Alum is used for pre-treating fabric or paper before marbling.
It is also used as a mordant with some natural dyes.

INSTRUCTIONS

For fabric marbling:
Dissolve 2 teaspoons / 10 grams of alum in 3.79 L of warm water.
Soak the fabric in the solution, squeeze out excess moisture (do not rinse), and allow it to dry.
The fabric will be ready for marbling once dry.
It can be ironed before marbling.

For paper marbling:
Dissolve 2 teaspoons / 10 grams of alum in 0.95 L of water.
Spray or sponge the solution lightly over the paper.
Allow it to dry completely before marbling.

WARNING: Keep paints and chemicals out of children's reach. Avoid unnecessary contact. Do not ingest.