Hot Process soap making is the old method still used by soap makers. In this process heat is involved to initiate saponification. While preparing soap by hot process lye solution, fats and oils are boiled. Once the saponification occurs the soap is precipitated by adding salt to the solution. The remaining component in the form of liquid is drained of. Those modern soap makers using this traditional method of soap making add exact proportion of lye and water to hot oils and mix till saponification process is achieved. This can be tested by the experts by taste; where they touch it to the tip of the tongue or by eye; which only the experienced soap makers can be sure of. After the process is completed the solution is put into molds to set. The hot process soap can be used immediately after preparation, which is not in the case of cold process soap.
In earlier times during hot process of soap preparation salt was added to harden the bars- potassium hydroxide (aka potash), which made soft soap whereas the most commercially used Lye - Sodium hydroxide was used in making hard bars. Hot process is time consuming but the more beneficial factor in this is that it can use natural lye like potash. This helped the soap makers to prepare soap without knowing the exact concentration of Lye and still performed the process successfully. But nowadays this process is done with accurate amount of required ingredients.