Safety & precautions when handling lye and caustic soda

Making soap uses strong alkali (caustic soda) which can be dangerous if not used correctly. It can cause severe burns and can be fatal if ingested.

Call 111 (medical emergency) immediately if you or another person ingest caustic soda/lye or splash lye in the eye!

If you splash lye on your skin, rinse under cold water for at least 15 minutes. In case of severe burns, go to your nearest A&E (Accident and Emergency) or call 111 (NZ)!

Safety is important! Make sure that you and others stay safe by taking the following precautions:

  1. Firstly, know the correct safety and first aid procedures when working with and handling lye.
  2. Always store caustic soda in its original, correctly labelled, secure container away from children.
  3. Do not make soap if you are likely to be distracted or have children or pets running around.
  4. Never leave the caustic soda or prepared lye unattended!
  5. Don’t pre-make your lye! unless you are experienced enough to know how to master-batch. Always make just enough as you need.
  6. Always use cold water when preparing your lye! The dissolution of lye in water is an exothermic reaction. Using warm water will add extra heat, which can cause a severe explosive reaction.
  7. As a simple precaution, add solids to liquids. This is particularly important when using potassium hydroxide.
  8. Always work in a well-ventilated area and do not breathe in the fumes!
  9. Protect your eyes from splashes by wearing protective goggles.
  10. Wear long sleeved protective clothing, closed shoes, and gloves to protect your skin from splashes. If you do splash lye on your skin use COLD WATER to rinse it off. Do not use vinegar, which can cause a flash reaction and can cause a greater burn. Vinegar should only be used to neutralise spills – not on skin!
  11. Keep your soap making equipment separate from your cooking equipment, particularly plastic and silicone tools, which can take on the scent of the fragrances.
  12. Don’t fill the soap pot more than half full to avoid splashes and spillages when mixing.
  13. Always measure oils and caustic soda by weight and not by volume! Weight (grams, ounces) is more accurate than volume (mls, cups, spoons)
  14. And lastly, always test your soap before selling it or giving it away as gifts!

Soap makers use two types of lye: sodium hydroxide and potassium hydroxide. Sodium hydroxide is used to make solid bar soap, and potassium hydroxide is used to make liquid soap. A combination of the two can produce soap paste, which is often used in shaving soaps and cleaning soap pastes.

The words lye, caustic soda, sodium hydroxide etc are often used interchangeably by soap makers. However, the correct chemical names are sodium hydroxide for the solid form and lye solution for the liquid. Caustic soda is the industrial name given to sodium hydroxide, and does not come in liquid form.

The chemical formulae are NaOH for sodium hydroxide and KOH for potassium hydroxide.

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