Method of the iron ebonisation is based on the reaction between tanins and iron ionts.
In my case I used walnut wood (naturalyl full of tanins), iron wool, iron leavings and vinegar at the beginning. After some time I added the ferrous sulfate for acceleration of the reaction. I found out that slow reaction of the wood soaked only in the water with the iron is the best way, but it takes a longer time.
Another way is soaking wood in the broth of strong tea or in the solution with iron galls and after that in ferrous sulfate. It’s quick, but the shade of black is a little bit blue and the wood is blackened mostly only on the surface. The heartwood is very quickly black, but unsuitable for the soaking of solution, so it’s blackened only at the surface). The sapwood is easily colored trough very soon, but not stained black enough.
Here are photos. It took circa 3 months to blacken trough the material (3-6 mm).