A tongue test is a popular, old-fashioned way to test to see if your soap is too caustic (has too much lye in it). Tongue testing is simple, fairly reliable, and really not as bad as it sounds.
To do a tongue test, take a bar of soap and either:
- Touch the very tip of your tongue to the soap...or
- Lightly wet your finger and rub it on the soap. Then touch your finger to your tongue.
Whether your soap passes the tongue test or not, rinse your mouth out with plenty of water...especially if you got a zap.
Can the test be trusted?
The tongue test is imprecise, yes. It isn't going to tell you if you just mis-measured a little bit. Only phenolphthalein or pH strips are going to tell you that. But it is a reliable test for major problems like significant mis-measuring or leaving an oil out of a recipe.