Black Walnut Tree

This is the black walnut tree in my back yard. The nuts have already fallen, and oddly enough -- they're not black. They look sort of like hard limes. The outer covering falls off and the nut inside is black, the hull oozing with an inky blackish-brown substance that stains the skin (and everything else) brownish-yellow. People used to make everything from wood stain to hair dye using this walnut ink. You can still buy the ink in places, but it's crazy expensive. I would love to learn how to make it, myself.