Reply To: Educate me on tannic acids

The key is tannins to combine with the iron to stain the woods.

The traditional source here in the NW USA is hemlock bark dust, which is available at any garden supply as mulch. Hemlock ark for tannins for leather work was once a thriving industry which reduced the hemlock forests, but that’s over now.

Make a tea of it for a week or so, then paint he wood a couple times allowing it to dry between coats.

Then apply a tea made from iron and vinegar. (A pad of OOOO steel wool is a good source for the iron)

Repeat both processes until you get it as dark as you want.
Many woods will stain black using this process, including the oaks,
chestnut, Rowan, cherry, walnut, and even maple.

Here’s more:
http://www.popularwoodworking.com/techniques/ebonizing_wood