Cold process soap is a transformation process between a fatty matter (vegetable oil, vegetable butter) and soda (sodium hydroxide), potash (potassium hydroxide) or even as in the old days, ash ..
Oils and butters consist mainly of fatty acid triglycerides. When soda is added, these are transformed into soap and glycerin.
For the manufacture of solid soap, soda is used. Potash is used for the production of liquid soaps (black soap, liquid Marseille soap).