this is a really cool staining method that i learned from a blacksmith in west Virginia a couple of years ago. the first time i saw it i was amazed, not only are the results beautiful, but the process is just cool to watch. you start with a solution of 10% nitric acid and water (doesnt need to be exact just diluted considerably). i got nitric from the guy who taught, he used to work in a big chemical plant so he has a ton of it, otherwise im not realy sure where to get it but im sure you can find it somewhere. the acid has to be neutralized, the stuff i have was neutralized using steel wool. the solution will be totally clear and just look like water. once you make the solution here is the actual staining process.
you start out by wiping the surface down with the solution, it doesnt need to be drenched but should have an even damp coat across the whole surface. after that you let it set until the surface is totally dry. the guy who taught me said to lay it in the sun to dry, i dont think this actually does anything but help it dry a little quicker though. when the guitar is dry it wont look any different than when you started but that is where the heat comes in. the guy who showed me was staining knife scales so for heat he just used a red hot piece of steel, for a luthier a heat gun is much more practical. start in one small area of the guitar and work it with the heat gun. i dont know the exact chemistry behind it, but my understanding is that the heat triggers a reaction between the nitric acid and the sugars in the wood. as the wood begins to heat up all the sudden before your eyes it will start to take on a beautiful amber to honey color gradually getting darker and spreading with the heat. keep the heat in one area until the color stabilizes and stops getting darker. When one area reaches a stable color move onto the next area until you get the whole of the guitar complete. after words i will coat it with danish or linseed oil.
though i have only just begun experimenting with this staining method i love using it becuase it gives great depth and transparency to the wood and gives you that sense of being able to almost look into the finish. this of course can be accomplished in other ways but where the nitric wins out is in the ease of application. with very little work you can get a beautiful finish with lots of depth. where it falls short is in its limitations, you can only get one hue,so if you want some type of burst or for you guitar to be green then its out. the other pitfall is that no two pieces of wood are the same and even among the same species each piece will yield a slightly color from the next, and different species can yield very different results.