Once upon a time rainwater was filtered through hardwood ashes to make a Potassium Hydroxide solution. It’s actually a pretty cool process. The ladies who taught me how to make soap would let rainwater collect in an old tree stump, and add ashes to it periodically. They knew the water was alkali “enough” to make soap when a raw egg would float on the top.
Currently, bar soaps are made using Sodium Hydroxide. Sodium Hydroxide (NaOH) is created by running electricity through salt water.
Liquid soap is made using Potassium Hydroxide (KOH), sometimes known as caustic potash.