The Big Book of Homemade Soap

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.

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.

Source: The Big Book of Homemade Soap — The Gel Phase in Cold & Hot Process Soapmaking · Jasmine-Infused Hot Process Soap · Open the article

Most balanced cold process bars need 4–6 weeks; soft-oil bars need longer, and 100% olive Castile 6–12 months. Curing lets water evaporate, the pH settle and the bar harden.

During the cure, water evaporates, the pH stabilizes and the soap crystallizes into a hard, mild, long-lasting bar. The book's cure times:

High-coconut or utility bars: 2–3 weeks (a 100% coconut skin bar at 20% superfat, about 10 weeks). Balanced cold process bars: 4–6 weeks. Hot process: usable after the cook, with 1–2 weeks of hardening. Bars with 60–80% soft oils: 6–12 weeks. 100% olive Castile: 6–12 months. Salt bars: 4–8 weeks. No-gel, milk, high-butter or high-sugar bars: 8–10 weeks.

Source: The Big Book of Homemade Soap — Curing Times for Different Soap Formulations · Open the article

A deliberate excess of oil that the lye can't saponify. It stays in the bar as free oil, making it milder and ensuring no free lye remains.

The standard safety superfat is 3–10%, and most recipes in the book use 5–7%. Coconut oil is the exception. Because it's so saturated and stable, it can carry a 20% superfat for a skin bar without going rancid, while 0–3% makes a household or laundry bar. Oils high in unsaturated fats, such as olive or sweet almond, would go rancid at 20%.

Source: The Big Book of Homemade Soap — 100% Coconut Oil Soap: A Masterclass in Single-Oil Formulation · Open the article

That's soda ash: a harmless, powdery film of sodium carbonate formed by carbon dioxide in the air. Spray fresh soap with alcohol to prevent it.

Soda ash doesn't affect how the soap performs; it's purely cosmetic. To prevent it, spray the freshly poured surface generously with 91–99% isopropyl alcohol.

Source: The Big Book of Homemade Soap — Recipe steps and FAQs, e.g. Rosemary & Citrus Luxury Bar · Open the article