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This is a large crystal of alum, or aluminum potassium sulfate.

This is a large crystal of alum, or aluminum potassium sulfate.


Question: What Is Alum?

Answer: Usually when you hear about alum it is in reference to potassium alum, which is the hydrated form of potassium aluminum sulfate and has the chemical formula KAl(SO4)2·12H2O. However, any of the compounds with the empirical formula AB(SO4)2·12H2O are considered to be alum. Sometimes alum is seen in its crystalline form, although it is most often sold as a powder. Potassium alum is a fine white powder that you can find sold with kitchen spices or pickling ingredients. It is also sold as a large crystal as a "deodorant rock" for underarm use.

Types of Alum

  • 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. It's chemical formula is KAl(SO4)2.


  • Soda Alum
    Soda alum has the formula NaAl(SO4)2·12H2O. It is used in baking powder and as an acidulent in food.


  • Ammonium Alum
    Ammonium alum has the formula NH4Al(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 cements and vegetable glues, in water purification and in some deodorants.


  • Chrome Alum
    Chrome alum or chromium alum has the formula KCr(SO4)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.

Uses of Alum

Alum has several household and industrial uses. Potassium alum is used most often, although ammonium alum, ferric alum and soda alum may be used for many of the same purposes.
  • purification of drinking water as a chemical flocculant
  • in styptic pencil to stop bleeding from minor cuts
  • adjuvant in vaccines (chemical that enhances immune response)
  • deodorant "rock"
  • pickling agent to help keep pickles crisp
  • flame retardant
  • the acidic component of some types of baking powder
  • an ingredient in some homemade and commercial modeling clay
  • an ingredient in some depilatory (hair removal) waxes
  • skin whitener
  • ingredient in some brands of toothpaste

Alum Projects

There are several interesting science projects that use alum. In particular, it is used to grow stunning non-toxic crystals. Clear crystals result from potassium alum, while purple crystals grow from chrome alum.

Alum Sources and Production

Several minerals are used as the source material to produce alum, including alum schist, alunite, bauxite and cryolite. The specific process used to obtain the alum depends on the original mineral. When alum is obtained from alunite, the alunite is calcined. The resulting material is kept moist and exposed to air until it turns to a powder, which is lixiviated with sulfuric acid and hot water. The liquid is decanted and the alum crystallizes out of solution.

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