Sunday, March 8, 2015

Crystal growing: potassium alum

Potassium alum, an old favourite in crystal growing at home. In childhood, I read a book on crystals, and it mentioned this compound a lot. Unfortunately, alum was not available at my place, and I could only look at the pictures.

Formula of the alum, showing that it is a double salt: sulfate of potassium and aluminium.

Well, now I decided to make a few crystals and they grew great. Very transparent (though not without soem defects, alas) sparkling octaheders. Just look at them (full gallery).

With a scale to show size.
Transparency check
In front ob my lovely orthographic dictionary. The word in the center says "alum".


I used the simplest, traditional growing method: slow evaporation. Prepare a glass of saturated solution, make a small seed crystal (I poured a bit of solution to a glass and let it evaporate for a day) and suspend the seed on a thin thread (I used a very fine synthetic fiber, almost invisible). Then wait patiently - that's all.

Common problems in growing alum crystals:

  • Crust formation: crystalline crust grows slowly on the sides of the glass, depleting the solution and slowing down crystal growth. Solved by moving crystal to a clean glass (crust can be re-dissolved and used again).
  • Mold growth: some molds and bacteria can thrive in concentrated alum solution, dimming the crystal and reducing evaporation. I solved it by adding a few drops of iodine tincture.


Alum is a generally safe compound, so no special measures are required. Remember though that even table salt can become a poison if consumed too much.

On my hand. It's totally safe.