Ammonium alum is a prevalent natural compound that cannot be absorbed into the skin and doesn’t clog pores the way aluminum chloride does. While it doesn’t kill the bacteria or stop perspiration, ammonium alum inhibits bacterial growth that causes odor. It is the primary ingredient in deodorant crystals, a safe and effective alternative to antiperspirant and commercial deodorants.

Source: http://greenlivingideas.com/2008/02/26/natural-deodorants-and-antiperspirants/