This method is based on reaction between tanins and iron ionts. I used walnut wood, iron wool, dust and vinegar at the beginning. After some time I added ferrous sulfate. I found that slow reaction of wood only with iron is the best, but it take a long time. Soaking wood in broth of tea or iron galls and after in ferrous sulfate is quick, but shade of black is little blue. Heartwood is very black, but bad for penetration (sometimes it’s blackened only at the surface). Sapwood is colored trough very soon, but not black enough.
These are photos. It took circa 3 months to blacken trough material (3-6 mm).
It’s important to wash out remains of acid ionts.
Really black trough.
This wood used in intarsia autoportret (without finishing). Some part are not black enough because I pulled it out too soon.
-- Cabinetmaker, restorer