Cold-process soapmaking requires exact measurements of lye and fat amounts and computing their ratio, using saponification charts to ensure that the finished product is mild and skin-friendly.
A cold-process soapmaker first looks up the saponification value of the fats being used on a saponification chart, which is then used to calculate the appropriate amount of lye. Excess unreacted lye in the soap will result in a very high pH and can burn or irritate skin. Not enough lye, and the soap is greasy. Most soap makers formulate their recipes with a 4-10% deficit of lye so that all of the lye is reacted and that excess fat is left for skin conditioning benefits.
The lye is dissolved in water. Then oils are heated, or melted if they are solid at room temperature. Once both substances have cooled to approximately 100-110 °F (37-43 °C), and are no more than 10°F (~5.5°C) apart, they may be combined. This lye-fat mixture is stirred until "trace" (modern-day amateur soapmakers often use a stick blender to speed this process). There are varying levels of trace. Depending on how additives will affect trace, they may be added at light trace, medium trace or heavy trace. After much stirring, the mixture turns to the consistency of a thin pudding. "Trace" corresponds roughly to viscosity. Essential oils, fragrance oils, botanicals, herbs, oatmeal or other additives are added at light trace, just as the mixture starts to thicken.
The batch is then poured into molds, kept warm with towels, or blankets, and left to continue saponification for 18 to 48 hours. Milk soaps are the exception. They do not require insulation. Insulation may cause the milk to burn. 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 once again turning opaque. 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 saponification is complete. However, cold-process soaps are typically cured and hardened on a drying rack for 2–6 weeks (depending on initial water content) before use. If using caustic soda it is recommended that the soap is left to cure for at least four weeks.