Vinegar and steel wool solution.

[Robson Valley]

It's a mineral salt, iron acetate, which complexes with the organic tannins in the woods.
A presoak in very strong tea will add tannins to weak wood for a deeper stain color.

I made some, tried it on a variety of wood carvings and concluded that it just made the wood look dirty.
Nothing was ever exposed to sunlight (UV) so I don't know how that might bleach the tannin component
and what result you might then see.

[aarggh]

I found on Vic Ash which has a reasonable tannin content it would go a motley blotchy sort of dark grey and just looked dirty. However I've had great results on American White Oak with it pretty close to a pure black, so I've ordered some of that Quebracho powder to use and see how it goes on some oak and walnut along with the iron oxide treatment. If anyone's after the powder, this Aussie crowd was really cheap, and they also have a 30% off special atm:

Querbracho Natural Dye Powder
– Fabric of Humanity

cheers, Ian