I am aware of the highly toxic nature of hexavalent chromium, but it is perfect for creating a very convincing aged surface on wood [elm] that has been pretreated with alum, and after drying coated with cutch [Catechu, an extract of Acacia, high in tannins and used as a food dye]. Applying dilute potassium dichromate to this prepared surface will yield the desired tone. My question is what has transpired chemically - is the chrome mordant on the surface now in trivalent form, and thus the object and any sanding dust relatively non-toxic? The other question I have is how to reduce the hexavalent chromium containing rinse water from the treated surface to a trivalent form. I have read that it can be done if the water is high in dissolved iron. Thank you.