What is Cold Process Soap?

Cold-process soapmaking requires exact measurements of lye and fat amounts and computing their ratio, using saponification charts to ensure that the finished product does not contain any excess hydroxide or too much free unreacted fat.   It really is an art and requires time, dedication and attention to detail.  Most soap makers formulate their recipes with a 4–10% deficit of lye so that all of the lye is converted 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 the oils are liquified and the lye is fully dissolved in water, they are combined. This lye-fat mixture is mixed until the two phases (oils and water) are fully emulsified.Emulsification is most easily identified visually when the soap exhibits some level of "trace", which is the thickening of the mixture. (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 and fragrance oils can be added with the initial soaping oils, but solid additives such as botanicals, herbs, oatmeal, or other additives are most commonly added at light trace, just as the mixture starts to thicken.

The batch is then poured into moulds, kept warm with towels or blankets, and left to continue saponification for 12 to 48 hours. (Milk soaps or other soaps with sugars added are the exception. They typically do not require insulation, as the presence of sugar increases the speed of the reaction and thus the production of heat.) During this time, it is normal for the soap to go through a "gel phase," wherein the opaque soap will turn somewhat transparent for several hours, before once again turning opaque.

After the insulation period, the soap is firm enough to be removed from the mould and cut into bars. At this time, it is safe to use the soap, since saponification is in essence complete. However, cold-process soaps are typically cured and hardened on a drying rack for 2–6 weeks before use. During this cure period, trace amounts of residual lye is consumed by saponification and excess water evaporates.