Per Tom's request I'll start a soap thread.
First thing to know this isn't your dear old Grannie's harsh soap. It is so good I quit buying shampoo and just use the soap instead. Haven't bought soap in 5 years. It is nearly as gratifying as making beer! Almost...
I'll address cold-process soap first. The base recipe is similar to beer. One uses a base fat and can augment it with various other types of fat to comprise a recipe. I like to make Castille soap (100% vegetable based) out of 80% vegetable oil (soy) and the other 20% can be olive oil plus some coconut oil. Or you can add melted lard to the mix- this will result in harder bars- which somehow correlates to "clarity" in beers to me. Just for drawing comparisons. LOL
How it works:
My house soap usually is 1000 grams of vegetable oil (very neutral light oil); 250 grams of olive oil (makes the suds very silky); 300-400ml of h2o; and 161 grams of pure sodium hydroxide (lye) to comprise about 5% excess fat. I don't usually add scents, but have used anything from cinnamon to basil flowers. Tea tree oil is nice.
Dissolve the lye in the water until it is clear. Add to the fat and mix with a stick blender or in a regular blender. Be a grown-up and be careful. It is very caustic at this stage.
Mix well until one begins to see signs of "trace". This is when the mixture comes together and emulsifies. Looks like custard and will leave a line when you sweep the blender through it. There is such a thing as a "false trace" so make sure you don't quit when it initially thickens. I like it super thick before it goes into the mould(s).
It's still not soap yet. It has to saponify
. That is to go through the exothermic saponification process which converts itself to soap. Sound familiar? The soap will harden quickly once in the mould. It will look like beautiful creamy soap. It isn't soap yet! In an hour or two it will start to saponify. The appearance will become gel-like and it will begin to put off a significant amount of heat. This is just the chemical reaction. We are not physically heating anything with this method. I like to insulate my moulds which makes for a better finished product.
After a couple days or longer I'll tighten the cap on my mould and use an air pump to push the soap out like a piston. It will look creamy again.
I will then air-dry it in log form until it is firm and dry to the touch. Might be a few days but usually at least a couple weeks or more. Depends on water content. Similarly, like most beer the soap only gets better as it ages. I slice bars off as needed.
Here is a great calculator
for making up soap recipes.