Alum block

I confess I had never heard of alum until I had a shave at The Gentleman's Shop in Hungerford.
So what is it?

An Alum Block is a crystal-like stone that is moistened with cold water and gently rubbed over the entire face to act as an antiseptic for razor burn. It can also help to stop bleeding of nicks and cuts. The Alum block also has antiseptic properties that not only cool and refresh the skin after shaving, but also act as a balm. Simply wet the block, apply to face, and then dry with towel.

I use it most days now. But I rinse it off before applying after shave products. Doesn't cost much and will last ages.