Cold process and hot process use the same oils, lye, and SAP math. The only difference is whether you add heat to speed up saponification.
Cold process (CP)
Mix lye solution into oils, bring to trace, add fragrance/color, pour, and insulate. Saponification finishes on its own over the next 24-48 hours; you unmold and cut, then cure 4-6 weeks. Gives the smoothest surface and the best swirls — the classic artisan look.
Hot process (HP)
Same start, but you cook the batter (typically a crockpot) so saponification completes in the pot within an hour or two. The soap is technically safe to use sooner, though a cure of a couple of weeks still makes a harder, milder, longer-lasting bar. The look is more rustic/textured.
How to choose
| Cold process | Hot process | |
|---|---|---|
| Heat after mixing | None | Cooked |
| Ready to use | After 4-6 wk cure | Sooner (still cure for quality) |
| Look | Smooth, best for swirls | Rustic |
| Lye math | Identical — same calculator | |
Whichever you use, run the recipe through SoapCalc first for the exact lye. See how long soap takes to cure.