It is very informative and can help you discover how to make soaps that you know are free of the allergens you don't want around your child or yourself.
The following is the text from the directions that she posted there:
Some of you expressed interest in learning how to make your own soap (or at least in how I manage to do it, LOL), so here's the bare-bones run-down. I promise you it's actually very easy; there are a lot of steps, but they aren't hard and they're mostly quick little things you can sneak into your existing routine.
First things first: a bit of chemistry. Soap is a salt of a fatty acid, which, in layman's terms, means it's the result of a chemical reaction between a strong base (lye) and a triglyceride (oil, lard, whatever). The lye breaks down the fatty acids into their constituent chains, which have both a polar and a nonpolar end. The nonpolar end resembles grease or fat, and therefore can bond with it. The polar end of the molecule bonds with water, which enables you to use soap to wash grease off of dishes or body soil out of clothes.
Now, there are several options for the lye: some people use sodium hydroxide (NaOH) solution, which is the most convenient because it has the most consistent effectiveness. You can also use ash, but that's a pain because you have to distill your own lye solution from it and add lime (the white mineral, not the fruit) and whatnot. More on how to do that in a comment later. Personally, I use solid NaOH, because it's more convenient than making my own lye, especially as we don't burn wood, but it's significantly cheaper than buying NaOH solution. How much lye you use depends on how much of what oils you use. Generally, your soap should have 5-8% excess (unreacted) fat; less if you're making a laundry detergent or other deep cleaning/clean rinsing soap and more if you're making a body or baby soap. I use this calculator to help me: www.thesage.com/calcs/lyecalc2.php
There are scores of different fat sources and thousands if not millions of ways of combining them. Each different fat has slightly different qualities as a soap. Go here to see different oil qualities: candleandsoap.about.com/od/soapmaki...
Choosing and balancing your fats is the tricky part of soapmaking, so I'll break this part down as clearly as possible:
1. Decide what kinds of soap you need. You can make any bar soap into a powder or liquid, so don't bother thinking about that yet. What do you use soap or detergent for? Who uses the soap? What qualities are important in various soaps in your household (gentleness, deep cleansing, antimicrobial action, luxurious lather)? In my house, we need a well-balanced high-lather hand/body soap, a deep cleansing antimicrobial dish/laundry soap, and a gentle baby soap.
2. For any basic hand or body soap, start with a simple and well-balanced recipe. You can adjust it from there. A good basic soap for washing the hands and body is 30% tallow, 25% coconut oil, and 45% olive oil. A vegan equivalent would be equal parts of palm, coconut, and olive oil.
3. For specialty soaps, address the most important quality first. For example, if you need a laundry soap that can be used with cloth diapers, you need an oil that with rinse clean, like coconut oil. Next, consider how important it is to balance the soap: in the cloth diaper detergent case, we don't need to balance the soap for rich lather or moisturizing qualities, so we can use a single-oil soap if we want to. Finally, think about other qualities it might be nice to add to the soap; I make my laundry soap slightly antimicrobial and insect repellent by using a small amount of neem oil.
My basic hand/body soap is 5% cocoa butter, 20% olive oil, 20% coconut oil, 55% vegetable shortening. This is a moisture-rich soap with a thick lather. My laundry/dish soap (which is cloth diaper safe) is 98% coconut oil and 2% neem oil. I would never use it to wash my hands because it dries out my skin in large exposures, but it doesn't leave any oil on my diapers and that's the important part. It's also very low-foaming. My baby soap is 30% olive oil, 30% coconut oil, 25% palm kernel oil, 10% castor oil, 5% pumpkin seed oil. This is a very gentle soap with excess fat and some premium moisturizers (N has dry skin and cradle cap).
Now, once you have a recipe, there's the question of how you make soap. I use cold process because hot process is annoying and not as useful for small stovetop batches.
This is REALLY EASY, I promise.
1. Heat your fats to 100F, stirring to mix.
2. Heat your lye to 100F (or, if making your own lye solution from solid NaOH, let reacted solution cool to 100F).
3. Slowly and carefully add the lye to the fats.
4. Mix with a handheld blender (easiest), strong metal whisk, glass rod, or wooden spoon (hardest) until you reach "trace". Trace simply means that when you let soap drip back onto the surface of the mixture, you can see the outline of where the drop fell. At trace, you can add any colorants, fragrances, or other extras.
5. Pour mixture into molds or glass baking dishes.
6. Insulate molds by wrapping in towels, placing in empty coolers, etc. Leave completely alone for 48 hours to set.
7. Remove soap from molds (if hardened--- if still soft, let sit an additional 24 hours). Cut into bars, set on a wire rack and leave to cure for 4-6 weeks.
And that's it. The end.
Unless you want a powder or a liquid, that is. If you do, grate your soap very fine, use your fingers to break up any chunks, and leave to dry out for at least 4 hours.
To make powder laundry detergent, combine 2 parts grated soap with 1 part baking soda and 1 part borax. Mix well and store in an airtight container. Use 2 tablespoons per full machine load.
To make liquid dish soap, combine 2 parts grated soap 2 parts hot water and let sit overnight. Thin as necessary by adding water, then add 1 part lemon juice or distilled vinegar (a grease-cutting agent). Shake before using because separation will occur.
To make liquid hand soap, pour 2 parts near-boiling water over 1 part grated soap. Mix until all soap is melted. After completely cooled, remix if necessary to dissolve clumps.
Questions? Post and I'll try to answer.