Lye safety

What lye is

Soap is made by reacting fats and oils with a strong base — sodium hydroxide (NaOH, “caustic soda”) for hard bar soap, or potassium hydroxide (KOH) for soft and liquid soap. Both are highly caustic: they cause chemical burns on skin and permanent damage to eyes, and they react with aluminium to release hydrogen gas.

Before you start

• Splash goggles and chemical-resistant gloves. Long sleeves.
• Good ventilation — lye fumes when first mixed with water.
• Heat-safe, non-aluminium containers (stainless steel, HDPE plastic, or glass rated for the heat).
• A clear, child- and pet-free workspace. Label everything.
• Have running water and white vinegar nearby for spills.

Mixing

Always add lye to water, never water to lye. Adding water to dry lye can cause it to erupt violently. The solution heats to near boiling within seconds — stand back and stir gently.

Why a test batch

SAP values are averages. Real oils vary by crop, region, and refining, and historical lye (e.g. from wood ash) varies even more. The calculator gives you a sound starting point, but the only way to know a recipe behaves is to make a small batch first and adjust. Treat every number here as an estimate, not a guarantee.