How Is Soap Made?
Before the end of World War II, soap was manufactured by a "full-boiled" process. This process required mixing fats and oils in large, open kettles, with caustic soda (NaOH) in the presence of steam. With the addition of tons of salt, the soap was made to precipitate out and float to the top. Here, it was skimmed off and made into flakes or bars. This process required large amounts of energy and over six days to complete one batch.
After World War II, a continuous process of soap manufacture became popular. In the continuous process of soap manufacture, fats and oils react directly with caustic soda. The saponification reaction is accelerated by being run at high temperatures (248°F; 120°C) and pressures (2 atm). Glycerin is washed out of the system and soap is obtained after centrifugation and neutralization. This process has several advantages over the "full-boiled" process. It is more energy efficient, time efficient, allows greater control of soap composition and concentration, and the important by-product, glycerin, is readily recovered.
Both manufacturing methods yield pure soap. Certain chemicals can be added to this pure soap to improve its physical characteristics. The foam in soap is enhanced by additives such as fatty acids. Glycerin is added to reduce the harshness of soap on the skin. Other additives include fragrances and dyes.