What Is Cold Process Soap, and Why Is It Different?

Cold process soap is made by mixing oils with a sodium hydroxide (lye) solution without cooking the batch. The mixture is poured into a mould, finishes reacting on its own heat within a day or two, and is then left to cure for several weeks. For anyone buying a bar, the point is not the temperature. It is what the method leaves inside: the glycerin, and a small share of oil that never turned into soap.

All real soap starts with the same reaction

Soap comes from one chemical reaction, saponification. Fats react with a strong alkali and produce two things: fatty acid salts, which are the soap itself, and glycerol, better known as glycerin. The FDA describes traditional soap the same way — fats or oils combined with an alkali such as lye — and notes that when soap is made correctly, no lye remains in the finished product.

"Cold" describes the absence of outside heat, not a cold bar. Hot process soap is cooked for hours to speed the reaction up; traditional kettle soap is boiled in large vats. All three methods make genuine soap. The differences begin after the reaction.

The real difference is the glycerin

In the traditional kettle method, salt is added once saponification is complete. The soap separates out, is filtered off and washed several times, and the glycerol is recovered separately from the wash water. The glycerin formed inside the batch leaves the soap and is sold on as a raw material of its own.

Cold process has no salting-out step and no washing. The glycerin stays where it formed, in the body of the bar. Glycerin draws in moisture, which goes a long way to explaining why a handmade bar feels softer and less drying than a milled commercial one.

Some oil is left unsaponified, on purpose

A 2018 study in *Molecules* analysed commercial natural soaps made by cold saponification with a 6% superfat. Between 60% and 100% of the unsaturated fatty acids in the starting oils remained unsaponified in the finished bars. The soapmaker deliberately uses slightly less lye than full saponification would need, so no free lye is left and some oil stays in the bar. In an olive oil based bar, that leftover oil is olive oil.

Why it takes weeks

A cold process bar is firm enough to cut within a day or two, but it is not finished. During curing, the last of the reaction completes and water slowly evaporates. Less water means a harder bar, and a harder bar dissolves more slowly in use. Milasso's soap cures for a minimum of six weeks after unmoulding — six weeks of stock that cannot be sold, which is part of why it costs more than a supermarket bar.

"Cold process" on the label proves nothing

Cold process is not a defined or policed term, so printing it on a wrapper commits nobody to anything. The ingredient list is what you can check. Under Article 19 of the cosmetics regulation, which applies in the EU and in retained form in the UK, ingredients are listed in descending order of weight. A short list that starts with sodium olivate means the bar is built on olive oil; sodium palmate or sodium tallowate first means palm or tallow, whatever the method. We explain the five-second label check in is olive oil soap sold in supermarkets.

Milasso's handmade olive oil soap is made in Milas, Muğla, on an olive oil base by the cold process method, with no fragrance, colourant or preservative. The oil it starts from is the same one we bottle; see the olive oil range.