Hot process

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Hot process is a traditional method of making soap, still used by some soapmakers.

In the hot process, fats and oils are boiled in a lye solution (either sodium hydroxide or potassium hydroxide). After saponification has occurred, the soap is precipitated from the solution by adding salt, after which the liquid component is drained.

Most modern soapmakers add the correct amount of lye and water to the hot oils and stir until full saponification occurs- either by taste (a quick touch to the tip of the tongue - lye has a very bright and distinct taste which goes away upon saponification) or by eye (more experienced soap makers know what gel stage and full saponification looks like). The hot, liquid soap is then poured into a mold. Hot process soap can be used right away (cold process needs to be cured so the saponification can finish).

Historically salt has been added to harden the bars- postassium hydroxide (aka potash) makes a soft soap. Sodium hydroxide (most commercially available lye) makes a quite satisfactorily hard bar.

Although more elaborate than the cold process, the hot process was used in the time before pure lye was available, as it can use natural lye solutions such as potash. The main benefit of hot processing is that the exact concentration of the lye solution does not need to be known to perform the process successfully.

Other processes used by soapers are the cold process, the melt and pour process, and rebatching.