When dyeing with plants, a mordant is needed to fasten the colour in the wool. Alun is the most common mordant to use when plant-dyeing. It is heated up to boiling and then the wool is added in for at least an hour. Cooled down. Before putting alun-bathed wool into the dyeing pot, it is necessary to rinse it with clean water to get the extra alun away. Otherwise, it will take colour from the wool.
Alun is also used when tanning skins.