How do I make soap at home safely?

Soap making sits oddly in the craft world: the process is straightforward and the results are excellent, and one of the two ingredients is a strong alkali that causes serious chemical burns. Treating the lye stage with the seriousness it deserves is what separates people who make soap for years from people who have one bad afternoon.

Safety

What you'll need

• Sealed chemical splash goggles, not glasses
• Nitrile gloves and long sleeves
• Sodium hydroxide sold for soap making
• Digital scales accurate to a gram
• Stainless steel or heatproof plastic containers — never aluminium
• A stick blender kept for soap only
• A thermometer and a mould

Step by step

1. Run the recipe through a soap calculator, every time.Each oil requires a different quantity of lye to saponify. A recipe cannot be scaled up, down or have an oil substituted without recalculating, because too little lye gives soft greasy soap and too much gives a caustic bar that burns skin. This is not a step to improvise.
2. Include a superfat allowance.Calculators let you specify a small percentage of excess oil, which guarantees that all the lye is consumed and leaves a little unreacted oil for conditioning. It is the margin that makes home soap safe to use.
3. Set up the room before you open the lye.Ventilation on, children and pets out and the door shut, goggles and gloves on, sleeves down, everything weighed out and in position, and a source of running water available. Once lye is dissolving you should not be moving around.
4. Always add lye to water, never water to lye.Adding water to solid lye can cause the solution to erupt out of the container. Lye slowly into water, stirring, is the only correct order. The solution will heat itself to near boiling within seconds.
5. Mix the lye solution outdoors or under extraction.The first minute produces irritant fumes. Stand back and to the side while stirring, and let the solution cool in a safe place where nobody will knock it. Label the container clearly.
6. Bring the lye solution and the oils to a similar temperature.Both somewhere around body temperature to slightly above, and within a few degrees of each other. Wildly mismatched temperatures cause separation or a seized batch.
7. Blend to trace.Pour the lye solution into the oils, never the other way round, and blend in short bursts. Trace is when the mixture thickens enough that a drizzle from the blender sits briefly on the surface. Over-blending past trace gives you no time to pour.
8. Add fragrance and colour at trace, and pour.Some fragrances accelerate the reaction dramatically and seize the batch in seconds, so add, stir briefly and pour without delay. Pour into the mould, tap out air, and insulate it.
9. Leave it, then cure it for weeks.A day or two in the mould, then unmould and cut. Then cure on a rack with air all round for a period of weeks. Curing is not just drying — the bar continues to change and a fresh bar is harsher and dissolves faster.
10. Test the finished bar before giving it away.A correctly made bar feels like soap. A bar that stings, feels caustic, or has white crystalline patches has excess lye and should not be used. Zap-testing by touching a bar to the tongue is traditional and is not a method to recommend — pH strips are safer.

Tips

• Keep every tool used for soap separate from kitchen equipment permanently, including the stick blender and the scales bowl.
• Write the recipe down with the exact weights before you start and follow the written version. Working from memory while wearing goggles with lye on the bench is how mistakes happen.
• Start with a simple recipe of two or three common oils and no fragrance. Fragrance oils are the main cause of batches seizing, and a first batch is complicated enough.

Common mistakes

• Adding water to lye instead of lye to water — The reaction is violent enough to spatter caustic solution out of the container, at face height, at near boiling temperature. The order is not a preference.
• Scaling or substituting a recipe without recalculating — Each oil needs a specific amount of lye. Swapping olive oil for coconut oil at the same weight produces either soft greasy soap or a caustic bar. Every change goes through the calculator.
• Wearing safety glasses instead of sealed goggles — Glasses do not stop a splash from below or from the side, and an alkali splash in the eye causes rapid and often permanent damage. Sealed goggles cost very little.
• Using an aluminium pan or spoon — Lye attacks aluminium, generating hydrogen gas and contaminating the batch. Stainless steel, heatproof plastic, silicone or glass only.

If it doesn't work

Alternatives

• Melt-and-pour soap base: Soap that has already been made and saponified, sold as a block. You melt it, add colour and fragrance, and pour. No lye, no cure time, no hazard beyond hot liquid. It is the sensible starting point and a legitimate craft in itself.
• Rebatching: Grating an existing soap, melting it with a little liquid and re-moulding with additions. No lye handling, and it is also the rescue route for a batch that went wrong.
• Hot process soap: The same chemistry as cold process but cooked, so saponification completes in the pot and the bar is usable much sooner. Same lye hazard, rustic texture, shorter wait.

Questions people ask