This kettle has an open steam coil in the bottom to supply the heat. Part way up the side is an outlet pipe with a valve through which the crude soap is drawn into the crutcher after the saponification is complete. At the bottom of the kettle is another outlet through which is drawn the watery residue after the soap has been separated out. Following the charging of the kettle with fats a measured por tion of a dilute lye made with caustic soda is run in and the steam turned on, and the con tents of the kettle vigorously boiled. As the soda unites with the fat more lye.is added at intervals, care being taken that at no time until the saponification is almost complete shall there be an excess of alkali. When the contents of the kettle appear clear and homogeneous, dry salt is shoveled in. As soap is but sparingly soluble in brine the action of the salt is to separate the mass in the kettle into two layers, the upper one being of the separated soap and the lower one a mixture of brine, glycerine and excess lye. This step in the process is called because of the granular appearance of the soap. The amount of salt necessary de pends upon the kind of oil and fat used as the raw material, some fats requiring much more than others. The salt is added in small instal ments until the separation is complete. After standing several hours the lye is drawn off from the bottom of the kettle and clear water turned in. If the soap is to have an admixture of resin, it is added at this stage. A propor tion of caustic soda is added to remove im purities, and the boiling is renewed, and the soap again becomes a clear mass. It is salted out a second time, and the process repeated, if it seems desirable, with the addition of lye only. The next step in the process is °fitting.° Water is added by degrees and boiling continued until the soap gains the proper appearance, readily recognized by the experienced soap boiler. It is then allowed to settle, which if the batch is large may take four or five days, more or less, according to the raw materials used and the season of the year. The batch is then found to be in four distinct layers: (1) a crust of or soap froth; (2) the good soap, con taining 63 or 64 per cent of fatty acids; (3) the or weak, dark-colored soap, con taining about 33 per cent of fatty acids; (4) watery lye. In making cheap soaps the second layer is run directly into the soap casings in which it stands until cold and solid. For a finer grade it is run first into the a mixing machine, where it may be by adding sodium carbonate, sodium silicate, sodium borate, naphtha (up to 10 per cent), perfumes and coloring matter. After some hours of crutching it goes to the frames to solidify. Special cooling machines have re cently come into use whereby the cooling proc ess has been reduced from several days in the ordinary frames to two how's, or even less. The device consists in forcing the liquid soap into metal casings the sides of which are con tinually cooled by the passage of currents of cold water. By the turning of a screw the casings are released, and the blocks of soap readily removed. When hard enough the blocks are put through the which divides them into slabs by forcing piano-wire cutters through the mass. They are then cut twice again, lengthwise and crosswise, into cakes which are placed on racks to dry. When they have dried out to the proper hardness they go to the presses, where each cake receives definite form and the imprint of the maker. Thence they go to the machine which wraps them preparatory to boxing for the market.
Toilet soaps are usually the process beginning with shaving or chipping of the blocks of soap as they come from the frames. These chips are allowed to dry hard, perfume and coloring matter are added and they are then run several times through a mill of rollers which transforms them into thin flakes while perfecting the thorough mixing of the ingre dients begun in the crutcher. When the soap flakes are entirely homogeneous they go into a machine called the "plodder;' a cylindrical or conical casing in which revolves a longitudinal screw. The screw forces the soap forward under great pressure and it is finally exuded through a form plate in a continuous bar hav ing the outline of the cake in which it is eventu ally sold. The bar is cut into sections of the proper size and these receive their final shape in a press.
The second process of soap-making, in which the glycerine is set free but not separated, is used in the manufacture of soft soaps. Large quantities of soft soap are used in the textile industry. They are made chiefly of linseed oil, corn oil, whale and fish oils and in the finer grades of olive oil. Special white soaps are made with cocoanut oil and some tallow. As previously noted the lye used in making soft soap is made from caustic potash. The lye is added by degrees as in making hard soap, and the boiling is done with a closed instead of open steam coil. The color of the finished soap depends largely upon the kind of oil used, but to a considerable extent upon the care taken in the making. Good soft soap contains about 43 per cent of fatty acids, 10 per cent of alkali and 47 per cent of water. In some grades a per centage of saponified resin is added, but this is highly objectionable in certain textile soaps, and the practice is, therefore, limited.
The third process, that in which the glycer ine is not formed, is known also as the "cold process.' The soap is not boiled, the oils and alkali being carefully proportioned to form complete neutralization. While this process was formerly restricted to cocoanut oil, it has proved workable with other oils and fats when the lye is made strong. It is required, how that the fats be absolutely clean, as all of the ingredients remain in the soap. The caus tic, also, must be of the highest grade of purity. The fats and oils are warmed to the melting point and a little beyond, and placed in a crutcher, the lye, heated to 80° F., being run in gradually with constant stirring. Replacing a part of the soda lye with potash, up to 20 per cent, improves the soap. When the appearance of the soap indicates that saponification is well established, the soap is run into the frames and left to stand for a week, being covered to pre vent it from cooling too rapidly. This method is very popular with makers of toilet soaps, as it is available for small batches, and does not require the costly equipment of the soap-boiling plants.
Specific Silicated soaps are produced by mixing silicate of sodium (soluble glass) with ordinary soap. These soaps are cheaply produced, and have very considerable detergent power.
Toilet soaps are produced by perfuming the best ordinary curd-soap with essential oils, or sometimes by saponifying lard, beef-marrow or oil of sweet almonds with caustic soda-lye, in the cold and perfuming the products. They are preferably made of vegetable oils which lather freely.
Shaving soap is made of cocoanut oil as a base with some stearic acid to give hardness. Both soda and potash lyes are used and gum added to prevent its drying quickly. Glycerine is a common ingredient and of ten a little sugar.
Castile soap when pure is made of olive oil and a soda lye. It soon becomes very hard, however, and to remedy this cocoanut oil or linseed oil is used in the mixture. The cheaper grades of Castile soap are made from cotton seed oil.
Light or floatant soap is produced by agitat ing a solution of soap, to which a fifth or sixth part of water has been added, until the lather has risen to a considerable height, and then transferring it to a mold, -where it remains until the quality of transparency is fixed.