From the beginning of soapmaking, people have made it “hot.” In fact, professional soapmakers used to be called soap boilers and shared a patron saint with firefighters.
As far as I can tell, the kind of soapmaking now called hot process was the rule for both family and commercial soapmaking all the way up to around 1940. At that time, companies that manufactured lye began to market it for home soapmaking, which was falling out of fashion. The new method came to be known as cold process—a term I’ve found from as early as that same period, in a lye company pamphlet.
Though in cold process the soap mixture isn’t “cooked,” it isn’t really cold either, as its temperature usually falls somewhere between room temperature and 110°F (43°C). Still, cold process seemed simpler, possibly safer, and was less intimidating to beginners. So, as craft soapmaking became popular, cold process was the technique favored in many books.
More recently, soapmakers adding milk to their soaps have come up with a version of cold process that truly does involve lower temperatures. In my book Milk Soapmaking, I called it Cool Technique. It uses frozen liquid to counteract the heat generated by the dissolving lye. This aims to keep the milk as cold as possible, to avoid browning the milk sugars and darkening the soap.
After writing that book, I continued to refine Cool Technique. But more important, I discovered that its usefulness goes far beyond milk soap. In fact, it can help wherever high temperatures cause problems of scorching, fumes, acceleration, or other unwanted reactions.