Soap has a very long history dating back to the Babylonian Empire. The chief ingredients of soap are: coconut oil, palm oil and tallow, a by-product of the frozen and canned meat trade. Sometimes oil from cotton seeds and soya beans is used. Resin from certain sorts of pine-trees goes into household soaps.
The oils are mixed together in the right proportion according to the type of soap to be made. Caustic acids are added to separate the fat from the glycerin in the oil. (Sodium hydroxide is used for household soaps. Potassium hydroxide is added for soft toilet soaps.)
The mixture is pumped into a large tank, through which pass steam pipes with holes in them. The steam pours out of the holes into the soap mixture. At the end of the process all the caustic substances get mixed with the fatty acids to make the soap, and the glycerin floats off free. Salt is added to the mixture to remove the glycerin.
Then the pumping of steam stops and the whole mixture is left to cool and settle, while the glycerin in the salty water is drawn through the tap at the bottom of the tank. After this it is washed and purified and used as medical glycerin. As the glycerin does not come off from the soap mixture at once, the soap mixture is washed several times in strong and weak salty water, with the pure soap coming to the top, and salt, and dirt running off from the bottom.
After standing for two or three days the soap, still warm, goes to another machine. There perfume is added while the soap cools. The soap goes into a large frame where it gets solid. Next it is cut into blocks, stamped and placed to get hard before it goes to the shop.
Toilet soaps take purer oils. They get more perfume and coloring. Their technology is more complicated.