Tannin is used to assist the mordants of cellulose fibres and fabrics. Alum does not bond with cellulose fibres as well as it does with protein fibres. However, tannin bonds well with cellulose. and once treated with tannin, alum will combine with the tannin-fibre 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. It will leave a very light red-brown colour on cloth. Use at 10-20% WOF as part of the mordanting procedure.

Tannins can be clear or they can add colour to the fibre, 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.

See Maiwa's Guide to Natural Dyes.