The chemistry of soap
Soap is made by saponification: a strong alkali, the lye, reacts with the fatty acids in oils and butters to form soap and glycerin. Sodium hydroxide (NaOH) makes hard bar soap; potassium hydroxide (KOH) makes soft or liquid soap. Each oil needs a specific amount of lye, given by its saponification value, so a recipe must be calculated oil by oil.
How the calculation works
For each oil: lye = weight of oil × saponification value. The calculator adds the oils, then applies:
- Superfat: a percentage of oil left unsaponified, usually 5 to 8%, for a milder bar and a safety margin. The lye is reduced by that percentage.
- KOH: for liquid soap, the NaOH amount is multiplied by 1.403, the ratio of their molar masses, and divided by 0.9 because commercial KOH is about 90% pure.
- Water: as a share of the oil weight, typically 30 to 38%, giving a lye solution of about 25 to 40%.
Example
A recipe of 16 oz of olive oil, 10 oz of coconut oil and 6 oz of shea butter, with 5% superfat and water at 33% of the oils:
- Lye (NaOH): (16 × 0.135 + 10 × 0.183 + 6 × 0.128) × 0.95 = 4.52 oz, or 128 g
- Water: 10.56 oz
- Lye concentration: 30%
- Total batch: about 47 oz, or 2.9 lb
Saponification values used
Per gram of oil, for NaOH: olive 0.135, coconut 0.183, palm 0.142, shea butter 0.128, cocoa butter 0.137, castor 0.128, sunflower 0.134, sweet almond 0.136, canola 0.124, avocado 0.133, lard 0.138 and beef tallow 0.143. Values vary slightly between batches of oil, which is one reason to keep a superfat.
Safety
Lye is highly caustic and can cause severe burns and blindness. Wear splash goggles, gloves and long sleeves. Always add lye to water, never water to lye, in a heat-resistant container, in a ventilated space away from children and pets. Weigh everything on a digital scale, never by volume, and let cold process bars cure for 4 to 6 weeks before use. Keep vinegar for cleaning surfaces, but rinse lye on skin with plenty of running water.