Part of the problem and confusion with all these concoctions is the incorrect nomenclature often being used here. Aqua Fortis is the medieval, alchemical name for nitric acid...just the acid...not iron or anything else dissolved in the acid. The acid, when applied to wood and heated, even with no iron present, will indeed color the wood darker. The Ferric Nitrate solution (very improperly called "aqua fortis") generated when iron is dissolved in nitric acid, will produce a darker color when it is applied to wood and heated as the iron is now part of the chemical process that imparts color to the wood. So if you ask a chemical supply house for "aqua fortis" they will give you nitric acid, not the ferric nitrate solution most are looking for. So, as Dan Phariss correctly points out, you make ferric nitrate stain using aqua fortis (i.e. nitric acid). Or, you can make ferric nitrate stain with ferric nitrate crystals...either way, it's the same stuff and neither of them are "Aqua Fortis". The stain is not "Aqua Fortis" (which, by the way, means "strong water" in Latin. They called it that because it dissolves a lot of stuff !)
From time to time, I also see the term "Aqua Regia" bandied about. Aqua Regia" ("Royal Water" in Latin) is a mixture of nitric acid (HNO3) and hydrochloric acid (HCl) ("Acidum Salis" in Latin because it was made with salt and sulfuric acid). It is called Royal Water because it will dissolve gold. Something neither nitric nor hydrochloric acid can do alone. I recently had someone tell me how dangerous hydrochloric acid was. They were surprised to learn that they had a stomach full of it and that a lot of people, who own pools or spas, throw a big slug of "muriatic acid" (i.e. hydrochloric acid) in the spa and then jump @!*% near naked into it. The HCl in your stomach is strong enough to dissolve a horse shoe in fairly short order.
Dave C
P.S. If you are looking to buy chemicals, unless you need it for some very specific reason, don't buy "reagent grade" anything. Reagent grade chemicals are of very high purity and are used for chemical analysis where it is important to know exactly what you have in the solution. Reagent grade chemicals are many times more expensive than lower grades. "Laboratory" or "Technical" grade is what you want for home made wood stains.
P.P.S If you just throw steel wool or nails in nitric acid to make ferric nitrate stain and let it get too hot, you will make an inferior stain. You must slow down the reaction in an ice bath and keep the temperature cool while the iron is dissolving.
