View Full Version : A short discourse on traditional metal finishes

Barry Lee Hands
07-20-2008, 09:43 AM
These are some notions I wrote down in response to a discussion on metal finishes with some gunmaker friends, and primarily is written from a American gunworker point of view. I thought it may be of interest to some folks here who are just beginning to learn about finishing.
These opinions come from restoring a lot of guns, including using heat blue in air, in nitre salts and with my home grown carbonia method developed by trial and error to imitate the old American gas furnace company methods.
And my opinions come from stripping these original finishes off old guns and observing the differences in their resistances to acids and abrasives.
And also listening in on Dan Goodwin and Monte Mandarino discussing period finishes back in the 80's.
Perhaps someone else has differing opinions, or knowledge and would like to share them. I would like to hear something about how these things are done in Europe today, if anyone has knowledge of that.

Rust bluing, or blacking, is among the oldest finishes, it was used and still is, primarily on barrels, as it will not attack the solder,and though it has a romanticly inspired reputation for being very durable, is only about the same as a well applied modern hot dip blue.
It is appied by swabbing on an acidic solution, allowing the part to rust over a periond of hours or days, then boiling to kill the rust, and then carding, usually with a wire wheel. The process is repeated again and again, until a good black finish is developed.

Heat bluing was used to draw parts back after case hardening.
Screws,and hinge pins as they were made of the easiest to machine steel, were hardened as hard as possible, then polished and then drawn back to spring temper by heating to a blue color which with most steels occurs between 650 and 700 f whether you are using flame,or a nitre bath.
The case hardening carburizes the hardened part, so even after is is drawn in the salt bath, it is more resistant to rust then it was before case hardening.
The Salt bath simply provides a way to heat the part evenly,as opposed to a flame or primitive oven and has the advantadge over an oven or lead bath in that you can see the part in the bath, and I think the nitrates were cheaper than lead, back in the day, as they still are.
The salt bath also does not alter the chemical composition of the steel to any real degree, and the blue, whether by flame, or salt bath is very, very thin and can be removed very easily.

Charcoal blue, done in a box with bone charcoal where the part, usually a triggerguard or some other furniture, that has probably not been case hardened because it may warp, and is not as imortant in its hardness being spring temper as a screw or a hinge pin, adds some carbon to the part being colored, and leaves the part in a soft state.
A soft part rusts easier, and this explains why triggergurads and forend latches on old english shotguns are often far more corroded than the parts next to them, including the screws and actions.

Around the time of the civil war the carbonia process was developed leading to the american gas furnace company 's patented rotary furnaces in common use by the major american companies by 1905 or so.
The carbonia blue, which is a creamy black color commonly, is by far the most durable of the traditional finishes. It is applied using an oil to coat the parts, which are heated in the rotary furnace, the rotation acting as a convection oven for even heating and color, and a large amount of carbon transfers from the oil into the steel and makes it very difficult to strip with acid. Winchester appied this so heavily that we have all seen the old winnies with the black flaking off the frame. The black is actually in this case the carburized oil, baked onto the frame.
Colt used the same furnaces and chemical charge to put down a much thinner finish, and lowered the temp to get the fantastic, but durable bluish finish on the early automatic pistols.

Much of this fabulous knowledge was tossed in the second world war, or shortly after in favor of quicker methods.

07-20-2008, 02:17 PM
Thank you sir!

Saved that off for further study / reference.

rick woodward
07-21-2008, 03:43 AM
Thanks for this education on finishes. I have been thinking about finishes for a while and reading what i could find. Could you expound on modern/easier/less expensive/Quicker ? metal finishes ? Some things I could do here at home without thousands in investment ? In another forum i read about an aqua ammonia coloring of brass. Haven't had time to try it yet. I have checked out Brownell's sytem's..... Pricey to get into. Also read about Casey Birchwood stuff I think its called. Will probably experiment with that stuff at first. I would like to leave a center area "clean" and blue around the oval to the edges.But then how to protect the bare metal in the center? Don't know if an airbrush would be needed or a regular brush could be used. Don't know what it would do to an airbrush either.... I have read about bead blasting also. If you or someone could point me in the right direction for a pencil blaster recommend I'd appreciate that very much . Can a carbide tip be had for it ? what diameter orifice. I can get a cheapo small benchtop cabinet for $110.00. I recently sold my 48"x 30" sandblast cabinet. But its just up the street. Your tutorials and posts continue to enlighten me and helps build my knowledge. Cant thank you enough. Rickw