This is from the late Bill Adair's web site:

Bluing Methods, Definitions and Processes
by Bill Adair
[snip]
CARBONIA Heat/Chemical
Now here's one of the most mis-used, least-understood words in the entire bluing lexicon. 'Carbonia' Blue was a S&W proprietary method used in the period from before WWI thru the 1960's. It was also known as 'Smith & Wesson blue'. It was ONLY done by Smith. Never by Colt or any other manufacturer. Carbonia bluing resulted in that deep-black/glossy high-polish finish that Smith was noted for during the years they used it. It's similar to 'DuLite' and Charcoal bluing as far as the process goes, but certainly not the same.
The Carbonia oil (a product of American Gas Furnace Co.) was used by many gun manufacturers in their own versions of 'DuLite' bluing, but the use of Carbonia oil does not make it 'Carbonia Blue' as only S&W did it. DuLite bluing, such as Colt did on their 1918/1919 military model 1911's is an industrial/utility finish. It was generally done over a fairly coarse-polished and/or sandblasted surface, and is a dullish, dark-grey or near-black color when used in that way. It was also far less durable than the S&W Carbonia Blue.
And there's a funny story to go with the S&W Carbonia Blue. I'm telling it like I heard it, and I have no idea if it's true.
The basis of S&W Carbonia Blue was an oil mixture (pine-tar based) made by the American Gas Furnace Company, and they supplied the oil in bulk to S&W, who mixed it with bone charcoal and other 'stuff' to make their own Carbonia product. Years ago, by the way, I contacted the American Gas company for info on the process, and they were kind enough to give me a list of the chemicals/ingredients used by Smith for the process, but it was just a list of chemicals, not a formula.
So, here's the story:
Apparently, only one old-timer at Smith knew the exact formula and he had it in a notebook which he kept. He eventually retired from Smith, and later died. His widow, so the story goes, contacted Smith and offered to sell them the formula in the notebook for $50k. I guess she knew that her husband had the only written copy of the secret formula. Well, Smith had gone into hot-bluing by then, and wasn't really interested in shelling out $50k to her for the Carbonia formula. So, she burned the notebook. And that was the end of Carbonia.
The moral of the story is that all of these companies who now say they do 'Carbonia' bluing, or worse yet 'Colt Carbonia blue', are just you-know-what. Maybe they can do something that looks similar to S&W Carbonia Blue, but it ain't. And Carbonia blue is not Charcoal blue. It's very black the way Smith did it, not blue, and please, Colt never did it.
Carbonia, when applied to a surface that is not expertly high-polished, results in just a so-so utility kind of blue. Time and temperature controls were critical in obtaining the exact color Smith desired.
I've still got the list of ingredients, but there are numerous items on the list, and you'd need to combine them in the correct measures to get the actual S&W formula. I've combined most of the ingredients (or similar ones) in various percentages and at one time did quite a bit of R&D with it, but I never got too interested in pursuing it much further. It was hard enough to find any whale, let alone a sperm whale, so I never had any sperm-whale oil. That was just one ingredient I couldn't locate. American Gas Furnace Company doesn't make the stuff they supplied to Smith any more, so it's a futile pursuit as well as further proof that true Carbonia bluing no longer exists. However, the Carbonia look can be simulated or duplicated by other means.
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