Branwen,that article you posted regarding Woad..Seemed to focus on Iron {and copper to a lesser extent} and the primary source of Woad pigment in antiquity. It got me curious enough to have a look see further. I found this WIKI page that seems to attribute it more to this particular plant.
http://en.wikipedia.org/wiki/Isatis_tinctoria

Maybe a combination of these things together?
I think the plant was maybe what people used to rub on with the intent of removal.
The oxide's maybe for Tatoo's?
Just a thought.