Working safely with lye
What lye is, why it's caustic, the gear and handling that keep you safe, and how Hi Ellie's lye acknowledgment gate and in-conversation math fit in — read this before your first cold- or hot-process batch.
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)
Water is roughly twice the lye weight, about a 2:1 water-to-lye ratio.
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 bars need 4 to 6 weeks to cure. 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.