Apparently a critical thing is to NOT stir it. The goal is to create a fluid with a lot of suspended alum. If you stir it, it will dissolve and you will get much smaller crystals due to the lack of suspended alum.
How to grow your own crystal necklace DIY with alum salt. Make crystals at home. This also works as a great science project for children or a cute original gift for a friend. Click through for the full tutorial / DIY including a printable step-by-step che