Sumac is a great source of catechic tannin and comes from the bark of the sumac tree.
Tannin or tannic acid is used to assist the mordanting process when using cellulose (plant) fibers and fabrics such as linen, hemp and cotton. The universal mordant alum does not bond well with cellulose fibers, therefore a tannin should be considered for dye consistency, light and washfastness.
Tannins can add color to your fabric or fiber. Please consider the following when working with tannins:
Clear Tannins, such as gallnut, some sumacs and tara
Yellow Tannins, such as pomegranate and myrobalan
Red-brown tannins, such as cutch, some sumacs and quebracho