Have you tried walnut ink? A strong ink can be made by soaking walnut husks, and then cooking down that water until thick. There may be more to it that that, but as I understand it walnut ink is pretty easy to make and more or less fool proof.
Then there is iron gall ink, a little more challenging to make, and probably not worth the extra efforts as its benefits over walnut ink are not a factor when marking joints (it is more stable in color, and is black instead of intensely dark brown)
But the question is, is European walnut suitable for ink? I'm not so sure it is...
