doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com How is nitre bluing done.

C. J.
You are right, Oscar was a good friend for over 30 years and it is not a matter of watching a clock but of watching the parts. The material is simple salt peter and a dash of aluminum sulfate. I once asked Oscar how much a dash was and he just grinned. Anyway I use a bread baking pan and the chemicals heated outdoors till they melt then put the DRY parts in and watch.
bill

CJ, Classic Guns says that Carbona and charcoal blue are the same thing. What have you learned that makes you think different?

http://www.classicgunsinc.com/

OB

Wow, I never heard that one, charcoal blue is a simple old time process, carbona was a complex industrial process used by mass manufactors that cost a lot of money to set up and run. You need volume to pay for carbona but it is a very good finish.
bill

Mal, the lenght of time in the bath is directly related to the size of the part,..small screws will reach temperature much faster than say...a thick trigger guard or a butt plate, so like Bill said it's a matter of watching the parts untill the desired colour is achieved.....Temp??...I'll just say that 625/650F is not hot enough for my colours

Miller, yes you are correct,. from what I was able to find out "Carbonia" was a trademark name
here is an article you might have read before that describes all different types of metal finishes, it concurs with most of the info I have found, if you ignore some of the sarcasm you might find the article interesting specially the story of how the formula was lost forever

http://www.restoration-gunsmith.com/artblue.html

This other link states very similar information but it somewhat contradicts some of the info in first article, so it's pretty difficult to say who is right or wrong,...what both seem to agree on is that charcoal and Carbonia are not the same

http://www.ronsgunshop.com/finishes.html

OB,...Classic gun call it "Carbona", not Carbonia so I'm not sure what they mean,...they are not very specific as to how they do it,..I just know that most people that do this for a living are not all too keen to talk about details.

CJ

Carbonia Blue is not the same as charcoal blueing, though it makes use of charcoal in the form of (bone)-charcoal in it's process. Carbonia blue (sometimes called 'machine blue') is the later post WW1 process. Prior to that period, charcoal blueing was used. Charcoal blueing was/is a labor intensive process. Each part handled manually into glowing charcoal hearth and taken out and rubbed down lightly with a slightly rough material & oil,,usually at the time whale oil, though linseed will work. EAch mfg had their own techniques and set-ups and variations in applications differed but the basics of the process were the same. Take a look at a pre-WW1 mint condition Colt to see what real charcoal blueing looks like. The first few years of 1911 production were charcoal blued. Carbonia blue (Carbonia being the brand name of the oil used in the process at the time) was a major step in advancing production and eliminating labor and costs at the time. Many parts were able to be blued at one time. Other industrys used the process as well. Remember when screws, washers and other hardware were blued? Most if not all were 'machine blued' in bulk with this process through the 20's and 30's. Most likely they used cheaper linseed oil or even pine pitch mixed with the bone char to get an acceptable result. Some gun mfg. used linseed also when blueing small parts (pins, screws, washers, internal parts, etc) with this process. Carbonia was being replaced by hot salt blueing in the late 1930's as a next step in looking for a way to produce an acceptable blued finish quicker and cheaper. Though one was a substitute for and very close match in color and looks for the other, Charcoal Blue and Carbonia Blue are different even in the basic way they are applied.

In a post from 1999, Oscar Gaddy pointed out that all the various bluing processes did the same thing which was to grow a film of iron oxide on the metal surface.

The color variations run from the first temper colors produced by an oxide film 1/2 wavelength of light thick to blue-black to black as the film becomes thicker. The temper colors actually repeat as the oxide film goes from 1/2 to 1 to 1-1/2(and so on) wavelenghts of light. However, the thicker film also grows more opaque and thus absorbs more and more light reflected from the metal surface (as opposed to the oxide surface).

Re: Carbonia blue, a 1937 Winchester employee in the finishing shop said that their "machine bluing" process used sperm oil and bone charcoal in the rotating retort. He said that the smoke produced by the oil was what caused the coating to form. They had to be careful to not let too much air into the furnace or the oil smoke would catch fire. They got two heats per 12hr. day. Small parts bluing used pine tar instead of sperm oil. Some charcoal bluing was still being done at that time but he doesn't elaborate. Hot salts bluing(Du-Lite) began to be phased in in 1939.

The above is from "Winchester, The Way it Really Was" by P. Muerrle

OB

I do some charcoal blueing in one of the classes that I teach and it isn't too difficult with the right materials and equipment; I do find it a bit too shiny for some applications however. It is not a good choice for heat treated parts since, at least the way I do it, the temperatures involved approach 900 degrees. Here is a somewhat bad photo of a cylinder I did last summer, the surface is actually very deep and consistant, something the photo doesn't show. The spots showing in the photo aren't really there.

Care to share your method Alex?

It's actually quite easy. I use pure charcoal from Brownells and a heat treatment furnace, the type isn't important as long as it can hold 900 degree temperatures and you can control it. The parts are given a high polish to at least 600 grit and than I degrease them. The parts are buried in the charcoal in a container that can be sealed, placed in the furnace and brought to 900-950 degrees and held at temp for 30 minutes to an hour. After this soak turn the furnace off and leave the parts inside until they oool down to about 200 degrees. At this point I generally take the parts out and degrease them again (this was probably the stage when they were traditionally rubbed with lime) repacked in the charcoal, resealed, and placed back in the furnace and reheated to the 900-950 degree range for another hour or so. Lastly the furnace is shut off and the parts are cooled down to 200 degrees and than rubbed with oil (I generally just use any heavy oil on hand, sometimes motor oil). The parts are left alone over night and the color tends to get deeper and more even during this time. Not hard, but you need to have a furnace that you can control for good results, also the charcoal from Brownells is the only thing that has given consistant results in my tests, same for color case hardening.

ALex,..Do you use wood or bone charcoal?

CJ