Hot-processed lye-based soap simply means that the soap is made using heat. This can be done in the old-fashioned way, as in a big cast iron pot over an open fire, it can be a more modern home-made soap method, in a pot over your stove's burner, or it can be a much more sophisticated, industrial version of hot-processed soap making. The definition of cured soap is this: Soap that has reached a good pH balance and has hardened enough to be usable as a bar soap. However, hot-processed lye-based soaps differ depending on the recipe used and the exact amount of lye used in proportion to the fat and water in the soap recipe, Twenty-four hours is usually a good cure time to get the pH balance to a good level.

Cold-processed lye-based soap is made without any heat... kind of. You do use heat--actually, twice, in the cold-process method. First, you use heat to make a lye-water mixture; then you use it to melt your solid fats until they liquify. However, you do not use heat when mixing the lye-water with the liquified oils, which is the actual "soap-making" part of the whole process. Once the fats are liquified, you remove the pot containing the fat from heat, add the lye-water carefully, and blend the whole mixture with an immersion blender (quick and easy) or by hand (a much longer process). Cold-processed soap takes much longer for the pH balance to reach a usable level, when the caustic properties of the lye have been chemically balanced by the fat and water with which it is combined. Anywhere from several days to four weeks is recommended. To be on the safe side, cold-process soap should cure for at least four weeks.

The other factor in whether or not soap is cured, or ready to use, is the level of hardness it has reached. For both hot and cold-processed soaps, the amount of time needed to cure to a good bar soap consistency is about 24 hours. The soap should be hardened, and you should be able to pop it out of the mold at that point; with hot-processed soap, the soap will be completely cured at that time. With cold-processed soap, you will want to set the soap bars aside for another 25 to 28 days to let it cure completely.