Another maple coloring technique calls for the application of dilute nitric acid ("aqua fortis")followed by heat, such as holding the stock over a gas burner. The wood becomes "greenish" and then brown. Repeated applications darken the wood further and eventually can turn the wood a brownish black. This technique was often used by the old Pennsylvania rifle makers. At one time. pre-mixed "aqua fortis" stain was commercially available but it does not seem to be, now.