Mordanting Processes for Cotton and Other Cellulose Fibers

Mordanting cotton or linen with potasium alum requires a 4 to 1 ratio of fiber to alum, plus a 4 to 1 ratio of alum to soda ash. In other words, for one pound of cellulose fiber, thoroughly scoured, use 4 ounces alum & 1 ounce soda ash. Simmer fiber in the mordant bath, with both ingredients, for one hour, stirring occasionally. Let cool in the bath over night. Thoroughly rinse in the morning, then dry and store until ready to dye, or dye immediately.

For the Alum/Tannin/Alum mordanting process (some dyes work better with this three-step process for cotton -- it chemically opens up more bonding sites for the dye to attach to): Do the above alum recipe the first day. The second day simmer one ounce of tannic acid (dissolved first in hot water) with cotton for one hour, stirring frequently, as the tannin can streak, then let cool over night. Third day thoroughly rinse out fibers, then repeat fist day's recipe with a fresh bath of alum and soda ash. Fourth day thoroughly rinse, then you are ready to dye.

This sounds like a long process, but it really doesn't take much time each day, and it does hold onto the color much better for dyes on cotton.