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The Chemistry of Saponification

The Chemical Reaction

Soap is the salt of a fatty acid. It is created by a chemical reaction called Saponification:

\[ \text{Triglyceride} + \text{Strong Alkali} \rightarrow \text{Soap} + \text{Glycerin} \]
  • Triglyceride: Your oils/fats.
  • Alkali: Sodium Hydroxide (NaOH) or Potassium Hydroxide (KOH).
  • Soap: The cleaning agent.
  • Glycerin: A natural humectant byproduct that stays in handmade soap.

Saponification Values (SAP)

Every oil requires a specific amount of lye to turn into soap. This is its SAP Value.

  • Coconut Oil has a high SAP (~0.183 NaOH), meaning it needs more lye to saponify.
  • Olive Oil has a lower SAP (~0.135 NaOH), meaning it needs less lye.
\[ \text{Lye Needed} = \text{Oil Weight} \times \text{SAP Value} \]

soap-calc stores these values for hundreds of oils to ensure safe conversion.

Lye Purity

Commercial lye is rarely 100% pure. * NaOH is typically ~97-99% pure. * KOH is typically ~90% pure (impurities include water and potassium carbonate).

soap-calc assumes standard purities (NaOH 100% theoretical, KOH 90% adjusted) unless configured otherwise. Note: Check specific soap-calc settings for exact purity defaults.

Water Ratios

Water dissolves the lye to allow it to react with the oil. It evaporates as the soap cures.

  • Water:Lye Ratio: Parts of water per part of lye.
    • 3:1 (High water): Slow trace, long cure, good for swirls.
    • 2:1 (Medium): Balanced.
    • 1.5:1 (Low water / Water Discount): Fast trace, fast cure, can be tricky.