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Old 09-09-2009, 10:58 AM
2152hq 2152hq is offline
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Charcoal blueing and Carbonia/Machine Blueing are not the same process though they produce nearly the same,,but not quite identicle finishes. As already stated, any blueing, including rust and hot salt,,the final look depends alot on the prep polish, though the rust blueing techniques will normally knock the gloss down. (There are ways to do glossy rust blueing though)

Charcoal blueing is done in just that,,charcoal. No furnace, no tumbling, no magic mumblings by the 'smith, save for what ever proper wording is necessary to offset the heat, sweat and the occasional burns that you'll receive doing it.

An open hearth, pan, trough or what ever shape container of small pea sized chunks of wood charcoal are brough to heat by a fire under the container,,not in the charcoal itself.
It will start to burn slowly but you do not want it to get to a glowing BBQ type fire in the container itself. The underfire is raked down to control the heat.

Clean parts are buried in the coals and left to heat, then pulled one at a time and quickly carded down. The carding was/is done with anything from wool cloth soaked in oil, burlap, a bit of rottenstone (polishing) added, old recipes talk of 'tow' and oil. Don't use any synthetic cloth as it'll melt on the hot metal and you'll have to start over after repolishing.
The idea is to remove any scale and burnish the remaining blue color and then back into the pit. You work up the color in coats.

Working this can 'be the pits'. It's hot work and can be exhausting if you are doing alot of parts. Obviously cautions against burns and eye protection a must. Early factorys quite often had child labor doing the work overseen by a boss.

The results can be spectacular, especially with a proper polish underneath. Take a look at any mint condition pre 1913 Colt revolver or automatic. That is Charcoal blueing. The first couple of years production 1911's were charcoal blued.

The only production long gun barrel I can think of that was charcoal blued was the Henry rifle IIRC. Quite a few muzzle loading Longrifle barrels were charcoal blued by the 'smiths that made them in what we'd call makeshift settings.

Not hard to see why Carbonia Blue/Machine blue was welcomed as a labor savings device. One machine could turn out hundreds of pieces per/day.

The process was used in other aspects of mfg including the hardware fittings/fastening industrys.
Carbonia Oil was used to impart the fine finishes on firearms and was a propietary finish with that name but the American Gas Furnace (rotating furnace) could and was used by the factorys with other oils including linseed and even pine pitch to impart blue finishes on less important parts like pins, screws, ect (just like the hardware industry used).

Tru Carbonia Oil is no longer available, at least it wasn't when we were trying to reinvent the process, but Mobil supplied a substitute oil that they said was it's modern equivelent. Looked like 5W Tar!

The 'charge' placed into the furnace is ground bone charcoal, as opposed to wood charcoal used in the Charcoal blueing process.
The oil is added in the correct amount after first heating both the char and oil outside the furnace to drive off any water in them,,a definate must do. Then it's mixed and added to the furnace.

The parts are racked up inside or on racks outside and place into the drum. The furnace w/ parts and charge is closed up tight save for a small vent. Enough to allow for continual pressure escape but no free oxygen intake of air from the outside while the process is running.

Any oxygen getting into the chamber during the process, or initial water vapor steaming off oto the parts during heatup will discolor the final finish. Usually a red haze. Chemicly clean of oil, finger prints, etc a must before start also.

Temps and times are part of the 'secret' that each mfg had for themselves and like case color hardening makes each mfg's blue (and case color) look just enough unique to be able to be identified from anothers, though they all reached beautiful results.

At it's peak, the temp will be in the 800F range. There are slight cool offs and heat ups sometimes used. Winchester had problems with color matches and used slight variations in process according to the parts being done.

In the end, the parts are left to cool. It is actually an annealing process at those temps so some though SHOULD be given to the parts, their original heat treating specs, etc. The process may actually weaken the strength of some heat treated parts.

Steel at 800F for a couple of hours and then left to cool slowly inside the furnace is a dead soft annealing for alot of steel.

Hot salt blueing replaced Carbonia at Winchester in about 1938/9.
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