Cold process
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Cold process is a method of making soap (saponification) which does not require use an external heat source to force the saponification process to begin. (Heat is often still applied in order to melt fats or oils that are solid at room temperature and to speed the reaction.) For this reason, this process is often used by soapers, or home soapmakers. Lye, either sodium hydroxide or potassium hydroxide, is mixed with an appropriate amount of fats and/or oils to start the saponification process that leads to soap.
Once the lye solution has cooled to approximately the same temperature as the oils, the two are combined and stirred. A stick blender is often used to speed this process. The two thin, clear substances become cloudy and begin to thicken. Soapmakers refer to the thickening process as "tracing". Often, tracing can occur very suddenly: after many minutes of stirring, the mixture can turn to the consistency of pudding in a matter of seconds.
Essential oils, herbs, and fragrances are added at "light trace" (just as the mixture starts to thicken) to minimize the lye's neutralizing effect. Soap is then poured into molds and allowed to harden overnight. While the soap hardens, it's normal for it to go through a "gel phase" where the opaque soap will turn clear (like glycerine soap) for several hours before turning opaque again. The soap will continue to give off heat for many hours after trace.
The soap is hard and can be removed from the moulds after typically 48 hours, but it should be allowed to cure for a few weeks to allow excess moisture to evaporate. Contrary to popular belief, perpetuated in several popular crafting books, the soap is perfectly safe to use before curing—no excess lye will be present if the soap was properly formulated.
This process lets the chemicals do the work. For successful cold-process soapmaking, one needs to be able to measure the exact amount of lye to be used and know the saponification values of the oils in the soap. Excess unreacted lye in the soap results in a very high pH and will burn your skin. Not enough lye, and your soap is a greasy mass of goo. Most soapmakers formulate their recipes to react all of the lye and to leave about 3-10% of the fat unreacted.
Other processes used by soapers are the hot process, and the melt and pour process, and rebatching.
