Potash alum is a double salt of potassium sulphate and aluminum sulphate. It is highly water soluble. To prepare pure crystals of potash alum, dissolve the sample in distilled water and remove the insoluble impurities by filtration. This solution is concentrated to the crystallization point and cooled. Colourless transparent crystals of potash alum separate. The soluble impurities are left behind in the mother liquor. Potash alum has an octahedral geometry.
This video explains how to prepare pure crystals of potash alum form an impure sample through crystallization.