Tannin is a large, astringent (meaning it tightens pores and draws liquids out), molecule found in plants that bonds well with proteins.
It is concentrated in the bark layer where it forms a barrier against micro-organisms such as fungi and bacteria. There are three types of tannins we use in natural dyeing;
Clear tannins (gallic), yellow tannins (ellegic) and red-brown tannins (catechic). The two lather ones can also act as a colour in itself. Use as a first layer before dyeing with more fugitive natural dyes and with iron for various shades of brown-blacks, moss-greens and greys.