Potassium Alum: Potassium alum is also known as potash alum or tawas. It is aluminum potassium sulfate. This is the type of alum that you find in the grocery store for pickling and in baking powder. It is also used in leather tanning, as a flocculant in water purification, as an ingredient in aftershave and as a treatment to fireproof textiles. Its  chemical formula is KAl(SO4)2.

Soda Alum: Soda alum has the formula NaAl(S O4)2·12H2O. It is used in baking powder and as an acidulant in food.

Ammonium Alum: Ammonium alum has the formula N4Al(SO4)2·12H2O. Ammonium alum is used for many of the same purposes as potassium alum and soda alum. Ammonium alum finds applications in tanning, dyeing textiles, making textiles flame retardant, in the manufacture of porcelain cement and vegetable glues, in water purification and in some deodorants.

Chrome Alum: Chrome alum or chromium alum has the formula KCr(S O4)2·12H2O. This deep violet compound is used in tanning and can be added to other alum to grow lavender or purple crystals.

Selenate Alums: Selenate alums occur when selenium takes the place of sulfur so that instead of a sulfate you get a selenate, (SeO42-). The selenium-containing alums are strong oxidizing agents, so they can be used as antiseptics, among other uses.

Aluminum Sulfate: This compound is also known as papermaker's alum. However, it is not technically an alum.