The semi-boiled process differs from the cold process in the fact that the saponification mixture is heated to 70 - 90° C using a steam-heated coil to accelerate and complete the saponification reaction. Dyes, perfumes, and additives are added at the end of the process to prevent them from evaporating.
The process allows the quantity of soda undergoing saponification to be adjusted before the crude soap is drawn off. It also allows manufacturing waste to be recycled, better incorporation of the additives and a wider choice of raw materials. Generally speaking, saponification is more complete and the hardening time of crude soap in cooling frames is slightly reduced.
These various advantages, combined with shorter production cycles and reasonable production costs, make the semi-boiled process a flexible process which is particularly well-suited to developing countries. It is often used in industrial-scale production of widely marketed low-grade soaps.
As for the cold process, the semi-boiled process does not discharge any effluent into the environment.