You can darken oak using vinegar and rust. If you soak iron in vinegar it creates a solution that you can wipe onto the wood and created a darkened effect. Two notes: 1) vent whatever container you use to react the iron / vinegar as gas is created during the reaction and allowing in to escape allows the reaction to continue more quickly (and is probably safer) and 2) If you happen to use steel wool, rip it into several pieces as it is sometimes treated to prevent corrosion (which is what you want to happen) and ripping it exposes non-treated metal surfaces.
Also note that the effect of treatment is not immediately obvious and you should test it and leave it overnight to check if you've created a solution with high enough concentration and also what colour you are going to end up with.
This solution works in a similar manner to ammonia, that is, by reacting with the tannins in the wood, so you will get a greater or lesser effect depending on the tannin content and the number of applications.
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