Saponification is the chemical process of making soap that involves an exothermic reaction between lye (sodium hydroxide) and a fat (usually oils). What is commonly known as cold process soap making yields a glycerin-rich soap, which used to be referred to as lye soap. People may associate lye soap with something that is unpleasant to use because too much lye was used in the soap formula and lye (sodium hydroxide) remained in the bar of soap to irritate and burn the skin. Soap cannot be made without using lye. When made correctly, no lye will remain in the bar of soap.
Soap cannot be made without using lye but when done the right way, no lye will remain in the bar of soap.
The History of Lye Soap
For a long time, soap was made by rendering available animal fats and adding natural lye (leached from ashes) to make soap. Without the scientific data readily available today, the soap makers of the past estimated the amount of lye to add to the fats. If not enough lye was added, with too many fats remaining, the mixture would separate and not be useable. If too much lye was added, some extra lye would remain in the soap and still not be of any value. While the latter method of adding extra lye to ensure the soap would be useable was preferred – it was worse because the lye would irritate or burn the skin.
Today’s “Lye” Soap
Nowadays, it is much easier to make the correct measurements of a fat to lye ratio, so there will not be any extra lye in the soap that will irritate the skin. The number of milligrams of lye (also known as sodium hydroxide) required to completely saponify one gram of a specific fat is referred to as the saponification value.
Note: Laboratories usually refer to the saponification value of potassium hydroxide, not sodium hydroxide. Potassium hydroxide is commonly used to make liquid soap and sodium hydroxide is necessary for making solid bars of soap.
Below is video courtesy of About.com that demonstrates the soap-making process.