All soaps follow the basic algorithms below. If you're interested in more advanced recipes, (Neutrogena, for example) let me know!
INGREDIENTS:
1 1/2 Cup melted tallow
1/2 Cup olive oil (can substitute 2 cups tallow,
but I wouldn't use it on my face!)
6oz cold, soft or distilled
water
4 TBSP LYE (flakes or grains...a common brand in the US is
"RedDevil")
Molds for pouring (a saran-wrap lined bread pan is fine, or a
vaseline coated
square dish. Candy molds work fine, too.)
Soap forms when fats are converted into fatty acids through the process of saponification. This occurs in the presence of water and a strong base. Lye is simply Sodium Hydroxide, (NaOH) and is an extremely powerful base. Wear rubber gloves and BE CAREFUL when handling this substance. If you notice microscopic areas of skin that begin to burn after using lye, you have small grains of NaOH on your skin that are picking up water from the air and trying to saponify your skin. Wash immediately with water or a 50:50 mix of water and vinegar (a weak acid.) If you get it in your eye, flush copiously and constantly and high-tail it to the eye doctor, as alkaline substances are more prone to injuring the eye than acidic ones. Soapmaking is fun and safe if the ingredients are handled properly. I mean, we're dealing with strong inorganic chemicals and hot, melted fat. Could be a disaster if one was careless.
ALGORITHM:
Melt some tallow in the oven at around 150 deg F (65deg
C.) Measure appropriate quantity into glass caserole dish and save the rest for
the next batch. Add olive oil, and stir to mix. Set dish in a 95 degF water bath
(35 degC) to cool. A "lukewarm" water bath is close enough, if you don't have a
good thermometer.
Add Lye to cold soft water, using an old mason jar as a vessel. You can soften hard water with a few drops of ammonia, or a few grains of Boric acid (Borax,) or use distilled water. Stir quickly after adding lye to water, with a wooden utensil, such as a chopstick. Never use any metal utensils during the actual soapmaking process, as lye will etch the surface and introduce metal ions into your soap (yuck.) Set lye solution in water bath to cool.
When fat and lye are leukwarm (judge the lye by feeling the outside of the glass container, rather than by sticking your finger into the solution, as the latter will cause you to draw back a stump) slooowwly add the lye solution to the fat, stirring constantly with a wooden utensil.
Now, the mixture will become cloudy immediately, but may not thicken for up to an hour. Stir constantly, avoiding splattering saponifying fat and raw NaOH all over the place, for 15 minutes. You can then take a break, if necessary, for 2-3 minutes, but return to stirring until the mixture rehomogenizes. Purists will tell you you must stir constantly, but I've found that intermittent stirring is fine as long as you keep at it, and not be too intermittent about it.
Soap is ready to pour into the mold when it's thick enough to support an object stuck into its midst...that spoon you're holding, for instance. The consistency is somewhere between whipped cream and cake frosting.
Carefully pour into molds, and set in a warm place, covered, to continue the reaction. In about eight hours, use a sharp knife to cut into squares, rectangles, whatever. In 24-48 hours, remove from mold (if you used saran wrap, you can just lift the whole thing out of the mold) and stack bars on a plate pyramid style to cure.
Cure for 2-4 weeks and then store wrapped in tissue, ziplock bags, a metal candy box, or whatever. Homemade soap has a higher water content than store bought soap, and also a higher glycerine content (a natural by product of the reaction.) This does mean that the soap bars can dehydrate and decline in quality if just left out for months on end.
If an ash forms on the surface, just scrape it off with the dull side of a knife before showing off your work. You'll find soap is easy to carve, but scrape, rather than cut the surface for a perfect finish. Wiping with a linen cloth, or scoring the surface with a serrated blade makes for truly presentable bars. Every bar of soap is unique. In this way, soap is a metaphor for humanity. (Hee!)
Touching the soap to your tongue is a good test for completed curing; if there's no "burn," it's ready to use.