Working safely with lye

From-scratch soap — Cold Process, Hot Process, and Liquid — needs lye. Melt & Pour and lotion, bath bomb, and scrub recipes don't. If you're about to make your first from-scratch batch, read this first.

What lye is

Lye is sodium hydroxide (NaOH) for hard bar soap, or potassium hydroxide (KOH) for liquid and soft soap. It's what saponifies your oils into soap in the first place. Lye is caustic — a small measuring or arithmetic slip makes lye-heavy soap that chemically burns skin, so it deserves real respect and careful handling.

Gear you need

Gloves and eye protection (goggles, not just glasses).
Long sleeves, and good ventilation in the room.
A dedicated, accurate scale — lye work is a weight-based recipe, not a volume one.
A heat-safe container that is not aluminum — lye reacts with aluminum.
Kids and pets kept well clear of the workspace while you mix.

The rule you never break

Always add lye TO water, never water to lye. Doing it backwards can cause a violent, boiling, caustic reaction. Mixing lye and water is exothermic — it heats up fast and gives off fumes — so do it in a ventilated space and avoid breathing the vapor directly.

Getting the amount right

Each oil has a SAP value — how much lye it takes to fully saponify it — and superfat leaves a small surplus of unsaponified oil for a gentler bar. The math is:

lye_weight = sum(oil_weight × SAP) × (1 − superfat / 100)

Hi Ellie resolves the water from the alkali rather than from one rule for both. A bar (NaOH) recipe takes the cold-process convention — water at 38% of the oil weight. A liquid (KOH) recipe takes a 25% lye solution (three parts water to one part KOH), because KOH is the more soluble and the more exothermic of the two and liquid soap is cooked and diluted again afterwards. Both are documented craft starting points held in code rather than figures read off your records. Neither is a per-recipe field you edit: the water follows the alkali, and the recipe records which convention it used.

Ellie will walk this math with you in conversation as you build a recipe, but she never hands you a final authoritative lye weight — that number is too important to trust to a single unverified source.

Tip: always carefully double-check the exact lye amount yourself with a dedicated calculator before mixing. Never treat any single number — Ellie's, an app's, or your own memory of a past batch — as gospel for a caustic ingredient.

The acknowledgment before you save

Every from-scratch Cold Process, Hot Process, or Liquid recipe shows a lye safety acknowledgment before you can save it — the same gate at every experience tier, with no bypass. It displays your batch's computed lye and water weights so you can check them one more time before confirming. Melt & Pour skips this gate entirely, since it starts from a pre-saponified base and never involves handling raw lye.

Cure and cleanup

Cold-process soap needs weeks, not days. Hi Ellie's own guidance is 28 to 42 days for cold process and hot process — cold process needs the full window to finish saponifying and harden, and while a hot-process bar is usable sooner (it saponified in the pot), it still benefits from a cure. Ask Ellie about cold process in conversation and she'll put that same window as 4 to 6 weeks.

Each recipe then carries its own cure target, and that target — not a rule of thumb — is what starts the clock. Hi Ellie only clamps it into a sane range for the method, 14–60 days for cold process and 7–60 for hot process. Those floors are the outer edge of what the app will store, not a cure length to aim for: treat the 28–42 day guidance as the real answer. Fresh bars are still high-pH early on, so keep handling them carefully until they've cured. Clean any spills promptly with plenty of water. If lye touches skin, flush immediately with water — don't reach for vinegar on skin; keep it on hand for surfaces only.