Alum is a a chemical compound that forms part of the binding materials for many types of paper. The specific compound is the hydrated potassium aluminium sulfate. Alum can also refer to a group of similar compounds.

Alum is generally the cuase of acid in paper, which for many art & craft purposes is not describe, see: Acid free paper.

Alum also has a very wide range of other uses from industrial, cosmetics, medicinal and culinary.