What is Charcoal Bluing?
Capturing the proper shade and texture of original charcoal bluing (sometimes referred to as machine bluing or carbona bluing) finish depends largely on the underlying metal polish. Metal parts are placed in a gas-fired furnace with animal bone and sperm whale oil, or equivalent. The end result is a hard-wearing finish with colors that vary from a blue-black (highly polished) to a lighter blue (low polish) depending on the level of polish. A charcoal blued high polished part will produce a mirror-like finish that is second to none.
Acknowledgement: Parts of this article are adapted from “Antique Metal Finishes” by S.P. Fjestad and Doug Turnbull, found in the upcoming 2nd Edition of the Blue Book of Antique American Antiques & Values. (link takes you to 1st edition at Midway USA)