Sodium hydroxide, also called caustic soda or lye, is the chemical compound NaOH. It is used to turn oils and fats into soap. Before that reaction takes place, however, sodium hydroxide and its concentrated solutions are corrosive: they can cause severe skin burns and eye damage. A ready-made liquid solution still requires full precautions.
What is sodium hydroxide?
Pure sodium hydroxide is a white solid. Commercial forms include pellets, flakes and aqueous solutions at different concentrations. It is produced industrially by electrolysis of brine, a solution of sodium chloride in water. Electricity drives the chemical process; the fact that the starting material is salt water does not make the resulting caustic chemical mild or harmless.
In French, lessive de soude means an aqueous sodium hydroxide solution. Despite the word lessive, it is not ordinary laundry detergent. The archived Atelier Populaire recipes specify a ready-made 30% solution. Concentration matters: the same weight of a different solution will not supply the same amount of sodium hydroxide.
Not every “soda” is the same chemical
| Common name | Chemical identity | Use in these recipes |
|---|---|---|
| Caustic soda or lye | Sodium hydroxide, NaOH | The specified 30% aqueous solution is the alkaline reagent. |
| Baking soda | Sodium bicarbonate, NaHCO3 | Not a substitute for sodium hydroxide. |
| Washing soda or soda crystals | Sodium carbonate, Na2CO3 | Not a substitute for sodium hydroxide. |
| Sodium percarbonate | Sodium carbonate peroxyhydrate | Not a substitute for sodium hydroxide. |
What does it do in soap?
In saponification, sodium hydroxide reacts with the oils and fats to form soap and glycerin. A recipe must match the amount of alkali to the particular oils being used. Replacing one oil with another, changing the lye concentration or estimating quantities by eye can upset that balance. Use a reliable scale and the exact specified ingredients.
The finished soap and the raw lye are different materials. Correct formulation and manufacture are essential; simply leaving a mistaken batch to cure does not guarantee that excess alkali has disappeared. The original recipes specify a one-month cure, but this is not a substitute for checking the recipe and process.
Sodium hydroxide also has industrial uses and is found in some drain and oven cleaners. Those household products may contain other ingredients and are not interchangeable with a specified soapmaking raw material. This article is not a cleaning or drain-unblocking protocol.
Handle a ready-made solution with care
- Wear suitable chemical-resistant gloves, splash goggles and protective clothing, including long sleeves. Keep children and pets away.
- Work in a ventilated area and avoid breathing mist or aerosols. Keep the work area separate from food and drinks.
- Use compatible containers and tools. Aluminium and several other metals react with sodium hydroxide; choose equipment whose supplier confirms compatibility.
- Keep the product in its clearly labelled original container, tightly closed, in a cool, dry, protected place away from incompatible chemicals.
- The archived recipes use ready-made lye and require no dilution. Never pour water into concentrated lye or onto solid sodium hydroxide: dilution releases heat and can splash.
A strong alkaline solution must be treated as corrosive; a single pH number is not a replacement for the concentration, label or safety data sheet. These precautions follow the INRS recommendations (in French).
What to do after an exposure
Skin or eyes: rinse immediately with plentiful running water for at least 15 minutes and obtain urgent medical help. Remove contaminated clothing. For eyes, remove contact lenses if easily possible and keep rinsing. Do not apply vinegar, an acid or a cosmetic to neutralise a splash.
Swallowed: rinse the mouth, do not induce vomiting, and call a poison centre or emergency medical service immediately. Do not give anything to drink unless instructed by a medical professional. For inhalation or breathing difficulty, move away from the exposure without putting anyone else at risk and call emergency services. Keep the container or label available.
Read the complete recipe before handling lye
The Douceur recipe and the Détox recipe contain different oil mixtures and lye quantities. Do not combine them. For a finished bar rather than home manufacture, the cold-process soap guide compares scent, superfatting and format.