Gdrmorris7, do you make soap? Or maybe more to the point, how long do you leave soap on your body before you rinse it off?
The point I'm trying to make is that soap is a wash-off product and it doesn't stay on the skin long enough for therapeutic purposes. It's soap, not medicine.
Another thought I have is that certain parts of the body would not react well to rubefacients, even if you do successfully make such a soap.
Deep heating rubs contain rubefacients, which act to dilate the capillaries and increase blood flow to the affected area. Fine and dandy for a local application, but certainly not for all over the body!
All are leave-on products. You either rub them in to be absorbed by the skin, spray them on, or you use a patch. In all cases, the warnings are that with any adverse reactions, discontinue use, wash them off with soap and water, and to seek medical attention if the reaction persists. Also for many of them they are not to be used by children. And for all of them, not to be used in mucus membranous areas.
Additionally, how would the ingredients react with lye? I have no idea, but as far as I know very little that is exposed to lye remains unchanged. I know there are some people who have used menthol in soap, and there are certain spices and essential oils that do have rubefacient properties, that have been used it soap. But as far as using them for therapeutic purposes in soap, it is beyond my realm of experience. Perhaps someone who has done so can chime in.
But for me, I would not try to make a deep heating type soap because:
1. it's a wash off product
2. I wouldn't be able to use it on my whole body
3. children couldn't use it
4. IMO it would be a waste of money for the ingredients and
5. the my soap would become a drug
I have made a cold process soap with cayenne pepper, comfrey, and a few other herbal remedies for muscle soreness (opening up blood vessels and truly heating up skin). Granted, my cream containing the same botanicals works much better (and gets nice and hot like menthol when used), but I believe it can work in a soap as well. But you can't just wash it off. Like shampoo, which I don't immediately rinse out of my hair, when using my "muscle ache soap", I let it sit for awhile before rinsing. In fact, sometimes, I don't rinse it at all from the targeted spot.
Does it help? Who knows, but it surely does more than doing nothing.