Just to clarify (because it seems incredible to me):
If you don't use alcohol, the burn lasts for hours / days and if you do use alcohol, burn that would otherwise last hours / days is gone in say 10 minutes???
Hm, I don't want to cause burn especially for experimentation purposes... But I might. Whenever I use alum, it does burn the face if I have been too aggressive - but that's not too often. Even if I get some red spots (sometimes from soap reaction, aggro stretching, actual blade scrape, whatnot...) I am usually fine after a short time
And, for the record, I would encourage anyone to try to avoid burn in the first place :D
Cheers
Ivo