Use a solution of Potassium Dichromate. Garrett Wade (among others) carries it. It works particularly well on white oak, box elder and pine. It does not dye the wood, it reacts with chemicals in the wood to simulate aging.
Others have suggested strong solutions of tea or coffee rubbed in with steel wool. Still others have used a vinegar solution in which steel wool has been soaked for a couple days. I'd think though, that this would work best with a high tannin wood, like oak.
'The true soldier fights not because he hates what is in front of him, but because he loves what is behind him.' G. K. Chesterton
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