Tannin or tannic acid is used to assist the mordants of cellulose fibers and fabrics. Alum does not bond with cellulose fibers as well as it does with protein fibers. However, tannin bonds well with cellulose. and once treated with tannin, alum will combine with the tannin-fiber complex. Many dyestuffs contain tannin (black oak, pomegranate, cutch, fustic, etc) and do not need an additional tannin. This is a blend of tannins mainly composed of quebracho, which will leave a soft red-brown color on the cloth.