Hi, beautyinabasket,
I use BB's Peppermint EO (1st distillation) and really like it.
It took me a while to settle on a usage rate. I was so afraid of using too much because Peppermint EO is so strong and can be a skin irritant in excess, so I thought it best to err on the side of caution. After a couple of batches of slowly upping the amount of peppermint EO I used, I finally settled on 1/2 tsp per pound of M&P base, which, if my math is right, works out to about a .5% usage rate, maybe a bit more.
Don't know if that amount is to your liking -- you may want more or less. Fragrance is so personal, but with EOs, certain cautions must be kept in mind, and with some you can use more, and others you should use less. I have seen recipes calling for as much as 2% or more of peppermint EO, but that just seems like a lot of peppermint, and I wouldn't risk it irritating someone's skin or someone finding it just plain overpowering and unpleasant.
Looks like Anne-Marie calls for 1 ml peppermint EO (2nd distillation) per 4 ounces M&P soap base in her new tutorial:
http://soap-queen.blogspot.com/2009/12/ ... canes.html. (A milliliter equals about .034 ounces.) So, again if my math is right, I guess that's a bit more than a .75% usage rate. (Please check my math.) Don't know if there's a big different between 1st distillation and 2nd distillation, usage rate-wise.
(I'm curious, fellow soapers --- what usage rate do you like for peppermint EO? Just wondering if I'm high or low or about right ... )
Make sure you warn customers not to use peppermint soap near their eyes or other sensitive mucous membranes or areas.
Also, pick up a book on essential oils. I like "The Directory of Essential Oils" by Wanda Sellar. Not specific to soaping, but gives you good EO profiles and warnings.
Hope that helps!