What is Superfatting? What is the Zap Test?

If you have ever used or made handmade cold process soap before, you may have heard the term "superfat," or a similar term.  But what does this word mean?  
As we have examined with our Basic Soap Chemistry series, when the oils/butters react with the lye solution, the oils effectively neutralize the lye.  This is in essence a chemical reaction, with the variables in a careful balance.  If done perfectly, the lye should be completely rendered safe.  In a large laboratory, this would be a simple matter to test.  However, most cold process soaps are made in smaller facilities, including makers' residences.  So how can we make sure our soaps are safe to use?
The simplest answer is to superfat our soaps.  What this means is that the soap maker will add additional oils and/or butters to the recipe.  The chemical reaction proceeds as usual, but some of these added oils/butters do not react with the lye.  These free-floating molecules are what soap makers refer to as a superfat.  These are generally measured in percentages, with higher percentages meaning that more oils are left unreacted.  
Once again, the soap maker has to carefully work with their superfat level to make a good bar of soap.  If a superfat is too low, the soap might be too harsh for use.  If a superfat is too high, the soap stands a higher chance of going rancid.  I typically superfat my soaps at 7-8%, which I find to be a good range.  You may commonly find soaps in the 3-10% range, or even up to 20% in some exceptions.  
Another way a home soap maker tests the safety of their soap is by performing what is known as the "zap test."  This involves the soap maker touching their tongue to the surface of the soap.  If there is a "zap," then the soap is lye heavy, and not safe to use.  This zap is similar to the feeling of when you touch your tongue to a 9-volt battery.  If you've never been zapped, believe me, you'll know when it happens.  As a soap maker gains experience, they generally do not have to zap test their soaps unless there is a specific reason to do so.  My recipes have been tested thoroughly, and you can rest assured that they are safe for use.  

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