Superfat: why soap uses less lye on purpose
Superfat is oil you leave as oil, by using a little less lye than your oils could turn into soap. Worked through: oils needing 140 g of lye get 133 g at 5%.
Lye calculators ask for a superfat (some call it a lye discount). This short explains what it is: the share of your oils you leave as oil, by adding a little less lye than they could turn into soap. Each oil’s saponification (SAP) value is an estimate, and real oils and lye vary. Holding some lye back is a safety cushion that makes sure the lye gets used up. The video works it through on a batch whose oils need 140 g of lye: at a 5% superfat, 95% of it, 133 g. For a classic oil blend, 5% is a good place to start; much higher and the bar gets softer and lathers less.
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Transcript
Hook
What is superfat? Less lye than the math says. On purpose.
Definition
Superfat is oil you leave as oil, by adding a little less lye than your oils could use up.
Why
Each oil needs its own amount of lye: its SAP value. Those are estimates, and oils vary. Holding lye back is a safety cushion: the lye gets used up.
Example
Say a soap maker’s oils need 140 grams of lye. At 5% superfat, they use 95%: 133 grams. The spare oil stays in the bar.
Takeaway
For a classic oil blend, start at 5%. Much higher, and the bar gets softer and lathers less.
Sources
- Superfat is fat that remains in a soap after saponification; a soap must include enough fat to make sure all of the lye is used up: Classic Bells (Kathy Miller), Superfat (opens in new tab)
- Recipes are based on estimates of the saponification values and alkali purity, so soapmakers add a little extra fat to make sure the alkali is used up: Classic Bells (Kathy Miller), Superfat (opens in new tab)
- A saponification value is the amount of alkali needed to saponify one gram of a fat; published values are ranges (coconut oil 248–265 mg KOH/g), and bar soap uses sodium hydroxide: Wikipedia, Saponification value (opens in new tab)
- SAP values used by lye calculators are averages drawn from published ranges: The Sage, About the Lye Calculator (opens in new tab)
- 5% superfat is a good place to start with a classic blend of fats; a too-high superfat reduces lather and makes a bar softer: Classic Bells (Kathy Miller), Superfat (opens in new tab)
- Lye calculators apply superfat as a lye discount: 140 g × (1 − 0.05) = 133 g: Arithmetic; Ardent Seller Lye & Saponification Calculator (superfat / lye discount field) (opens in new tab)
- Raising superfat even 2–3% can noticeably soften the bar and reduce lather: Classic Bells (Kathy Miller), What do the Soapcalc numbers really mean? (opens in new tab)
General education, not formulation or safety advice. Lye is caustic: wear gloves and sealed goggles, and run every recipe through a lye calculator.
Last updated .