I was researching crystal and alum as deodorant and ran across this excellent post. Good info and a good laugh!
Alum salts like potassium aluminium sulfate are mild deodorants (they are not usually antiperspirants) in that they do kill certain bacteria by creating an environment that is too salty for them to live on. The crystal may work on those germs but other halophilic (salt-loving) bacteria are not going to be effected and will even be fed by your salt crystal deodorant and will continue to grow (and stink) on you and your crystal until you decide to to wash yourself and the ball again. You just end up killing the germs that are susceptible and selecting for those stinky salt-lovers and transferring the germs back and forth with massive inoculations from the ball. The crystal ball deodorants are relatively ineffective for this reason. That's relatively as in slightly better than nothing (maybe). I can tell you from experience with my hippie brother and all his hippie friends (and girlfriends !!! ) who use and swear by these crystal deodorants. Believe me, they don't work well at all. Imagine being trapped in close quarters with them as they spend hours earnestly trying to convince you otherwise by persuading you to "just give it a try" while generously offering up a dirty ball of crystal deodorant that smells as bad or worse than they do. So, unless you are a card carrying member of Voluntary Hairy or Veg Family Reunion or somesuch organization my personal experience would lead me to try something more effective. Of course, infrequent showering may have been a significant factor in the failure of crystal ball-type deodorants to live up to the hype as well.