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What is a Tongue Test?

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What is a Tongue Test?

You don't have to be a mad scientist to do a tongue test on your soap.

Brett Lamb / Getty Images
Definition: If a soapmaker that is having a problem with a batch of soap writes and asks about it, I often ask, "Did you do a tongue test?" But just what is a tongue test?

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:

  1. Touch the very tip of your tongue to the soap...or
  2. Lightly wet your finger and rub it on the soap. Then touch your finger to your tongue.
Whichever method you use, if the soap is fine, you'll just get a bit of soapy taste in your mouth. If the soap is lye-heavy, you will get a "zap" on your tongue. Yes, "zap" is really the best way to describe it. It's kind of like touching your tongue to the terminals of a 9-volt battery - a zing, a zap. It's not really a taste, it's more of a physical feeling.

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.

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