Alum is a chemical (aluminum potassium sulphate); it tends to be whitish powder with several uses, including:

1. As an astringent

2. As an antibacterial

3. As a food preservative

4. As a 'Styptic pencil' to heal shaving cuts

5. To stem the flow of minor blood loss and cuts

6. Soaked into materials to make them flame retardant

There are other uses, but these are some of the more common ones.