A solution of alum and water is commonly used to mordant paper* or fabric before being marbled. This prevents the paint from washing off after rinsing the surface marbled.
* If you are using DEKA Marble Paints this step may not be necessary. However, it’s recommended for getting optimal results.
** Be careful when working with alum: always use hand gloves, work in a well-ventilated area and keep it away from children.