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Creaturely Soap

Lye safety

There is no soap without lye. In cold process soapmaking, lye reacts with oils and fats during saponification, transforming them into soap.

Before that reaction is complete, lye is highly caustic and can cause serious burns. Read through the safety information before your first batch, and use the checklist as a quick reminder each time you make soap.

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What lye actually is

Lye, in cold process soap, means sodium hydroxide (NaOH) — a solid white alkali that comes as flakes, beads, or pearls. When dissolved in water, it becomes a strong caustic solution. Mixed with fats, it drives saponification. When saponification is complete, properly formulated soap should not contain free sodium hydroxide.

Two properties are especially important to understand.

It gets hot when mixed with water

Dissolving lye in water releases a large amount of heat. The solution can become hot enough to cause serious burns, so always use a suitable heat-resistant container and add lye to water gradually.

It can cause serious chemical burns

Lye is corrosive and can cause deep burns to skin and permanent damage to the eyes. Some exposures may not feel severe immediately, so rinse any exposure promptly.

Lye is also odorless. Do not rely on smell to tell you whether the air is safe.

The gear

Four things are essential for working with lye safely.

  • Splash goggles — sealed, not safety glasses

    Use goggles that seal around your eyes. Safety glasses leave gaps where splashes can enter.

  • Gloves that cover past the wrist

    Use nitrile or sturdy dishwashing gloves. Check them for holes before you begin, and replace them if lye gets inside.

  • Long sleeves, long trousers, closed shoes

    Cover your skin with clothing and closed shoes. Choose older clothes if possible, since lye can damage fabric.

  • Moving air

    Open a window, turn on an extractor, or work near an open doorway. Good ventilation helps reduce exposure to mist while the lye solution is being mixed.

Useful but optional: an apron and a bottle of water kept beside the sink for quick rinsing.

Your workspace

Lye reacts with metals including aluminum, tin, and zinc. Use stainless steel, heat-proof plastic suitable for hot solutions — polypropylene (#5) is a common choice — or heat-proof glass.

If you use glass, use borosilicate and retire it early

Lye slowly etches glass, and etched glass is weaker than it looks. Borosilicate laboratory glass resists it far better than the tempered soda-lime glass sold as kitchen Pyrex in the US. Whichever you have, check it for cloudiness, scratches, or chips before every use, never put a cold vessel under a hot pour, and replace it once the surface starts to look frosted rather than clear.

Lye is also corrosive to many surfaces. Protect countertops and flooring that could be damaged, and keep lye away from acids, food, and anything used for eating or drinking.

Have cold running water within easy reach before you begin.

Mixing, step by step

The one rule

Lye goes into water. Water never goes into lye.

Adding water to solid lye can cause rapid heating and dangerous splashing. Always add the lye to the water.

  1. 1Weigh your water into the vessel you will use for mixing. Use cold or room-temperature water.
  2. 2Weigh the lye separately in a dry container. Close the lye container when you're finished measuring.
  3. 3Take both to your ventilated workspace. Put on your goggles and gloves before opening the lye.
  4. 4Add the lye to the water gradually, stirring gently between additions.
  5. 5Stir until the solution is clear. Keep your face away from the rising mist.
  6. 6Place the solution somewhere safe and undisturbed to cool. Label it and keep it away from children, pets, and the edge of your workspace.
  7. 7Wash anything that came into contact with the lye solution while still wearing your gloves.

A note on frozen milks and juices

Recipes that use oat milk, coconut milk, or fruit purée often freeze the liquid before adding the lye. This helps prevent the sugars from scorching during the reaction. Add lye slowly to frozen liquids and expect the mixture to develop an unpleasant smell as it reacts.

Pre-flight check

Run through this every session, not just the first one.

Before you open the tub

The pre-flight check

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On your body
In the room
On the bench
In your head

When it goes wrong

Read this before you need it, because you will not be reading carefully when you do. The single most important thing on this page: do not try to neutralize a lye burn with vinegar or anything else acidic. Neutralizing releases heat into tissue that is already burning. Flush with water, at length.

  • Flush immediately with cool running water for at least 15 minutes

    1. 1Remove contaminated clothing.
    2. 2If there are dry lye flakes on the skin, gently brush them away while protecting your eyes and face.
    3. 3Flush the affected area with cool running water for at least 15 minutes. Do not delay rinsing to look for a neutralizing agent.
    4. 4Do not use vinegar, lemon juice, or other substances to neutralize lye on the skin.
    5. 5Continue rinsing while seeking medical advice if there is pain, irritation, or visible injury.

    Seek medical attention for significant burns or any skin that is white, blistered, numb, or broken. The full extent of a chemical burn may not be immediately apparent.

Professional resources

For additional information or help after an exposure:

Emergency: Call 911 for severe symptoms or a serious exposure.

Outside the US? Emergency numbers differ by country — look yours up here, and find your nearest poison center in the WHO world directory. It's worth knowing both before you need them.

This page is general safety information, not medical advice.

Stick it on the cupboard door

Print this page and keep it where you make soap. Every recipe I sell also comes with a one-page safety card, so the important information is easy to find when you need it.

See the recipes