Mordants are one of the keys to successful eco printing, botanical printing, and bundle dyeing. They help plant color attach to fabric, improve washfastness, and can change the final color of your leaf prints. Alum is a good beginner mordant for wool and silk, while cotton and linen usually need tannin plus aluminum acetate for stronger results. Iron can be used in small amounts to create darker outlines and dramatic botanical prints, especially with tannin-rich leaves such as oak, sumac, rose, walnut, pomegranate, and cotinus.