The process of converting a fat into soap by treating it with an alkali.

This may be done by boiling fat and alkali in a kettle under controlled conditions.  They react to form soap and glycerin .  Modern methods make soap in continuous processes, either by treating fat with alkali or by first splitting the fat into fatty acids and glycerin by a process called hydrolysis.  The fatty acids are purified by distillation and then mixed with the correct amount of alkali to convert them into soap.