Tannin is the binder that helps Alum react with the natural dyes to set the, into the fabric. A lot of eco dyers use oak gall which comes from the wound made from a wasp that injects its eggs into the oak tree, We don't have that wasp in NZ so oak gall is often hard to source and quite expensive. So when my husband was give a large oak burl I wondered if I could extract the tannin from the outside edge for use in my eco dying. some previous research had informed me that a gall is made from an outside influence whereas a burl is made from the trees internal process but both are knobbly outgrowths from the tree. I took some of the knobbly bits and boiled them up and then following instructions from a Facebook page I follow I dipped a paper towel into the liquid. I painted a line of ferrous sulfate across the paper towel. Ferrous sulfate reacts and goes dark black/brown in the presence of tannin. The lighter brown line in the first dip. I repeated the dip and line drawing with ferrous sulfate. The darker line is from the second dip so I think I have found a good source of ferrous sulfate. I just have to convince Rob not to dispose of the outer knobbly bits as he processes the burl.