The Saponification Reaction
Cold-process soap making is a chemical reaction between fats (oils) and a strong base (Sodium Hydroxide / Lye). This process is called Saponification. If the math is wrong, the resulting soap can be 'lye heavy,' which can cause severe skin burns, or it can be 'oil heavy,' resulting in a soft, greasy bar that quickly goes rancid.
SAP Values
Every oil has a unique 'Saponification Value' (SAP). This is the exact amount of lye required to turn 1 gram of that specific oil into soap.
- Olive Oil: 0.135
- Coconut Oil: 0.190 (Requires more lye because its fatty acid chains are shorter)
- Shea Butter: 0.128
The 'Superfat' Safety Buffer
To ensure no unreacted lye remains in the finished soap, soap makers always add more oil than the lye can process. This is called 'Superfatting.' A standard superfat level is 5%. This ensures the bar is moisturizing and 100% safe for skin use.
The Formula
The total lye required is the sum of the weights of each oil multiplied by their respective SAP values, then reduced by the superfat percentage.
Lye Weight = Σ(Oil Weight * SAP Value) * (1 - Superfat%)
Lye Safety
Lye is extremely caustic. When mixing lye and water, ALWAYS add the lye to the water, never the other way around. Adding water to a bowl of lye can cause a 'lye volcano' due to the rapid exothermic reaction. Always wear goggles and gloves.