What you're talking about is a solution called "aquafortis" ( not sure if I spelled it correctly). It's made by disolving iron in an acid, (the fumes of which are toxic). It's a very traditional stain and penatrates deeply into the wood, especially on a suger maple stock. It will stail other woods, but it does best wkih maple as it reacts with the sugars in the wood.
Many of the professional makers still use it today. You can purchase the solution already made from any major muzzleloading supply house.
Once the iron was fully disolved, it was wiped on the stock then heat was applied to the stock after the solution had dried and it would bring out the stripes in a curly maple stock. Some say you need to apply a base to it, like baking soda to stop the oxidation of the iron particles embedded in the stock. It may be one reason why the stocks on old muzzleloaders are so dark. And the acid may react with any metal it comes in contact with it it hasn't been nutralized.
I've got some that I've tried on some scrap maple, but I'm not real thrilled with the way it looks, and the idea that it could continue to darken the wood, while it may be historically correct especially on a Southern Mountain Rifle, doesn't appeal to me. You also have to put a lot of heat on it to make the grain "pop" Not enough heat and the wood has a green color to it. And you run the risk of scortching the wood as you apply the heat. I paid a premium for an extra curly maple stock, I want those stripes to show!!!! Plus I'm building a new rifle, not one that looks like it's 200 years old.