Rust bluing is a topic unto itself. There have been all sorts of rusting solutions proposed and used from saltwater to urine and in some cases rusting was done by Mother Nature, but that is a really slow way to do it. I have seen acid fumes used as a rusting agent. It is laborious procedure to perform, but it does work well and looks good. I noted in another forum question response that Winchester did rust bluing for their guns until the 1930s. Some of the older gunsmithing books have chapters on how rust bluing and browning are done. I have thought about doing some experimentation on rust bluing. But I never worked up enough enthusiasm to actually do anything.