Cold saponification consists of carrying out the saponification reaction at room temperature or slightly higher (between 40 °C and 50 °C ). This method requires no additional heat source other than that used to melt and mix the oils. The exact quantity of soda needed to transform the fats into soap is included in the recipe. It is a total chemical reaction, which ends only when one of the components is completely exhausted. Additional fatty substances are added at the end of the preparation in order to guarantee a total transformation of the soda into soap, and a great softness for the skin . Cold saponification also produces a soap naturally rich in glycerin (glycerol), which has strong washing power and softening properties .

The paste is then cast and thermally insulated for 48 hours, during which time it will rise in temperature. After 48 hours the soaps are still soft and can be shaped. They then undergo a period of “cure” ranging from four to six weeks, in a dry and ventilated place, allowing the soap to dry and harden, or even the chemical reaction of saponification to be completed completely. A soap that contains too much water will melt quickly on contact with water, hence the importance of the cure.

This method is slow and less polluting than the hot method. It does not allow industrial production because of its manufacturing time. She produces a soap quality, which retains the properties of the oils used at the end of the total chemical reaction, since they are only heated to be mixed, unlike hot saponification.