Alum (Aluminum Sulfate)
Alum is often used as a pre mordant in natural dyeing to brighten colors and make dyes more lightfast.
For cotton, Alum Recipe #2 from Jim Liles Art and Craft of Natural Dyeing.
Quick mordant recipes:
Protein fibers: 50-100% WOF alum for silk, 20% WOF alum for wool. Dissolve alum in hot water. For wool, add 6.25% WOF cream of tartar (potassium bitartrate). After all components are dissolved, add wet material and minimal water necessary for coverage and hold at 140F (60C) for 30 min with occasional stirring. Air dry fabric without rinsing for storage; rinse thoroughly just before use.
For cotton: Dissolve 25% WOF alum in 400% WOF water. Dissolve 3.1% WOF washing soda in water. Add washing soda solution slowly to alum solution and stir until gas evolution ceases. Add pre-wet fabric and just enough water to completely cover the material. Hold at 180-195 (80-90C) for thirty minutes. Rinse fabric thoroughly then rinse/soak in a hot calcium carbonate (chalk) solution to ‘fix’.