Aluminum Sulfate, commonly referred to as alum, is an aluminum salt that has been used since antiquity as a mordant. Before marbling, alum is mixed with water, applied to the material, and allowed to dry. This helps bind the marbling colors to whatever is being marbled (paper, textile, leather, etc.). For paper, simply use a sponge to apply the alum solution. For other materials such as textiles, fill a bucket with solution, soak for a few minutes and wring out any excess. Always let the alum solution dry before marbling. Sold in 1 and 5 lb bags.

To prepare the alum solution, add ¾ cup of alum crystals to one gallon of clean warm tap water. Stir to dissolve completely and always stir before using. If kept capped, this solution has an indefinite shelf-life. 

Safety Note: Can cause skin irritation or dermatitis. Use rubber or latex gloves while handling.