Dual Lye Soap Recipe: NaOH and KOH Blends

Sodium soaps of the common bar fats are solid at room temperature. They stack into a firm lattice. That is the bar you can stamp and ship.

Potassium soaps of the same fats are softer and more soluble. Pure KOH and a high water phase is liquid soap. Used as a minority share in a bar, KOH does not liquefy the batch. It loosens the lattice. The bar wets faster, lather starts sooner, and the feel moves toward cream rather than tight bubbles.

Shaving soaps exploited this first. A face lather needs to load a brush and stay slick. A pure NaOH bar can be too stubborn unless the formula is already heavy in stearic acid and clay. A little KOH fixes the solubility without giving up the puck.

The working classic is 95% of the alkali as NaOH and 5% as KOH, counted on neutralization demand — not by tossing 5 grams of each into the cup. KOH is a heavier molecule. The same neutralizing power takes more grams of KOH than of NaOH. A calculator has to convert SAP, then split.

Five percent KOH is enough to change how a shaving puck drinks water. It is often enough in a bath bar that feels slow to lather: high tallow, high palm, high stearic. It is not a rescue for a recipe that is mostly soft oils. Those bars are already soluble. Extra KOH makes them smear.

Go higher — 10%, occasionally 15% in a shaving formula — only when you have already raised stearic acid or hard butters and the puck still will not load. Above that, you are making a soft soap and calling it a bar. Cream soaps and shave pastes live in that range on purpose. Stamped bars do not.

Dual-lye is fussiness when the goal is a swirl-friendly cold-process bath bar with ordinary coconut, olive, and a hard oil. NaOH alone will do that job. Add KOH and you add a second weigh, a second SAP path, and a softer cure, for a difference most users will not notice in the shower.

Each oil has a NaOH SAP and a KOH SAP. The KOH figure is the NaOH figure scaled by the ratio of the two molar masses. You do not average the two alkalis and multiply once.

The sequence is fixed.

1. 1Total the NaOH demand from every oil.
2. 2Apply superfat to that total.
3. 3Split the remaining alkali by the ratio you chose.
4. 4Convert the KOH share from NaOH-equivalent grams into actual KOH grams.
5. 5Weigh the two alkalis separately. Dissolve them in the same water phase unless you have a reason not to.

Skip the conversion and the batch is lye-heavy or lye-light by the amount of the error. That is the whole case for a calculator. Hand charts that list "5% KOH" as a spoon measure are not charts. They are guesses.

Superfat still applies once, to the combined alkali demand. Do not discount NaOH and then discount KOH again. Water is set for the full lye solution, not for each alkali on its own.

Trace can look different. KOH soaps thicken on their own timeline, and a dual-lye batch sometimes moves faster than the same oils on NaOH alone. Plan swirls with that in mind, or skip them.

The cure is still a water-loss cure. A dual-lye bar can feel done sooner in the hand and still be wet in the middle. Give it the same weeks you would give the NaOH version. High-olive dual-lye bars need the long cure, not a pass.

Labels change. The saponified portion lists both sodium and potassium salts of the oils — Sodium Olivate and Potassium Olivate, and the rest of the pair. Free oil from superfat stays in the INCI as the oil name. A calculator that already emits INCI for NaOH or KOH should emit the blend without a second naming pass.

Set the ratio, then read both lye lines before you open either container. The Saponis soap bench (/soap) runs the split SAP math for NaOH, KOH, or a blend, and keeps superfat as a single discount.

Write the ratio on the batch note as percent NaOH / percent KOH. Weigh each alkali to 0.1 g. The blend is only as precise as the second number on the scale.