Ammonium aluminium sulfate, also known as ammonium alum or just alum, is a white crystalline double sulfate usually encountered as the dodecahydrate, formula (NH4)Al(SO4)2-12H2O.

It is used in small amounts in a variety of niche applications.

The dodecahydrate occurs naturally as the rare mineral tschermigite.

As a mordant in hand dyeing
Making crystals
Is an astringent/styptic and antiseptic
A natural deodorant by inhibiting the growth of the bacteria responsible for body odor
General Laboratory Reagent

Minimum Assay: >99%
Molecular Formula: AlH4(SO4)2-12H2O
Molecular Weight: 453.32 g/mol
Melting Point: 93.5 C
Solubility in water (% weight): 14% in water
Insoluble matter: <0.001%
Reaction: 3.22
Chloride (Cl): <0.001%
Nitrate (NO3): <0.001%
Phosphate (PO4): 0.002%
Silicate (SiO3): <0.002%
Arsenic (As): <0.00005%
Copper (Cu): 0.00009%
Iron (Fe): 0.00034%
Lead (as Pb): 0.001%
Potassium (K): <0.0004%
Sodium (Na): <0.001%
Calcium (Ca): <0.0001%