The boiling process is very time consuming; settling takes days. To produce soap in quantity, huge kettles must be used. For this reason, continuous soapmaking has largely replaced the old boiling process. Most continuous processes today employ fatty acids in the saponification reaction in preference to natural fats and oils. These acids do not contain impurities and, as explained at the beginning of this section, produce water instead of glycerin when they react with alkali. Hence, it is not necessary to remove impurities or glycerin from soap produced with fatty acids. Furthermore, control of the entire process is easier and more precise. The fatty acids are proportionally fed into the saponification system either by flowmeter or by metering pump; final adjustment of the mixture is usually made by use of a pH meter (to test acidity and alkalinity) and conductivity-measuring instruments.

The continuous hydrolyzer process begins with natural fat that is split into fatty acids and glycerin by means of water at high temperature and pressure in the presence of a catalyst, zinc soap. The splitting reaction is carried on continuously, usually in a vertical column 50 feet (15 metres) or more in height. Molten fat and water are introduced continuously into opposite ends of the column; fatty acids and glycerin are simultaneously withdrawn. Next, the fatty acids are distilled under vacuum to effect purification. They are then neutralized with an alkali solution such as sodium hydroxide (caustic soda) to yield neat soap. In bath-soap manufacture, a surplus of free fatty acid, often in combination with such superfatting agents as olive oil or coconut oil, is left or added at the final stage so that there is no danger of too much alkali in the final product. The entire hydrolyzer process, from natural fat to final marketable product, requires a few hours, as compared with the 4 to 11 days necessary for the old boiling process. The by-product glycerin is purified and concentrated as the fatty acid is being produced.

Cold and semiboiled methods

In the cold method, a fat and oil mixture, often containing a high percentage of coconut or palm-kernel oil, is mixed with the alkali solution. Slightly less alkali is used than theoretically required in order to leave a small amount of unsaponified fat or oil as a superfatting agent in the finished soap. The mass is mixed and agitated in an open pan until it begins to thicken. Then it is poured into frames and left there to saponify and solidify.

In the semiboiled method, the fat is placed in the kettle and alkali solution is added while the mixture is stirred and heated but not boiled. The mass saponifies in the kettle and is poured from there into frames, where it solidifies. Because these methods are technically simple and because they require very little investment for machinery, they are ideal for small factories.

Finishing operations

Finishing operations transform the hot mass coming from the boiling pan or from continuous production equipment into the end product desired. For laundry soap, the soap mass is cooled in frames or cooling presses, cut to size, and stamped. If soap flakes, usually transparent and very thin, are to be the final product, the soap mass is extruded into ribbons, dried, and cut to size. For bath or hand soap, the mass is treated with perfumes, colours, or superfatting agents, is vacuum dried, then is cooled and solidified. The dried solidified soap is homogenized (often by milling or crushing) in stages to produce various degrees of fineness. Air can be introduced under pressure into the warm soap mass as it leaves the vacuum drier to produce a floating soap. Medicated soaps are usually bath soaps with special additives—chlorinated phenol, xylenol derivatives, and similar compounds—added to give a deodorant and disinfectant effect. As mentioned above, shaving creams are based on potassium and sodium soap combinations.

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