The unholy mess involved in making soap belies its use as an incomparable cleaner of just about everything for at least the past two thousand years. It has always been easy to make out of cheap, readily available materials: fat and wood ashes. Lime was sometimes used also.
You can make it the way the Romans did: Heat limestone to make lime. Sprinkle wet lime onto hot wood ashes and mix well. Shovel the resulting gray sludge into a caldron of hot water and boil it up with chunks of goat fat for several hours. When a thick layer of dirty brown curd forms on the surface and hardens upon cooling, cut it into cakes. That’s your soap.
Or, if you prefer, just go to the store and buy a cake of today’s highly purified commercial product. In addition to soap, which is a definite chemical compound, it probably contains fillers, dyes, perfumes, deodorants, antibacterial agents, various creams and lotions, and lots of advertising. Sometimes more advertising than soap.
Every soap is made by the reaction of a fat with an alkali, a strong, potent base. (A base is the opposite of an acid.) Instead of goat fat, today’s soaps are made from any of a number of different fats, including beef and lamb tallow and the oils of the palm, cottonseed, and olive. (Castile soap is made from olive oil.) The alkali used in making today’s soap is usually lye (caustic soda, or sodium hydroxide). Lime is another handy alkali, while wood ashes can still be used in a pinch because they contain the alkali potassium carbonate.
Having been created by the addition of an organic compound (a fatty acid) to an inorganic compound (lye), the soap molecule retains some features of both its parents. It has an organic end that likes to fraternize with oily organic substances, and an inorganic end that is attracted to water. Hence its incomparable ability to coax oily dirt into the wash water.
Whenever you see the following chemicals listed as ingredients on the label of a shampoo, toothpaste, shaving cream, or cosmetic, be neither alarmed nor impressed; they’re all just the chemical names of soaps: sodium stearate, sodium oleate, sodium palmitate, sodium myristate, sodium laurate, sodium tallowate, and sodium cocoate.
If the “sodium” is replaced by “potassium,” the soap has been made with caustic potash (potassium hydroxide) instead of with caustic soda (lye, or sodium hydroxide). Potassium soaps are softer and may even be liquids.