A pale yellow powder in granular form.
Gallnut is used to mordant plant fibres (cellulose) before the alum mordant is used. Alum does not combine readily with cellulose fibres so a pre-treatment with a tannin bath is required to enhance the fixation of the mordant during the mordanting step.
Gallnuts from the oak tree are the richest and earliest source for natural tannin. They are produced by insects who deposit their eggs in small punctures they make on young branches. As a protection, the tree excretes a tannin rich gum that hardens and forms a gallnut. These are then collected – dried and ground into a powder and used as a tannin mordant.
As a dye it achieves shades of cream -grey – black