Cold process
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Cold process is a method of making soap (saponification) which does not require an external heat source to initiate saponification. (Heat may still by applied in order to melt fats or oils that are solid at room temperature, and to speed the reaction.) This process is often used by soapers, or home soapmakers. A Lye solution, either sodium hydroxide or potassium hydroxide and water, is mixed with an appropriate amount of fats and/or oils to start the saponification process that leads to soap.
Once the warmed or melted oils or fats and the lye solution have cooled to about 80-90 degrees Fahrenheit, the lye solution is poured into the oils 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". After many minutes of stirring, the mixture turns to the consistency of a thin pudding.
Essential oils, fragrance oils, herbs, oatmeal or other additives are added at light trace, just as the mixture starts to thicken. Soap is then typically poured into wooden molds lined with heavy wax paper, covered and/or insulated with towels or blankets and allowed to continue saponification for 18-24 hours. During this time, it is normal for the soap to go through a "gel phase" where the opaque soap will turn somewhat transparent for several hours before turning opaque again. The soap will continue to give off heat for many hours after trace.
After the insulation period the soap is firm enough to be removed from the mold and cut into bars. At this time, it is safe to use the soap since the saponification process is complete. However, most soapmakers prefer to cure the bars for 2-6 weeks, depending on initial water content, to allow for the bars to harden significantly before using.
For successful cold-process soap making, one needs to measure the exact amount of lye to be used and know the saponification values of the oils being used in the soap. Excess unreacted lye in the soap will result in a very high pH and can burn or irritate the skin. Not enough lye, and the soap is greasy and oily. Most soap makers formulate their recipes with 3-15% excess oil so that all of the lye is reacted and that excess fat is left for skin conditioning benefits.
Other processes used by soapers are the hot process, and the melt and pour process, and rebatching.
