You can't turn alum crystals back into alum. Alum is already alum, both in its crystal form and in solution.

Here's why:

* Alum is a chemical compound: Alum (potassium aluminum sulfate) is a specific chemical compound with a particular formula (KAl(SO4)2). The crystals are just the solid form of this compound.

* Dissolving and crystallizing: When you dissolve alum crystals in water, you create an alum solution. The crystals simply break down into their constituent ions (potassium, aluminum, and sulfate). When you evaporate the water, the ions recombine to form alum crystals again.

* No chemical change: The process of dissolving and crystallizing alum is a physical change, not a chemical change. The chemical composition of alum remains the same throughout.

In short, you are always working with alum, whether it's in crystal form or dissolved in water.