Alum, potassium aluminum sulfate, is a mineral mordant used to help dyes bind to fiber. It's also used in pickles! And papermaking! In use since Ancient Egyptian times and beyond.
I typically use 10% alum for the weight of fibers to be mordanted. When dyeing cellulose, I give the fibers a tannin bath beforehand (see our wildforaged sumac). You may see a flocculate or cloudy appearance in the water: this is just the alum causing the minerals in your water to condense. Cool huh?
*As with many dye stuffs, make sure you practice standard safety skills with this material: don't inhale vapors or dusts, and wear gloves as it is a possible skin irritant.*
Please read FAQ page for shipping details.