Lye is the only truly hazardous ingredient in soap making, and every year soap makers are injured by preventable accidents. This guide covers everything you need to handle NaOH and KOH safely — from basic equipment to emergency procedures.
Lye (sodium hydroxide and potassium hydroxide) is a strong alkali with a pH of 13–14. Contact with skin or eyes causes chemical burns. Dissolving lye in water is exothermic — the solution can reach 85°C / 185°F. Never treat lye casually, regardless of how many batches you have made.
Essential safety equipment
- ✓Chemical-resistant gloves. Nitrile or neoprene. Thin latex gloves are not adequate. Cover your wrists — lye splashes can travel further than you expect.
- ✓Safety goggles. Not regular glasses. Full-seal goggles that protect from splashes on all sides. Lye in the eyes is a medical emergency.
- ✓Long sleeves and closed shoes. Cover your arms and feet. A full-length apron adds extra protection.
- ✓Heat-resistant container for mixing. Stainless steel or heavy HDPE plastic. Never use aluminium — lye reacts with it and produces hydrogen gas. Never use thin plastic or regular glass.
- ✓Dedicated equipment. Do not use soap-making equipment for food preparation. Label everything clearly.
- ✓Running water nearby. Know where your nearest sink is before you start. Rinse immediately and thoroughly if lye contacts skin.
- ✓Ventilation. Dissolving lye releases fumes that irritate the respiratory tract. Work near an open window, under a range hood, or outdoors.
- ✓No distractions. No children, pets, or phone calls while handling active lye solution.
Always add lye to water — never water to lye
This is the single most important rule in lye handling. When you add lye to water, the exothermic reaction is controlled — the large volume of water absorbs the heat. When you add water to lye, you get a violent, potentially explosive reaction that can spray caustic liquid in all directions.
A simple memory aid: "Snow falls into the lake" — the white lye falls into the water.
Add lye slowly, in small amounts, while stirring constantly. The solution will heat rapidly — this is normal. Do not lean over the container. Stir until all lye is fully dissolved and the solution is clear.
Safe lye mixing procedure
- 1
Put on all PPE before touching lye
Gloves and goggles go on first, before you open the lye container. Do not remove them until the lye solution has cooled and been mixed with oils.
- 2
Weigh your water first, into a heat-resistant container
Use distilled or de-ionised water where possible — tap water minerals can interfere with the reaction. Place the container on a heat-safe surface.
- 3
Weigh your lye into a dry separate container
Lye absorbs moisture from the air. Weigh quickly and close the lye container immediately. If your lye has clumped, it has absorbed moisture and its effective purity has dropped.
- 4
Slowly pour lye into water, stirring constantly
Do this outdoors or under good ventilation. Stir after each addition. The solution will become very hot and release fumes — turn your face away or wear a respirator if sensitive.
- 5
Set the lye solution aside to cool
Leave it in a safe place — out of reach of children and pets, not on a surface that could be damaged by heat. A sink works well. Allow to cool to 40–50°C before combining with oils.
- 6
Never leave lye solution unattended and unlabelled
Label the container clearly while cooling. Clear lye solution looks like water. An unlabelled container of lye solution is a serious hazard.
What to do if lye contacts skin or eyes
Skin contact
Rinse immediately with large amounts of cool running water for at least 20 minutes. Do not use hot water. Do not apply vinegar or other acids — this creates a secondary chemical reaction. Seek medical attention if any redness, burning, or blistering develops.
Eye contact
This is a medical emergency. Rinse immediately with large amounts of cool running water — hold your eye open under a running tap for a minimum of 20 minutes. Call emergency services or go to an emergency room immediately. Do not rub the eye.
Ingestion
Call emergency services or poison control immediately. Do not induce vomiting. Give small amounts of water if the person is conscious and able to swallow. Follow emergency dispatcher instructions.
Storing lye safely
- ✓Store in the original container or a clearly labelled, airtight HDPE container. Never in food containers.
- ✓Keep in a cool, dry location. Humidity degrades lye and reduces its effective purity.
- ✓Store out of reach of children and pets, ideally in a locked cabinet.
- ✓Check purity before use if stored for more than 6 months. If it has clumped, its effective purity has decreased.
- ✗Never store near acids, ammonium compounds, or flammable materials.
- ✗Never store in glass — lye slowly etches glass containers over time.
Once saponification is complete and the soap has cured, all lye has been converted into soap salts. Properly made soap has a pH of 9–10 and is safe for skin. The zap test confirms it: if cured soap does not zap your tongue, it contains no free lye. The hazard exists only during the manufacturing process.
Calculate your lye amounts precisely
Accurate lye calculation is the first line of safety. SoapCalc.pro gives you precise NaOH and KOH amounts for any recipe, with adjustable lye purity inputs.