On Wed, 25 May 2005 16:16:53 -0400, "Stephen M"
wrote:

The ammonia gas
in the "air" reacts with tannins in the wood to turn the wood dark. I assume
that the color change stops when the reactive chemical is the wood is "used
up".

try this - a couple of samples of oak from the same board. Gas fume
one, treat the other with a light wet coat of the ammonia solution (26%
"strong household ammonia" for my technique). The gas fumed oak has a
greyish brown colour, the wet-treated oak has a much darker near-black.

So the gas reaction may well proceed to an equilibrium and then stop,
but there's clearly a way to force it well beyond this, by applying an
aqueous solution. So whatever is limiting the reaction, I don't think
it's simply shortage of tannins.