Lye Safety: The Non-Negotiable Checklist

Sodium hydroxide and potassium hydroxide turn oils into soap. They also destroy skin, eyes, and aluminum. This is the working checklist. If a step is skipped, the batch waits.

lye

safety

PPE

cold-process

Before anything leaves the shelf

No children, no pets, no open drink, no bare feet in the room.
Work on a stable, wipeable surface. Not a wooden dining table you care about.
Aluminum is off the list: no aluminum bowls, spoons, scale pans, or foil. Lye plus aluminum makes heat and hydrogen gas. Use HDPE, stainless steel, tempered glass, or polypropylene.
Vinegar does not treat an eye exposure. It is a cleanup rinse for counters after the lye is diluted and wiped. Eyes and skin get running water. That is the whole first-aid rule.

PPE that actually covers the splash

Wear all of it before the lye lid opens.

Eye protection: sealed goggles, not street glasses.
Gloves: nitrile, long cuff. Not cracked dish gloves.
Sleeves and closed shoes. An apron that can be stripped off.
Optional face shield over goggles when you pour lye water into oils.

A long-sleeve shirt you will throw in the wash is better than a favorite hoodie.

Ventilation

Mixing lye and water releases heat and a brief caustic mist. Do it under a running vent hood, next to an open window with a fan pulling air out, or outdoors. Do not lean over the pitcher while it clears.

The mix is usually done steaming in 30–90 seconds. Stay out of the plume until it settles.

Mixing order is not a preference

Water first. Lye second.

Weigh room-temperature distilled or deionized water into a marked HDPE pitcher.
Weigh the hydroxide into a separate dry cup.
Pour the hydroxide into the water in a steady stream, stirring with a silicone or stainless spoon.
Never pour water onto dry lye. The first water to hit the pile superheats and can erupt.

Target lye-water temperature after the reaction settles: 80–120°F for most cold-process batches. If it spikes above 180°F, leave it alone until it drops. Do not fridge a 200°F pitcher sealed tight.

Potassium hydroxide for liquid soap follows the same order. KOH often runs hotter. Same PPE, same water-first rule.

Water and lye amounts

Use a scale that reads 0.1 g for batches under 2 kg of oils, 1 g for larger. Volume measurements of lye are how people mis-batch.

Standard practice is to calculate NaOH or KOH from the saponification values of each oil, then apply a lye discount (superfat) of 5–8% for bars unless a specific recipe says otherwise. Different calculators publish slightly different SAP tables. Use one tool for the whole batch. Do not mix a SAP from one chart with a discount from another.

Water as a percent of oils commonly sits at 30–38% for cold process (a 2.2:1 to 2.5:1 water:lye ratio by weight is a typical band). That is a formulation choice, not a safety choice. Safety is: known weights, labeled pitcher, no substitutions.

Storage

Hydroxide stays in its original sealed container, dry, up high, away from acids and away from food.
Do not store lye in unlabeled jars. A strip of tape with "NaOH — caustic" and the date is the minimum.
Lye water that you mixed and did not use: leave it labeled, let it cool, and use it in the next batch within a day, or neutralize with a large volume of water and pour to a utility sink — never a septic lecture, just do not dump concentrate on soil or aluminum.
Clean tools with plenty of water. Wash gloves before you take them off so you do not touch the cuffs to your face.

If it gets on you

Skin: running water 10–15 minutes. Remove soaked cloth while under the water. Then medical care if the area is more than a small splash.
Eyes: running water 15 minutes, eyelids open. Then emergency care. Do not neutralize with acid.
Inhaled mist: fresh air. If breathing is affected, emergency care.
Swallowed: do not induce vomiting. Emergency care.

Keep a charged phone in the room. Keep a clear path to the sink.

Batch-day order of operations

Clear the room. Put on PPE.
Weigh water. Weigh oils. Weigh lye last.
Mix lye into water. Set it aside to cool.
Melt and combine oils.
When both pans are in the 80–110°F band (or whatever the recipe calls for), combine.
Do not take the goggles off until the mold is filled and the pitcher is rinsing.

Milk, honey, and other sugars

Sugar in the water phase heats harder. Frozen milk, added in slush to the lye, is the usual control. Same PPE. Same water-first logic: the milk is the water. Work cooler, and do not walk away from the pitcher in the first two minutes. Honey at 1–2 oz per pound of oils goes in at trace, not in the lye water.

Key numbers

Water first, lye second. Never the reverse.
Goggles + nitrile + covered skin before the lid opens.
No aluminum anywhere in the chain.
Scale to 0.1 g on small batches.
Eyes and skin: water only, 10–15 minutes. Vinegar is for counters, not eyes.