I had forgotten how much using tannin and iron together colours the fabric and we have been playing around with tannins and weeds in Kathy Hays on line Weeds and Leaves class. I do not really like the effect of the dark background but there are times when it might be of use. This is more using the leaves as a resist rather than printing from them and my objective is to get as clear a prints of the veins and outlines (with colour from the leaves if possible). This is why most of my prints are made with the underside of the leaf to the fabric - I am trying to get the imprint of the veins and these are usually more prominent on the underside of the leaf.
However I do have a love hate relationship with oak. They have such a distinctive leaf shape and these ones might be fun to stitch into. Oak should have lots of tannin but I struggle to get a good print rather than resist print from them. One of the other things we have been doing is dipping the end result in a solution of water and baking soda to "shift" the PH. I didn't think this had made much difference but when the fabric dried it was a deep tan rather than the dark grey ( as shown above) associated with an iron and tannin mix. It is interesting that the two step tannin/alum mordanting process with cotton does not have this effect. I assume this is something to do with the reaction between the tannin and alum so the background fabric stays the original colour