I like to work with the least toxic materials I can find. And so for a while I have intrigued by the prospect of coloring wood with the well covered technique of applying a solution of steel wool (iron) dissolved in vinegar. I finally got around to trying it out a few months ago. This process has the effect of darkening, or ebonizing (making black) wood by reacting with tannin in the material. Obviously this works differently on different woods based on the tannin content. A natural choice for a high tannin wood is oak, a wood I work with a lot. For woods with less tannin, you can apply a solution of brewed black tea to the surface first (to add tannin), then the iron-vinegar solution. A quick search on the internet will yield tons of "DIY" sites talking about this process to "weather" wood to make it "rustic". Ugh. I'm just using it to make it dark.
Making the iron-vinegar solution is as easy as dissolving a pad of steel wool in a container of common white vinegar. It takes about a week to fully dissolve. The proportions aren't too important. For my solution, i put one pad of 0000 steel wool into about 16 oz. of vinegar. Once the steel wool is fully dissolved, I filtered it through a paint strainer into a clean container. 16 oz of this is enough to last me a loooong time, and I think the shelf life is pretty much forever.
With the solution ready, I applied it to surface prepped oak with a foam brush. At fist, it looks like water, but after about 5 minutes, the oak was a dark dark grey. With the test successful, I used the technique on a picture frame project. I applied the solution to a white oak frame, with red oak pegs. After the solution dried, I applied some blonde shellac, and a little clear paste wax buffed on & off. The dark dark grey looks black after the finish is applied. But the grain of the wood really shows through, and looks very rich. I love this process and will be using it in more projects in the future.