There is a very effective product out there called “Plum Brown
Barrel Finish” which can be found in gun shops and I’ve also seen it
in Wal Mart’s gun section. I can recommend it for its results, but it
needs to be handled very carefully as it is quite toxic. An
alternative would be ferric chloride. A liquid form of this can be
bought at Radio Shack or other electronics stores and is used for
etching circuit boards. Of course, chemical supply houses will have
it in dry form. It is not something you’d want to drink, but it is
fairly safe aside from its being an oxidizing agent. If you expose
fabric, wood or paper to it, you have to rinse it out thoroughly
before discarding it as it will become quite flammable when it dries.
You can apply it to a warmed article for faster effect, but repeated
applications on a cold iron or steel item willl result in rusting.
David L. Huffman