Soap Curing, Sweating, and Seizing

soap curing

Here are discussions of three terms related to soapmaking that don’t come up as often as we think they would. For the first term – Curing – the discussions is is mostly informational. For the second – Sweating – it addresses something that can concern soapmakers, but is in the end harmlessly natural. For the third term – Seizing – the discussion points out something potentially more serious, something that can sneak up on you.

Two Things at Once


Curing is a process involved in all kinds of products, most commonly meat and fish. Officially, it’s a process during which a chemical reaction or physical action takes place, resulting in a harder, tougher, more stable substance or bond.

For meat and fish, curing is a form of preserving. For cold-processed soap, it’s more a form of finishing. For the 4-6 weeks it takes cold-processed soap to cure, there’s a slight bit of pH finalizing, but mostly, curing consists of a physical action – evaporation – during which the bar becomes harder as it loses water. Soap can be used before it’s fully cured, but since it’s not really finished, and therefore not as hard as it will be, non-cured soap won’t last anywhere near as long as bars that are allowed to finish.

It’s in the Chemistry

Soap doesn’t really sweat. In fact, what happens is the opposite of sweating. But it can look like it sweats. And that sweating can take the appearance of “weeping” or the formation of a crystalline-like powder or haze. One source we found uses the term “glycerin dew.”

The truth lies, as much does in much soapmaking, in the chemistry. One of the natural byproducts of true soapmaking is glycerin. In that respect, all true soap is glycerin soap, just not in the way that term is commonly used. Glycerin qualifies as a humectant, a substance that attracts and retains moisture beneath the surface through absorption. Natural soap, with its glycerin retained, can attract humidity due to climate or where it’s stored. Especially in humid climates, sweating can be an issue. The best advice is the simplest: Let your soap harden and cure at natural room temperature and then store in a cool, dry place.

Trace Acceleration and Seize

Sometimes chemistry can happen too fast. With soap, too fast takes the shape of “trace acceleration” where the speed of thickening overwhelms the process of saponification. Instead of a naturally thickening mix, you get batter too thick to pour, or worse, you experience true seize and you wind up with a hard, unusable mass.

You’re right to think this is something to avoid. The main culprits are fragrance oils and alcohol. Both have been known to cause trace acceleration. If you’re making organic soap and using essential oils, neither of those should be a problem. A third possible cause is the original temperature of your lye and oils. A prudent, commonly recommended  range is 80-100 degrees. With the possibility of other factors, prudence in chemistry is wise, especially given the consequences of haste.

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