A fairly simple way of dyeing cloth in the medieval period was to use oak galls, although I have no idea how often it was done. The tannins within them act as a mordant, so you don’t need that expensive import, alum. Then if you add some iron you can vary the shade, iron being fairly easy to come by, at least by the late medieval period.
So I prepared two examples, using fairly fresh oak galls gathered at the weekend. Following the instructions given in “Colours from nature” by Jenny Dean, you can get black after adding green vitriol (Iron sulphate) to the tannin solution. I roughly followed her instructions; at this stage I don’t care about exact yields, just proof of process.
Taking 3 oak galls, I ground them up, put one half in a beaker, boiled it for a while (I see no need to boil as long as Dean suggests though) and strained off a brown solution into another beaker with some white wool cloth in it. This was then heated on and off for an hour or so, and left in the beaker for a day. Here it is, looking a bit like tea before adding the cloth:
The rest of the ground oak galls were added to the first beaker, heated with water, and then some green vitriol added, which turned it black (On the left. On the right is the cloth in the previous solution):
This was then strained through some linen like the first lot, and heated with the cloth in it. I took the cloth out three times to allow it to air, as Dean says, and boiled it again every time it went back in.
Finally I took it out and allowed it to dry.
This first photograph was taken before rinsing the black cloth out. On the left, the undyed fabric, middle is the oak galls alone, right is the galls and green vitriol:
When I had washed and dried the black fabric, it looked less dense in colour, because I’d removed some of the black dye:
(The other differences are that the first photo was taken with indirect morning light, the second I think with the flash, during later afternoon)
Simple enough anyway. The black looks pretty good even after rinsing, but because of the acidic nature of the green vitriol it will be interesting to see if it suffers any damage over the next year or two. The brown is more a light tan, which would itself be acceptable I am sure, although I’m not sure how to get the dark brown of some clothing. Perhaps walnut hulls or copper moderated madder, from what Jenny Dean writes. I think I need some more books on dyeing though, to cross correlate the information.