Potassium hydroxide KOH, is commonly called caustic potash. It is famously used to saponify fats into liquid soap, which is then firmed up with the addition of common salt.
"Historically KOH was made by adding potassium carbonate (potash) to a strong solution of calcium hydroxide (slaked lime), leading to a metathesis reaction which caused calcium carbonate to precipitate, leaving potassium hydroxide in solution" (Wikipedia). Also historically but not mentioned on Wikipedia, potassium hydroxide was leeched from wood ashes using rainwater and a barrel, a long and indeterminate process which I will explain further in another post. The addition of calcium hydroxide (hydrated lime), which came much later, potentially simplifies and streamlines the process even for the farm wives who historically made soap.
Other than it's famously historical use in home soap making, potassium hydroxide has fallen out of fashion in the chemical world in deference to it's cheaper sister chemical, sodium hydroxide, now also used for soap making. Potassium-based soaps are known to be milder than sodium-based soaps, though properly formulated soaps of either kind are mild enough for children and pets.
This excerpt is from a chemistry text on soap and candle making, written in 1856. The science discussed herein is still good, though does not make adequate use of one our great modern advances in alkalimetry, known as the pH scale. Potassium hydroxide can have a pH of 12 to 14, depending on its formulation and purity. A pH of 13 is considered the normal for it.
