How long does cold process soap need to cure?
Most cold process soap is cured for about 4 to 6 weeks, and soaps very high in olive oil, such as castile, are often cured much longer. The chemical reaction is mostly done within a couple of days; the cure is mainly water leaving the bar, which makes it harder and longer-lasting. Weighing a few bars until their weight stops dropping tells you when that water is gone.
Saponification and curing are different
Saponification, the reaction between the oils and the lye, takes about 48 hours. Curing is what happens afterwards: the extra water in the bar evaporates, leaving a denser, harder bar (Makesy: How long does bar soap need to cure?).
Typical cure times
- Most cold process recipes: at least four weeks; four to six is enough for most (Lovely Greens: How to cure handmade soap).
- Castile and high-olive soaps (60% olive oil or more): longer; castile at least six weeks, and Lovely Greens suggests six months to a year is better (Lovely Greens: How to cure handmade soap).
- Some makers find bars usable sooner; Makesy's own tests found well-formulated bars performed well after a few days, while also finding that longer cures last longer in use (Makesy: How long does bar soap need to cure?).
Where to cure
Room temperature, dry air and good airflow: space the bars apart on a rack, out of direct sun, and avoid closed boxes, cupboards and damp places that trap moisture (Lovely Greens: How to cure handmade soap).
Knowing when it's done
Weigh a few bars every day or so. When a bar weighs the same for three days in a row it has reached its cured weight, which is also the weight to put on a label (Makesy: How long does bar soap need to cure?).
Worked example: less water, less to evaporate
The same 1000 g of oils (Olive oil 50%, Coconut oil 25%, Shea butter 20%, Castor oil 5%) at three lye concentrations. The lye is the same; the water you put in, and later wait for, is not:
| Lye concentration | NaOH | Water in the batch | Raw batter |
|---|---|---|---|
| 25% | 138.5 g | 415.6 g | 1554 g |
| 33% | 138.5 g | 281.3 g | 1420 g |
| 40% | 138.5 g | 207.8 g | 1346 g |
A higher lye concentration (less water) gives a firmer bar sooner, but can also make the batter thicken faster. See water in soap recipes.
What curing doesn't do
Curing doesn't fix a recipe with too much lye, or lye that wasn't fully dissolved. Check the recipe before you make it, weigh carefully, and set aside any bar that looks or feels wrong.
In the Soap Ledger app, each batch gets a target review date when it goes into curing (28 days by default, or what you set per recipe), with reminders and a review step before bars are released. The date is a target for checking the bars, never a promise that they're ready or safe.
Lye safety. Verify every weight before you make this, and follow lye safety. Not a safety or compliance certification. Eye protection, gloves and covered skin; ventilation and a clear, uninterrupted workspace.
Try it in the lye calculator. Compare the water in your own recipe at different lye concentrations.
Sources
Written for Soap Ledger in our own words. Numbers in the examples are calculated by Soap Ledger's engine when the page is built. Last updated .