The fat is first rendered in a large cylinder with an inlet for steam, exits for water and melted fat, manholes for charging and withdrawing fat, a false bottom for the latter to rest on, and a safety valve weighted to a pressure of two or three atmospheres, that is, 301b. to 451b. per square inch. The rendered fat is then run into cold water and removed for soap making, which is usually carried out in immense pans heated by fire and steam, either alone or together. The amount of materials put in the pan should not more than two-thirds fill it, so as to allow of frothing. The lyes are made by adding caustic soda to water. Two lyes are often employed, and usually three, one at 10oTw. ( 4 per cent.), one at 16 Tw. (6 1/4 per cent.), and the other at 24 Tw. (8 3/4 per cent.). The fat is run into the pan, and the weaker alkali is gradually added while boiling; the stronger alkali is then added, and the mass boiled for several hours until clear. The pan is then allowed to settle, salt added, and, after thorough stirring, the waste lye may be run from the bottom of the pan.

The strongest lye may now be added gradually, boiling and stirring thoroughly until the soap boils clear; then allow to settle again, and run off the soap into frames, taking care that any waste Lye at the bottom does not go along with it. The strength and amount of the different Lyes vary, but on the average 151b. to 16 lb. of caustic soda are employed for 1001b. of fat.