Pure soap is made with a combination of sodium hydroxide, which is lye, and fats or oils. Combined in the right ratio, the ingredients are chemically transformed into glycerin and soap. Hand-crafted soap retains the glycerin, making it a richer, more soothing and moisturizing product than commercial soaps. You can choose from a wide range of oils and fats, creating a soap that is completely free from animal products. However, making your own soap from lye requires following strict safety rules.
Gather all your tools. Dampen a hand towel and place near the vinegar; this is a safety precaution in case any lye splashes on your skin (see Warnings). Gather all your ingredients. Wear your safety glasses and gloves.
Measure the water by weight, and put in the large glass measuring cup. Stir the lye into the water, and keep stirring until the lye is dissolved. Let it cool to about 120 degrees F.
Measure your oils by weight into the stainless steel pot. Place over medium heat and heat the oil to about 120 degrees F. You may have to keep the oil warm over very low heat while you wait for the lye to cool.
Pour the lye mixture into the oils when both mixtures are about 120 degrees F and start stirring. Always pour lye into liquid, never vice versa (see Warnings). Stir until "trace," which means that a little of the soap scooped up and poured back over the remaining soap will leave a trace on the surface for a few seconds. Using an immersion blender will speed up the process of stirring.
Pour your soap into the molds and put them in the freezer for 24 hours. Remove the soap from the molds and let the bars air dry for at least two weeks. Once the surface is hardened, you can store the soap bars in a cupboard out of the way until they finish curing. At the end of the curing process, there will be no remaining lye in the soap.
Do a patch test if you aren't sure about whether the lye and oil have completely saponified after two weeks by moistening one bar. Make a few suds, and rub the suds into the crease of your elbow. If there is no irritation after 24 hours, then there is no lye left in the soap.