Make soap from lye and fat

Soap is the highest-value thing in this domain, because handwashing is one of the few genuinely effective disease controls available to a household with no medical support. See handwashing and making soap last, which is the page about using it, and read that first: making soap last is easier and safer than making soap.

This page assumes you have read make lye from wood ash, because every warning on that page applies here in full and is not repeated at the same length.

How it works

Fat plus a strong alkali gives soap and glycerol. That reaction is called saponification, it is reliable, and it has been done in kitchens for a very long time.

The whole problem is proportion. Too little alkali and you have greasy, weak, half-made soap. Too much and you have a bar with free caustic in it that will burn the person washing with it, and it looks exactly like ordinary soap.

Everything on this page is about managing that, and the method you use depends entirely on whether you know how strong your alkali is.

Which alkali, and why it matters

Buy sodium hydroxide while you can. It stores for years in a sealed container, it costs very little, and it is the difference between soap-making as a recipe and soap-making as a guess. It is one of the highest-value things in the whole "buy it now" category.

Ash lye is the fallback, and this page treats it as such honestly rather than romantically.

The fat

Almost any animal fat or vegetable oil works.

• Tallow, rendered beef or mutton fat, makes a hard, long-lasting, traditional bar. See rendering fat.
• Lard makes a softer, milder bar.
• Used cooking oil works, and must be strained and cleaned first: heat with water, let it settle, and pour off the fat from the top once cold.
• Olive and rapeseed oil make a mild soap that takes longer to trace and longer to cure.
• Coconut oil lathers well and is drying on its own; a proportion of it improves most recipes.

Render and clean the fat properly first. Meat scraps, salt and water left in the fat make soap that goes rancid, smells and will not set.

Cold process, with bought caustic soda

The straightforward method, and only usable when you know the alkali's strength.

1. Use a soap calculator or a tested recipe for the specific fat, and superfat by around 5%, meaning use about 5% less alkali than the fat theoretically needs. That margin is what guarantees no free caustic in the finished bar, at the cost of a slightly softer soap.
2. Weigh everything. Soap is chemistry, not cooking, and volumes are not good enough.
3. Mix the lye solution: add the caustic soda to the water, never water to the soda, in a stainless or heavy plastic vessel, outdoors or by an open window. It will get very hot and give off fumes. Do not lean over it.
4. Let both cool to around 40 to 50 degrees.
5. Pour the lye into the melted fat slowly, stirring continuously.
6. Stir to trace: the point where a drizzle from the spoon sits on the surface for a moment before sinking. By hand this can take a long time; a stick blender takes minutes.
7. Pour into a mould lined with greaseproof paper, cover, and insulate with a towel.
8. Leave 24 to 48 hours, then turn out and cut into bars.

Hot process, which is what ash lye needs

Cooking the soap drives the reaction to completion while you watch, which means you can judge and correct it before it is set. That is exactly what an alkali of unknown strength requires.

1. Start with much less lye than you think, and keep the rest to hand. Beginning fat-heavy is safe; beginning lye-heavy is not.
2. Combine fat and lye in a stainless or enamel pan and heat gently, stirring.
3. Cook on a low heat for an hour or more, stirring regularly. It will go through a gloopy stage, then thicken, then take on a translucent, waxy, mashed-potato appearance. That change is the reaction completing.
4. Judge it as you go. If it stays greasy and refuses to come together, it needs more lye: add a little at a time, stir it in, and keep cooking. If it seems harsh, add more melted fat.
5. Test before you mould it, see below.
6. Spoon into a mould while still warm, press down to remove air pockets.

Ash-lye soap will be soft, because potassium hydroxide makes soft soap. That is correct, not a failure. Kept in a tub as a paste, or thinned with water into a liquid soap, it works perfectly well for washing hands, bodies and clothes.

Curing

Cold process soap needs four to six weeks, on a rack, in a dry airy place, turned occasionally so all faces dry.

Curing does two things: it lets any last traces of alkali finish reacting, and it evaporates water so the bar hardens and lasts far longer in use. A bar used at two weeks dissolves into slime in a few washes.

Hot process soap is technically usable sooner, because the cooking has already completed the reaction, but a fortnight of drying makes a noticeably harder and better bar.

Cure it out of reach. It is still soap-in-progress, and a child who chews an uncured bar has eaten something caustic.

Testing it before anybody uses it

Other checks worth doing:

• Look for pockets of clear liquid or white crystalline patches, both of which suggest the mix separated and there is free alkali somewhere in the bar.
• Wash your hands with it once, then rinse well, and see how the skin feels an hour later. Tightness or redness means it is too harsh.
• Test on hands before it goes anywhere near a face, a child or a wound.

If in doubt, do not use it on people. Lye-heavy soap is still perfectly good for washing floors, tools, laundry and pots, so nothing is wasted by demoting it.

When it goes wrong

• It will not trace, stays like soup: too little alkali, temperature too low, or the fat was wet. Keep stirring; try gentle warmth.
• It separates, oil floating on top: temperature was wrong or it was not stirred enough. Rebatch it, meaning grate it, add a little water, and cook it again.
• It seizes into a solid lump in seconds: usually a fragrance or an additive, or too hot. Press it into the mould and treat it as hot process.
• White powder on the top, called ash: cosmetic only, scrape it off.
• It is greasy after curing: too much fat. Usable for laundry and cleaning.
• It burns or stings in use: too much alkali. Do not use it on skin. Rebatch with added fat, or keep it for floors.