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. Completing the cook does not by itself certify the batch for use; verify the formula, process record, pH with a suitable method, and your release checks. A cure can still improve hardness and water loss. 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 SoapPilot first for the exact lye. See how long soap takes to cure.