Most curing times I see listed in soap groups (which commonly say 4 – 6 weeks now, though 6 – 8 weeks used to be the popular one) are a WILD ASS GUESS. As is the 1 year for the curing time for castile. If 100% olive oil soap takes a year to cure, how can a 60% olive oil take 4-6 weeks or 6-8 weeks and how can a coconut oil bar take the same time? They don’t.
Let’s get clear on what we’re talking about here because there is also a lot of confusion between saponification time and curing time.
Saponification is the chemical process through which fat and lye make soap. It varies some but most correctly made soaps are saponified by the time they are hard enough to cut. This process will continue in a very faint way over time but by the time the soap is hard it will generally pass a zap test. A lye-heavy soap can often be saved just by waiting but that’s a different part of this topic. I want to talk about soap that isn’t problematic.
We often hear that hot processed soap is saponified right after the cook and that cold processed takes days. However, there are many times when a CP soap will reach as high a temperature as a HP soap will because saponification is an exothermic process. The process itself creates heat.
Regardless of how long the process of “milding” goes on, for our purposes the saponification is done when the soap passes the zap test. You can safely use the soap any time after this.
Then what is curing?!
Curing is the drying out and hardening process which will make the soap last longer once it starts being used. Fresh soap has a higher water content than aged soap. There are a lot of variables that have an affect on this: the oils used, the amount of water discount, other additives, humidity and the size of the bar.
You will often hear that castile soap has a 1 year curing time. Soft oils do not make a soap as hard as hard oils do and will generally need to be cured longer to reach their maximum hardness but even that has many variables, one of which is the huge variety of different olive oils. I do not know how the chemical properties break down but I can tell you from trying and using different castile soaps that the differences can be as big as the differences between two different types of oil. (And to add to the confusion, there is even counterfeit olive oil). Whether they are hot or cold processed and what amount of water is used also makes a big difference.
For some soaps, especially if you have made them for a long time, you could tell by looking or touching the soaps but that’s not much help for a new soap you are making or a new oil or brand of oil you are using. There is only one good way that I know of and that is testing the soap by weight.
For every recipe you use you need to keep different notes. Also note that any difference in ingredients (including scents) and color type*, process, superfat or water discount constitutes a new recipe because all of these things will affect your curing time.
Take a bar or two from each batch and weigh it (preferably right after cutting). Note the weight on the bar. Use grams even if you usually measure in ounces because you need this to be as accurate as possible, You are measuring small amounts. Weigh the bars every week on the same day, if possible. I try to weigh all my bars on Saturday. When they go a week without losing any weight, you can consider the bar finished.
Because I never did this for the first 40 years of soap making, I only have good factual (as opposed to anecdotal) info on my soleseife recipe. I know that using that recipe with a full measure of water (38% water as percent of oil weight – the default in soapcalc) creates a soap that cures in just under 4 weeks in individual molds from 3 – 4 ounces. That is a very fast soap. It was obvious when I first made it that it would cure quickly and we started using it the day after it was made. At that point it resembled my olive and coconut soap at about a 6 week cure time. There is no way around cure time and rushing it will lose customers because your soap will get used up too fast and therefore be expensive. Just keep in mind that if you didn’t do the testing or trust someone who did… you have no idea what the cure time actually is. There is no one-size-fits-all cure time.
Note: At the time when I was weighing the bars pictured above I didn’t always write the last weight on them. Once they stopped losing weight I just moved them to the “finished” tray but now that I want to be able to keep and show better records, I do write the last date – with the same weight as the previous week. It’s also a good idea to put a batch number on it so you can relate it to the rest of the batch and any other notes you have on it – sometimes different batches look identical.
* color type: Let’s say you are using micas for color. The colors will probably have the same cure time as each other. But if you are using clay in one batch and infused oil in another, then the color type will make a difference.