Ok, I am going to stick my two bits in on this one. 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.
Heat bluing I believe was developed 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.
All this fabulous knowledge was tossed in the second world war, or shortly after in favor of quicker methods.
Last edited by Barry Lee Hands; 07/20/08 03:07 AM.
