What is going on in terms of chemistry is very different if you are using nitric acid on wood instead of iron dissolved in nitric acid. Most of us who use iron in nitric add iron until the acid is all consumed so we are putting an acidic solution of ferric nitrate on the wood. If you use nitric acid, the nitric acid is bonding to the cellulose in the wood to produce cellulose nitrate. You can also make cellulose nitrate by treating cotton with nitric acid. There's another name for that stuff, gun cotton, which burns explosively. I don't think there is anyway around the argument that if you're treating the wood with nitric acid you are destroying the surface integrity of the wood. My guess, just that, is that the colors you are getting is mostly the color of roasted wood, not kicking over a color like you are if you use ferric nitrate in whatever form. That would clearly explain why you aren't getting color in the bottom of the carving, no way to get the necessary heat to caramelize the wood into those crevices without burning the surface.
There are quite a variety of ways to get a golden ground color on a piece of maple besides turning it into caramel. Look at any book on violin finishing as many violin finishes start with a golden ground.
Tom
