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Ammonium alum (NH4Al(SO4)2·12H2O) or ammonium aluminum 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.[1] As a crystal deodorant (pierre d'alun), it has a bacteriostatic action, meaning that it inhibits bacterial growth (but without killing the bacteria).[2] Since it does not kill the bacteria, it has somewhat shorter effectiveness when compared to other commercial antiperspirant or deodorants. Unlike the molecules of aluminum chloride used in antiperspirants that are designed to lodge themselves in sweat glands, ammonium alum molecules are much too large to be absorbed by the skin. 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.[3] 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. http://en.wikipedia.org/wiki/Ammonium_alum