[Ron Smith]

The Russians just dunked them in a hot dip tank. It was applied over the original finish, and often appears to be a "stove black" painted finish. The dip can be removed down to the original finish. I know of a collector who cleans them with an Ultra-Sonic cleaner as used for jewelry, with good results. He does warn that the dip will adhere to bare metal and can gum up the cleaner.