So after months of trying to identify the cause of my severe razor burn and bumps... was it pressure? angle? lather? blade? razor? brush? I finally realized, it was alum. I have been applying alum daily after shaving (post shave is cold water, alum, witch hazel) and would leave it on for about 1 minute while I cleaned up. It would burn pretty bad most of the time for a few seconds then calm down. After rinsing it off my skin would look fine but within about 10 minutes my neck would be bright red and splotchy with some razor bumps which would last for several hours. No matter what I did I could not remediate this problem. Three days ago I nixed alum on a whim to see what would happen, and you know what? razor burn gone, redness gone, razor bumps gone. I believe I was having a reaction to the alum. So even if alum works for some people, do know that it is possible to have a reaction to it.