I have pretty sensitive skin and need to be careful in general when using most skin products. I get allergic reactions to a lot of moisturizers, for instance, that cause my hands to turn red and dry out really badly. I expect that with alcohol-based solutions, but I wasn't really expecting it from potassium or whatever it is that makes up the alum block. It felt like salt on a wound, tough to describe it otherwise, and it left my whole neck puffy and red for a couple of hours.
Thanks for the tips, I'll definitely keep them in mind for next time.