Nitric acid that has had lots of light steel or iron dissolved in it. The old-timers way was to dissolve nails in it till it would not take anymore.
Track of the Wolf has it ready to go they are online. IT makes a great Stain for curly maple but you will have to heat the wood after treatment and then neutralize unless you want it to turn black over the years. I would keep it away from walnut as it will become BLACK and show no figure.
If you are wanting to brown steel pass on it and get Laurel Mnt brown as it will be simpler to use and more controllable.
