Yep - you can get the same effect from Copperas, from the garden center. It's ferric sulphate, and it's what yields "rabbitwood" from sycamore - the only non-species wood type differentiation in the entire woodworking world. Its effect on sycamore is slight, though, compared to its effect on oak. I've completely blacked oak with it, blacker than a pair of fresh-shined military shoes.
Speaking of which... black shoe polish (which is carnauba wax mixed with lampblack) works very well to ebonize wood, too, provided you're good with the waxed surface.
-- Tim --
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