To start, as a fan of design, I really like the tough guy logo.
The block itself looks like some kind of crystal rectangle. The product claims to be 100% potassium alum and is made in China. I try to avoid products made in China whenever possible. (Am I being unfair?)
I already use potassium chloride crystal cubes, also known as KCl, in my water softener, and have looked a bit into KCl as, among other things, a source of beta radiation.
As it turns out, several years ago I used the equivalent of an alum block as a deodorant. The exact product is still available on Amazon: Crystal Body Stick Deodorant. (That link is for a pack of four.) The deodorant lists as sole ingredient “Potassium Alum” and comes from Thailand. As a side note, if I recall correctly, the deodorant lost its effectiveness at some point before a date, and I vowed not to use it again.
Clearly, post-shave face and armpits are quite different. In both cases, however, the potassium alum serves to tighten the skin and change the pH of the skin’s surface.
After shaving, the alum block is wet with cold water and then is gently slid along the skin’s surface. Many shavers like the “feedback” that the block provides; areas of irritation will sting slightly. The alum block also serves to hasten the coagulation of small nicks. Personally, I like the cold, stinging sensation of using the alum block. I am not a big fan the tightness of the skin afterwards.
Alum block is a product for external use only, though getting a bit on your lips is not out of the ordinary. Some shavers claim it has a sour taste. Some concern has been raised about the aluminum content of potassium alum. Tom’s of Maine sells a Crystal Roll-On deodorant. They address the use of potassium alum by claiming that it contains a “low level of aluminum.” Potassium alum is commonly regarded as less toxic than aluminum chlorohydrate. Interestingly, alum is traditionally called for in old pickle recipes, though this is more commonly aluminum sulfate.
One difference between potassium aluminum sulfate and aluminum sulfate is that you can grow crystal with the former. This is a common science project and is even documented by Ms. Martha Stewart herself: Crystal Egg Geodes.
What are your experiences with alum blocks and potassium aluminum sulfate in general?