I originally found 'recipe' for darkening wood on toymaking plans website, and have used it successfully on pine from 6mm to 25mm, also on sycamore 18mm thick. I have found that it can make the wood brittle, (the up side of that is how beautifully it sands) and can be bit of a lottery as to whether wood will warp, but usually start with half hour and keep checking as different bits of wood seem to be different, guess its down to moisture content or resin or something.
have also used it on bought wheels, and they darkened up nicely for more realistic look without painting