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Chemistry and workshop

Lye for soap making

Grams of sodium or potassium hydroxide and of water to saponify up to three oils or fats, with superfat.

g
g

0 or empty if not used.

g

0 or empty if not used.

%

Soda flakes or pearls are usually 98-99 %; common recipes assume 100 %.

%

Fat left unsaponified as a safety margin. 5 % is usual for bar soap.

%

Alkali ÷ (alkali + water). Between 25 and 35 %; 30 % is a good starting point.

Result

Alkali to weigh
— g
Water for the lye
— g
Total fat
— g
Total batter mass
— g

How it is calculated

Each fat needs a fixed amount of alkali to turn into soap: its saponification value (in brackets in the list, in grams of NaOH per gram of fat).

NaOH = Σ(mass of each fat × value) × (1 − superfat ÷ 100). For potash: KOH = NaOH × 56.11 ÷ 40.00 (= × 1.403). Then divide by the purity. Water = alkali × (100 − concentration) ÷ concentration.

Example: 1000 g of olive oil, caustic soda, 5 % superfat, 30 % lye. NaOH = 1000 × 0.135 × 0.95 = 128.25 g; water = 128.25 × 70 ÷ 30 = 299 g; batter = 1000 + 128 + 299 = 1428 g.

Example: 500 g of coconut and 500 g of olive, 90 % potash, 5 % and 30 %. NaOH equivalent = (500 × 0.183 + 500 × 0.135) × 0.95 = 151.05 g; pure KOH = 151.05 × 1.403 = 211.9 g; to weigh = 211.9 ÷ 0.90 = 235.4 g; water = 235.4 × 70 ÷ 30 = 549 g.

Keep in mind

  • Protection: sealed goggles and nitrile gloves from opening the caustic soda container until unmoulding; long sleeves. Work with ventilation or outdoors: dissolving the soda gives off an irritating vapour. No children or animals nearby.
  • Always add the soda to the water, little by little and stirring, never water to soda: the solution heats above 80 °C and can spatter or boil. Use heat-resistant stainless steel, polypropylene or polyethylene; never aluminium (it reacts and releases hydrogen) nor thin glass.
  • First aid: on skin, remove contaminated clothing and wash with plenty of running water for at least 15 minutes. In the eyes, rinse with plenty of water non-stop for at least 15-20 minutes and seek medical attention; do not use vinegar in the eyes. If swallowed, rinse the mouth and seek medical attention without inducing vomiting.
  • The values are averages from soap-making tables (such as SoapCalc's), with variations of the order of ±2 % between batches. That is why superfat is used: with 5 % the soap is not caustic even if the real value is somewhat higher. Do not go below 3-5 % in bar soap for skin.
  • Soap needs to cure for 4-6 weeks in a dry, ventilated place before use: it finishes saponifying, loses water and hardens. Beeswax only partly saponifies; use it in small proportions.
  • This calculation applies to caustic soda or potash (hydroxides). Wood-ash lye (potassium carbonate of unknown concentration) cannot be calculated this way: it is adjusted by testing.
  • Cross-check the recipe with another calculator or a table before making a large batch: a unit error with caustic soda gives a soap that burns the skin.

See also

More tools in “Chemistry and workshop”