I found the formulas from The original owner of Whakun Bay as posted on this forum a few years ago. The instruction contain formulas for an aqua fortes and an aqua regia based product.
"So, here we go... 300 ml (10 fl-oz) of conc HNO3 (15-16 Molar), 240 ml (8 fl-oz.) of conc. HCl (10.5-12 Molar), mix the acids, in large, heat-tolerant container, add about 30 grams (~2 oz.) of 4d finish nails, do this outside, depart the immediate premises and allow the iron to dissolve, liberating NO2 gas, very dangerous, so get out of there, and isolate the system from any metal objects you care for!! Let this exothermic, almost volcanic, reaction occur, allow to cool, then with dionized or distilled water, dilute this matrix to one gallon, or about 3784 ml, total solution. Store final product in amber glass or polyethelene container. This will give you enough browner FOREVER. To make the AQ stain, follow same procedure, except after reaction has ceased, dilute to only about one-third, or to about 1200 ml (~40 fl-oz) of total product or solution. Now, if you are only interested in creating a stock reagent, then one can approach it by taking only the conc. nitric acid, diluting about 2 or 3 to one with water, by adding acid to the water, NEVER, the reverse, and adding degreased steel wool to the acid solution until no more appears to react, then use it for final product in that state of dilution. Pure nitric acid will not dissolve the iron, it must have "excess protons" within the system, whether from another acid, like the HCl, we cited, or from the water, which obviously need not be that pure, as we need H+ ions, not water molecules...
That's the story, not bioengineering or rocket science, but be careful if you undertake this project, as the NO2 is dangerous, noxious, toxic, and some say even carcenogenic... Wear eye protection and rubber gloves when handling any strong oxidants, safety first, stock stain/barrel- browner second!!"
