How does the semiboiled method of soapmaking differ from the cold method?

The primary difference between the cold and semiboiled methods of soapmaking lies in the application of heat during the saponification process.1

In the cold method, a mixture of fats and oils is combined with an alkali solution at room temperature. The mixture is agitated in an open pan until it thickens, at which point it is poured into frames to solidify and complete the saponification process. This method often uses slightly less alkali than is theoretically required, leaving a small amount of unsaponified fat or oil to act as a superfatting agent.1

In the semiboiled method, the fat is placed in a kettle and the alkali solution is added while the mixture is stirred and heated, though it is not brought to a boil. Unlike the cold method, the mass saponifies directly within the kettle before being poured into frames to solidify. Both methods are technically simple and require minimal machinery, making them well-suited for small-scale production.1