Why Handmade Soap Has to Cure (And How Long It Actually Takes)
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I poured a batch of soap a few weeks ago that nobody will be able to buy until October.
That sounds like bad planning. It's actually just how soap works, and it's the single biggest thing that separates a handmade bar from whatever's in the three-pack at the store. But almost nobody explains it, so you end up wondering why the small-batch bar you bought cost more and then disappeared down the drain in two weeks, or why it didn't.
Here's the short version, and then the longer one.
The short version
Cold process soap needs four to six weeks of curing before it's ready to use. During that time the water evaporates out and the bar gets harder, milder and longer-lasting. A bar used too early will still get you clean. It also feel a little harsher on your skin and turn to mush in about a week.
What's actually happening in those weeks
Soap is what you get when fats and lye meet. That reaction is called saponification, and it starts the moment the two go in the pot together. Within about 24 to 48 hours the chemistry of it is essentially finished. There's no lye left in a properly formulated bar after that point.
So if the reaction is done in two days, why wait six weeks?
Water.
Cold process soap is made with a fair amount of water, because the lye has to dissolve in something. That water is still in the bar when you cut it. Curing is mostly evaporation, and evaporation does three useful things.
It makes the bar harder. A freshly cut bar is soft, like firm cheese. You can press a fingernail into it. Six weeks later it's dense and solid. Hardness is the whole reason a cured bar lasts in a wet shower and an uncured one doesn't.
It makes the bar milder. A very fresh bar can feel slightly sharp on skin. As it cures, the pH settles and the bar softens in character, not in texture. That matters most for anyone with sensitive skin, which is a fair number of the people who buy handmade soap in the first place.
It makes the lather better. A cured bar lathers more richly and more slowly. A fresh one produces a lot of bubbles fast and then gives up.
Four to six weeks, and why the range
It depends on the recipe and on your house.
Recipe. A bar with a lot of olive oil cures slowly, because olive oil soap is soft to begin with. Castile soap, which is all olive oil, is properly excellent at around a year and merely fine at six weeks. A bar with more coconut oil or a harder fat in it firms up faster.
Humidity. Curing is evaporation, so a dry room speeds it up and a humid one slows it down. A batch cured through a damp stretch of weather genuinely takes longer than the same batch cured in January.
Airflow. Bars need to be spaced out with air moving around all sides. Stacked in a box, the ones in the middle stay wet.
Four weeks is the practical floor for most recipes. Six is the comfortable number. Longer is better, right up until you run out of shelf.
How to tell if a bar is properly cured
The most reliable way is weight. Soapmakers weigh a test bar every few days and watch the number drop. When it stops dropping, the water is out and the bar is done. It's unglamorous and it works.
Without a scale, go by feel. A cured bar is hard, doesn't dent under a fingernail, and has a slightly matte, settled look rather than a damp sheen.
What this means if you're buying
Handmade soap is not a fast product. When a maker says a scent is back in six weeks, that isn't a stalling tactic. Soap poured today for the holidays is the honest reason small makers make their holiday decisions in September while it's still warm out.
A cured bar should last you a long time. If a handmade bar disappears in a week, either it wasn't cured long enough or it's sitting in a puddle. Which brings us to the last one.
Let it dry between uses. This is the single biggest thing you control. A bar left in standing water will dissolve no matter how well it was cured. A draining dish fixes it completely.
The short version, again
Soap is chemically done in two days and actually ready in four to six weeks. The waiting is what makes it hard, mild and long-lasting. It's the least exciting part of soapmaking and it's the part that decides whether you like the bar.
If you want to see what a properly cured bar feels like, the fall ones have been on a rack since the beginning of August:
And if your bars keep vanishing early, it's almost certainly the dish and not the soap: Bamboo Soap Dish