Alums form when a sulfate salt of a metal like potassium mixes with aluminum sulfate in water. As the mixture cools, the ions link up with water molecules to build a crystal. The 12 water molecules are not just stuck to the surface.
They fit inside the crystal and give alums their neat cube or eight-sided shapes. People have used alums for thousands of years. Ancient dyers added alum to cloth so dyes would stick better and last longer.
Alum also helps clean drinking water by making tiny dirt particles clump together so they sink to the bottom. Leather makers use alum to tan hides. Not all alums contain potassium.
Sodium alum and ammonium alum work in similar ways but show up in different products, from baking powder to fire extinguishers. Alum crystals form an octahedral arrangement around the aluminum ion, with six water molecules directly attached to it and six more held nearby. This precise arrangement is why alum crystals grow into such regular geometric shapes.
This feature made them popular for growing large, clear crystals in school science projects. Ammonium alum and chrome alum are two other common types, each suited to different dyeing and tanning jobs. A common misconception is that all alums act the same. In reality, the choice of metal ion changes how well the alum performs in each application.