Lye is the key ingredient in turning fat and oils into soap.
Lye is actually a common name for two different chemicals, sodium hydroxide (NaOH) and potassium hydroxide (KOH). Both of these forms of lye can be used in making soap, however they are not the same chemical, and are not interchangeable in soap recipes. This means they are required in different amounts in a recipe and will produce a different type of soap. While sodium hydroxide will produce a hard bar-type soap, potassium hydroxide is more often used in making a liquid or soft soap. Sodium hydroxide has a PH of 14 while potassium hydroxide is reported to have a PH between 12 and 14.
Sodium hydroxide was once readily available in grocery stores and hardware stores, most commonly packaged as Red Devil Lye, it was used mostly as a drain cleaner. Because of it’s illegal use in the production of methamphetamines, it has become harder to find. While I’ve read that it is illegal to sell in retail stores in the U.S., I have been able to purchase it in a few stores. The easiest way to purchase it is online through soap making suppliers.
Potassium hydroxide is made from wood ash and water. I have not yet made soap using potassium hydroxide, but this is on my to-do list.
Because of it’s high PH lye is a dangerous chemical and must be use with caution. If it comes in contact with bare skin it can cause severe burns. It will erode some metals, but it is safe to use in glass, plastic or stainless steal containers. If mixed improperly or with certain substances it can create dangerous gases.
Once lye has been properly mixed with the liquid and oils and the chemical changes occur lye becomes safe to use on the skin. Soap should have a PH between 7 and 10.
You don’t see lye, sodium hydroxide, or potassium hydroxide listed as ingredients on commercial soap products. Instead you will see thing like sodium cocoate, which would be the combination of coconut oil and sodium hydroxide after the chemical change, known as soaponification, has occurred. Other examples would be or sodium palmate or potassium tallowate. I, like many handcrafters, list the ingredients as the raw materials put into the soap even though the chemical change does occur before the product is finished.