Soaping Science - Mini Lesson
Cold process soaping is the art and science of making soap from scratch using oils, lye, and water. When you combine these ingredients, the process of saponification occurs which results in a bar of soap.
Saponification is essentially the conversion of fat (esters) into soap. It's a one-step process in which the fat is essentially forced to "open" it's ester bonds and form soap (carboxylate) and byproducts (alcohol, which is evaporated during curing). But, in order for the fat to turn into soap it needs lye and water.
Lye is the nickname for KOH (potassium hydroxide) and NaOH (sodium hydroxide). Either of these can be used in soap making, but NaOH is the most common because it produces a harder bar of soap. KOH can also be used, but it will produce a much softer bar.
Note: Yes, lye is often used as a drain cleaner and there are some drainer cleaners that are 100% lye. However, please keep in mind that food-grade lye should be used for soap making and not industrial lye.
When it comes to choosing fats for soap making, there are plenty to choose from! You can use anything from Crisco to goat milk to olive oil. Check out brambleberry.com for more information on different types of oils for soap making. I only use vegan oils for my soaps as a personal preference because animal fats have caused my skin to react poorly before. However, this is a personal preference and any type of fat you choose can be used to make soap :)
After saponification has finished, usually 1-2 days after you make your batch, you need to let the soap cure. Why cure? Curing lets the excess water and alcohol byproducts evaporate from the bar, resulting in a harder and milder bar of soap. You can technically use a bar of soap when the saponification process is complete, but it's recommended to let it cure for 4-6 weeks so you get the best bar of soap.