Natural (Cold Process) Soap Making Using Vegetable Oils and Fats - Overview
Soap is a salt and when fats and/or oils are combined with lye (sodium hydroxide) the saponification process results in soap.
Oil/Fats (Fatty Acid) + Lye (Base) = Soap (a salt)
To calculate your own soap recipes you will need to know the Saponification Values (also known as SAP factor) of the various oils you may want to use in your soap recipe. This is the real benefit of making your soap from scratch. Not only do various oils have different saponification values, some oils can offer therapeutic values and when combined together can make a truly beautiful bar of soap. This is where you can have a little (or a lot) of fun and be creative. You can make your own special soap for your skin type.
To use the SAP chart you need to figure out how much of an oil you want to use in a soap recipe. Chemically speaking: a fat or oil is converted into soap with an alkali (lye). Each fat or oil requires a certain amount of lye to saponify.
If you're just starting out and want an easy soap mold for one batch of soap (1 lb) - I first started using the heavy plastic "Rubber Maid" drawer organizer (9 x 3 x 2)". This requires 16 oz of fats/oil to fill to the top of the mold. You can get these molds just about anywhere (Walmart, Save-On Foods, London Drugs, Home Hardware, etc.) You can also use most any heavy cardboard (milk cartons, egg cartons), plastic, pvc pipe, etc. for a soap mold. Avoid using metal containers such as aluminum, tin, and zinc. They will react with lye.
Don't get hung up on the lye thing. When soap is properly made and saponification takes place, the soap should cure for a minimum of 3-4 weeks. During this time the lye is neutralized resulting in a salt - which is soap. If you've used too much lye then you will have a very hard and brittle soap and it will be caustic and drying to the skin. Not worth using on the skin.
LYE SAFETY CHECK
Soap Making Instructions