Five weeks into cure, sitting on a wire rack in a spare room next to the bathroom, and it is a humid summer here. Recipe is olive heavy with 5 percent superfat and I used sodium lactate in the lye water for the first time. Every morning the bars are beaded with clear droplets and by early afternoon they look completely normal again. Do I keep waiting, move them, or write the batch off?
That is glycerin dew and the batch is fine. Soap makes its own glycerin, glycerin pulls water out of humid air, and the droplets evaporate again when the room warms up. It is a room problem, not a soap problem. Do not wrap or bag those bars while they are doing it.
It fixes it. Get the room reliably under about half relative humidity and the dew stops entirely. Airflow across the rack matters almost as much as the number.
Good, I was ready to rebatch the whole loaf. Would a dehumidifier in that room actually fix it or just slow it down?
Before you settle on humidity, do the boring check. Touch a wet fingertip to the flattest part of a bar and touch it to your tongue. If you get a distinct electric zap, that is free alkali and the bars are not safe to use yet regardless of what the droplets are. If nothing happens, humidity is your answer and you can relax. It takes five seconds and rules out the one explanation that actually matters. Five weeks with olive heavy soap should be well past that, but I have seen a mis-measured lye batch masquerade as a humidity problem.
Fair, and I should have said that. Dew that appears overnight and vanishes by lunch is almost always glycerin, but the zap test costs nothing.
I bought a cheap hygrometer for the curing room after fighting this for two summers, and it turned an argument into a number. My spare room was sitting in the seventies for relative humidity overnight, which is where every batch dewed. A small dehumidifier holding it around forty five to fifty stopped it completely, and as a bonus my bars finished harder and lost weight faster during cure. Two seasons of guessing solved by a device that cost less than one batch of oils.
Stand them on edge with a gap between each bar and put a fan on the rack for a few hours a day. Cure is water leaving, and water cannot leave a bar that is sitting flat on a solid surface in still air.
I shrink wrapped a batch that was doing exactly this because I had an order to post. Every bar sweated inside the film, the labels lifted at the corners, and two of them had a slimy film by the time they arrived. The customer was polite about it, which made it worse. Bars go in wrapping only when they have been dry to the touch for several consecutive days, and I now wrap in paper bands rather than film so anything that does dew can escape.
@darning_dot · 2mo ago
The sodium lactate is not innocent here either. It is hygroscopic on its own, so a batch with it will dew a little more readily than the same recipe without, especially at higher usage. That is not a reason to stop using it, because it does real work on unmoulding and hardness, but if you are already curing in a damp room it stacks on top of the glycerin. I dropped mine when I moved to a house with a damp utility room and the morning dew mostly stopped.
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