Hello, I've been using vinegar and steel wool to color oak and
walnut bowls but never knew much about the process. Here's a bit of
trivia for those interested in the Black color formed with acids, iron
and tannins in wood.
Kremer Pigments http://www.kremer-pigmente.de/englisch/homee.htm
call the black color by the historic name "Atramentum". They sell it
as a powder to be mixed with water or oils, linseed, tung, etc.
"Produced from the tannic acid of oak bark in a reaction with iron
salts, Atramentum is a deep black material that differs from carbon or
iron oxide blacks. Historically also called 'ink stone', it appears
greyish black in watery media, which can act corrosive on paper
supports. However, in linseed oils or resinous glazes this pigment
develops its incomparable depth and lustre. Very lightfast." Kremer
sells Atramentum for $11.20 / 10 g.

Another informative website describes the chemistry of several inks,
including Atramentum. Click on the "Ink" hypertext at this website
http://home.att.net/~numericana/answer/chemistry.htm#ink
I also found the discussion of Gum Arabic interesting. Fascinating
what people figured out in the "old days"....

Tony Bryhan