"You can also add tannic acid prior to applying the vinegar/iron oxide solution. This allows you to get much darker, even on woods that contain little tannic acid. If, like me, you are too impatient to wait days for the steel wool to rust in the vinegar, add a bit of hydrogen peroxide to the mix. You'll be ready to go in a few minutes.The chemistry is pretty simple. Look up iron gall ink. Same idea."