Cold process soap explained: saponification, lye calculators and curing time

Cold process soap is soap made from scratch: oils and butters mixed with a solution of sodium hydroxide (lye), poured into a mould and left to turn into soap over the following days. It gives you complete control over what goes into the bar, and it is the method most people mean when they talk about traditional handmade soap. It also involves a caustic chemical, so understanding what is happening matters before you start.

Oils and butters are mostly made of triglycerides: three fatty acids attached to a glycerol backbone. When those fats meet a strong alkali such as sodium hydroxide, the alkali breaks the fatty acids away from the glycerol. The fatty acids combine with sodium to form soap (sodium salts of fatty acids), and the glycerol is released as glycerin, which stays in the bar. That reaction is saponification. The important point is that lye is not an ingredient left in the finished soap. If the amounts are right, the lye is used up in the reaction. If there is too much lye for the oils, the leftover alkali stays in the bar and can irritate or burn skin. That is why accurate measuring is not optional.

Every oil needs a different amount of lye to turn fully into soap. This is its saponification value, and coconut oil, olive oil and shea butter all have different ones. A lye calculator holds those values and works out how much sodium hydroxide and water your particular mix of oils needs. Use one every time, even for a recipe you found online:

Always weigh oils, lye and water on digital scales, by weight, not by volume. Cups and spoons are not accurate enough. Run the recipe through a calculator, check the result, and keep a written record of each batch.

After the lye solution is blended into the oils, the mixture thickens. When drizzled batter leaves a faint trail on the surface, it has reached trace, the point at which it is ready to pour. Fragrance and colour are usually added around this stage. In the mould, the soap heats itself as the reaction continues. Some batches go through a gel phase, turning darker and translucent before setting again; this is normal and mostly cosmetic. Within a day or two the soap is usually firm enough to unmould and cut. At this point it is still caustic, so keep wearing gloves when handling it.

The bulk of saponification happens in the first day or two, but the bar is far from finished. Curing is the period during which the soap sits in a cool, dry, airy place while:

The figure most soap makers work to is around four to six weeks for cold process soap, and some recipes, such as bars made mostly of olive oil, are often cured longer. A cured bar lasts much longer in the shower than a fresh one, which can turn soft and wear away quickly. A practical check is to weigh a test bar weekly: when its weight stops falling, the water has largely gone. How to cure well:

Read up on lye safety, set up a clear, uncluttered workspace with no children or pets around, and have your protective equipment on before you open the lye. Start with a small, simple recipe checked through a lye calculator. Have you made your first cold process batch, or are you still deciding? Ask your questions in the comments.