Safe soap has no free (unreacted) lye left — every bit of the sodium hydroxide has reacted with oil. Soap is lye-heavywhen there's excess lye, usually from a measuring error or a recipe not run through a calculator. Here's how to check.
The zap test
On a fully cured bar, touch it briefly to the tip of your tongue. A sharp zap— like touching a 9V battery — means excess lye. No zap (just a soapy taste) is a good sign. It's the fastest field test makers use.
pH strips and phenolphthalein
Wet the bar and press a pH strip: finished soap sits around pH 8-10. Above that suggests excess lye. Phenolphthalein drops turn bright pink on high-pH (lye-heavy) soap and stay pale/clear on safe soap.
What keeps soap safe: superfat
A 5% superfat is your buffer — the recipe deliberately uses less lye than would react with all the oils, so a small error still leaves oil (not lye) in excess. This is why an accurately measured, calculated recipe reliably comes out safe. See the superfat calculator.
If a batch is lye-heavy
Curing will notfix it. Rebatch it (grate, add a little more oil, and re-cook) or discard it — don't use a zappy bar on skin.
Prevent it
- Weigh everything in grams on an accurate scale — never measure lye or oils by volume.
- Run every recipe through a lye calculator with a 5% superfat.
- Double-check the oil weights and the lye amount before mixing.
This is general maker guidance, not a substitute for proper lye-handling safety (gloves, goggles, ventilation).