I have seen aqua fortis for sale on various sources TOW and Kibler's website to name a few.
Are they all the same? Are any better than the others?
Kilted Cowboy
No, they are not the same. Track's "Ye Original Aquafortis Reagent" contains H2O, NH2O3, HCl, and Fe. Track adds some
hydrochloric acid with the nitric acid and iron. Most others would just be nitric acid with iron and water, of course.
Even looking at just the solutions with nitric acid and iron, the concentration of the nitric acid that they start with can vary.
The basic idea is to dissolve iron in nitric acid until no more iron will dissolve and nearly all of the acid is spent. You can probably imagine that a stronger acid solution will dissolve more iron. The end result is a ferric nitrate solution that may or may not be slightly acid still. The iron ion in the solution is free to react with the tannin in the wood when heat is applied making a dark brown or black compound in the wood.
"Are any better than the others?" That's difficult to say. Peoples' tastes are different, so what looks good to one person may not be desirable to another. Wood is also variable. You may get different results with the same bottle of ferric nitrate on different pieces of wood due to the different amount of tannin in the woods.
Here are a couple of examples of some tests I conducted. I had some old "Ye Original Aquafortis Reagent" from when Wahkon Bay Outfitting Co. was making it. I don't believe it has the hydrochloric acid that Track's version does. I also had some Frontier Flintlocks "Aqua Fortis" (
https://www.frontierflintlocks.com/).
Below is a piece of wood I tried each stain on.
I also tested applying some Laurel Mountain Forge Maple stain on top of the other stains after they were blushed with heat. You can see the the old Wahkon Bay "Aquafortis" produced a much darker brown than the Frontier Flintlocks "Aqua Fortis". On this piece of wood, both produced a brown stain. The LMF Maple stain added a hint of red to each which some people like.
Another option that a lot of people like is to buy ferric nitrate crystals from the Science Company (
https://www.sciencecompany.com/Ferric-Nitrate-100g-P6384.aspx) which they mix with either water or alcohol. This way they can experiment with the amount of the crystals per given volume of liquid until they get the results they want with a particular piece of wood.