Michael H. Mara Had this to say on Facebook yesterday^^
Michael H. Mara:
We paint Potassium Permanganate crystals dissolved in water on the antler to darken it. It's not a paint nor dye, simply a very powerful oxidizer. Takes quite a few coats to get the really dark brown color.
Like · · Share · 19 hours ago
He also added that it will stain anything it touches, and will dissolve the brush slightly.. :p