Cold process soap needs to age or "cure" before it is finally ready to use. Like firewood, as the soap cures, most of the water used in the recipe evaporates out of it. Cured soap is harder, milder, and more "finished."
Time Needed to Cure Homemade Soap
How long does soap take to cure? It seems like the answer should be straightforward, but there are two different reasons for curing soap and the amount of time you let your soap cure depends on the results you want. We cure soap to:
Complete the Saponification Process
Making sure the saponification process is completely complete generally takes anywhere between 24–48 hours. In layman's terms, saponification is the chemical reaction that occurs when the oils react with the lye and create actual soap.
Some people worry that soap isn't safe to use in the days after it's poured. This isn't true; soap is fine to use after a couple of days. It does become milder as it ages, but only slightly. 99 percent of the pH changes related to saponification happen within the first 48 hours after pouring.
Harden the Soap
The more important reason to cure your soap is for the water to slowly evaporate over time, which causes the soap to harden. A harder bar of soap will last longer, produce more lather, and just be an overall better bar of soap. Generally speaking, this takes three to six weeks, but there are some cases when it can take even longer.
Castile soaps or any other soap made with high amounts of olive oil benefit from an even longer cure. Many soap makers let castile soap cure for six to eight months. After you make soap for a while, you'll be able to compare a bar that has cured for several months with one that has just cured a couple of weeks and quickly tell the difference between the two.
Shortening the Curing Process
One way some soap makers shorten the amount of time needed for curing is by using a water discount. This is a technique in which soap makers use less water than a recipe calls for in order to reduce the amount of time needed for the water to evaporate out and the soap to fully cure.
This might sound good in theory, but it requires very careful calculation and it's not a technique for beginners. Water discounting accelerates trace, making soap difficult to work with. Making soap with a lower water content also means that a higher temperature is needed for it to go through a full gel phase.
If you're not ready to tackle water discounting and decide to stick with a regular soap recipe instead, know that after the first day or two after pouring your soap, it can be used safely. Waiting for your soap to cure any longer than is not necessary for safety, although it does result in a higher quality soap.