Concentrated nitric acid (69 to 71%) is a transparent, colorless or yellowish fuming, suffocating, hygroscopic, corrosive liquid. Attacks almost all metals. Miscible with water and decomposes in alcohol. Dangerous fire risk in contact with organic materials. Highly toxic by inhalation, corrosive to skin and mucous membranes, strong oxidizer.
Nitric acid is commonly used to dissolve gold from printed circuits boards by making aqua regia, typically 1 part Nitric acid to 3 parts Hydrochloric acid.
It is used to oxidize Propolis by the method of William Fulton. It can be used to enhance the color of any wood and does not rely on the presence of tannins in the wood for its magic to work.
For instructions on how to use Nitric Acid see the section in the book The Art of Coloring Wood by Brian Miller.