Here's some info on Ammonium Alum et al...

Antiperspirants, even so-called "natural" ones contain aluminum that work to clog our pores and stop us from perspiring. Aluminum is a poison. It has been linked to Alzheimer's disease. There is new research which indicates that there may be a link between Breast Cancer and antiperspirants. In particular, studies are showing that it travels more freely into our lymphatic system when underarms are shaved. Look on your antiperspirant label for aluminum as:

• aluminum chlorohydrate
• ammonium aluminum sulfate
• potassium alum
• potassium aluminum sulfate
• mineral salts or crystal

Of less concern is aluminum chloride.

Ammonium alum is different. It is a prevalent natural compound. It cannot be absorbed into the skin and does not clog pores the way aluminum chloride does. It does not kill the bacteria and does not stop perspiration, however, ammonium alum works to inhibit the growth of bacteria and therefore those critters that cause bad odor. This is the primary ingredient in deodorant crystals which are found to be safe and effective.

http://www.deliciousorganics.com/Controversies/antiperspirant.htm