What's the difference between cold process and hot process soap?
Short answer
Cold process soap saponifies in the mold and needs a full cure. Hot process soap is cooked until saponification is nearly complete, so it only needs 1–2 weeks to harden.
Explanation
In cold process, lye solution and oils are combined at moderate temperatures (from room temperature to about 46 °C, depending on the recipe), blended to trace and poured. Saponification finishes in the mold, and you can encourage or prevent the gel phase. The bars need a full cure, usually at least 4–6 weeks and longer for soft-oil recipes.
In hot process, the batter is brought to a light trace and cooked on low heat (a slow cooker on LOW, an oven or a double boiler at about 60–70 °C). It passes through a foamy stage, then a glossy, translucent gel, and ends as a waxy paste like mashed potatoes. That takes 45–60 minutes in the book's jasmine recipe. Superfat oils, delicate scents, milks and honey go in after the cook, once the lye is used up. The bars look rustic and textured, which suits layers and embeds but makes smooth tops and fine swirls harder.
What to do
- Hot process: don't stir during the initial gel, or the texture turns grainy.
- Check hot process soap is done with pH strips: a diluted dab should read 8–10.
- Cold process gives you smoother surfaces and finer designs.
Source: The Big Book of Homemade Soap — The Gel Phase in Cold & Hot Process Soapmaking · Jasmine-Infused Hot Process Soap