The hydrogen peroxide and saturated salt solution without the vinegar is doing a very good job of rusting a couple pieces of steel for me right now. I was going to do several more passes over the next couple days and then boil them to see if the rust turns black. You have answered that question, but I expected that it would turn black.
What I'd really like to know is if old barrels that have turned an even brown color could be degreased and boiled to reconvert the red oxide to black. I have been told that this will not work, but I don't see any good reason that it shouldn't. On some old guns that have a very deep and even patina, being able to reconvert the red oxide to black by doing a quick boil and if Damascus, an etch, might save a lot of time and effort to polish and re-blue. Anyone ever try this?
The simple answer is NO................When barrels are rust blued or browned, the rusting action is stopped by one of several methods, so it does not continue rusting inside the weep holes/ribs etc. Barrel corrosion processes need to be halted and most barrel blackers and browners usually use a Caustic Soda to halt the rusting process when they are finished, so by merely boiling some years later, no further color changes will take place after the oxide layers have been stabilized.
Different alloys with different carbon quantities produce different colors especially when "browning" damascus barrels. You can watch the process as you continue to add more coats, but they will "stop changing colors" when the alloy pores are full. Most stop when this level is reached.
As an example, when working on damascus barrels and browning.... when halting the rusting process one must be careful NOT TO used a water temperature with the Caustic Soda over 158 degrees F (70 C)OR ALL YOUR HARD BROWNING WORK WILL TURN BLACK
...........at which time you will need to start all over again......!
Hope this helps.....