Ammonium alum (NH4Al(SO4)2·12H2O) or ammonium aluminium sulfate dodecahydrate is a white crystalline double sulfate of aluminium, used in water purification, in vegetable glues, in porcelain cements, in natural deodorants and in tanning, dyeing and in fireproofing textiles.
The pH of the solution resulting from the topical application of ammonium alum with perspiration is typically in the slightly acid range, from 4 to 5. It is a popular deodorant because of its high availability and low cost. A 120 gram stone lasts for at least a year of daily usage - much longer than other deodorants and antiperspirants. It is also hypoallergenic and non-staining. Potassium alum is also used for this purpose.