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.


Unknown said…

Great blog and absolutely amazing crystals! I'm trying to grow some singles now but even the small ones have defects and are uneven :(
Did you use distilled or de-ionesed water to grow them? I'm currently growing them in de-ionesed water.

Kind Regards

WaffleStaffel said…
Your crystals are all beautiful, they inspire me to try. Thanks for sharing all the information.

I have a question. I meant to grow alum seed crystals, Before bed I dissolved 3 tablespoons into 150ml warm water. I woke up to one giant crystal. The part I don't understand is there seems to be a convection current established, with blurry lines rising from the crystal.
Even if the crystallization was exothermic I wouldn't expect it to be so obvious, it's still happening, it's been going all day. Is this current set up by the change in density from the aluminum sulfate leaving solution and sticking to the crystal? It's a double walled glass but there's no vacuum, it's not as well insulated as a thermos.

Can you shed some light on what I'm seeing?
Thanks a lot for sharing this amazing knowledge with us. This site is fantastic. I always find great knowledge from it. Crystal Sydney