Gallnut is used as a tannin for pre-mordanting cellulose (plant) fibres such as cotton. Made from tannin rich galls which form on trees as a natural defence mechanism against insects. The galls resemble nuts, in that they are hard and generally round, but are not truly a nut.
Used as a pre-mordant for cellulose fibres at 6-8% wof, the tannin step is very important for proper, colourfast dyeing when using plant fibres such as cotton or linen. It is not necessary when using animal fibres such as silk or wool.