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 and sumac, which will leave a soft red-brown color on the cloth.

Tannins can be clear or they can add color to the fiber, and this is an important consideration when selecting a tannin. 

• Clear Tannins: “Gallic” - Gallnut, Tara, some Sumacs
• Yellow Tannins: “Ellegic” – Myrobalan, Pomegranate, 
• Red-Brown Tannins: “Catechic” – Cutch, Quebracho, Tea leaves, and some Sumacs.