Came up on a subject of rust bluing (as opposed of hot caustic bluing) and did some research and tests on the subject.
In USA I've heard that parkerizing is very popular, and apparently it's more durable finish on a firearm, but bluing is also always present on loads of firearms.
So what is bluing ? It is a process of turning the surface of the metal to a layer of "controlled" iron oxide that is a bit passive and also the crystalline microstructure of the "rust" helps to hold the oil on the surface of metal. So, it's not some kind of rustproof or stainless coating (what a paradox, the coating is type of rust) !
Blued surfaces should always be wiped with oiled cloth after touching and using them, oryou would get new layer of fresh rust.Since it's a bit passive surface, rust won't be so quick to form like on clean shiny metal surface, but it's unavoidable.
Bluing developed in 19th century in the form of rust bluing, but in 20th century this was replaced by hot caustic bluing.The question is why because the rust bluing is more durable type of finish than what hot caustic bluing gives.Answer is; hot caustic bluing is quick process suitable for industrial scale of production, and rust bluing is slow process that takes a week to finish on a firearm so it's naturally very expensive for mass production of firearms and it's financially suitable only for gunsmiths and hobby gunsmiths like on this forum.
In general, rust bluing is performed on the firearms which surface is cleaned to a shiny metal (no more than 320 grit sandpaper is necessary) than rease is removed with some solvent and than the surface is treated with some acid type solution that will induce rust after 12-24 hours in moist environment.You cannot just sprinkle the metal with water and left it to rust because it would not corrode in controlled manner like with some acid.
Than when rust is formed on the surface, the part or firearm is placed into the bowl of simmering (distilled) water and "cooked" for an half an hour till the red iron oxide converts to black iron oxide.After that it's put out of the water, dried and than the layer of black rust is removed using the steel wool.
This process is repeated for 5-6 times till sufficient amount of controlled black rust layer is on the firearm.That's why is this time and money consuming (if it does some professional gunsmith charged by the hour).
One of the drawbacks of caustic bluing is that you gotta handle various dangerous chemicals and boil them which is potentially dangerous situation (fumes, corrosion on the containers etc.) and with rust bluing there is no such things.
Joined: January 04 2010 Posts: 393
Posted: July 17 2010 at 5:32pm | IP Logged
The important part of rust bluing are the chemicals or solutions that induce rust on the surface.
Traditionally, there is a "formula" of the rust bluing composing of nitric acid, hydrochloric acid and distilled water and also iron "nails" are placed in the solution to dissolve, to probably induce some new chemicals in the solution and maybe neutralize a bit the power of the acids.
Note: if the acids are too strong when re applying the solution to the surface for next coating of rust the acids will dissolve some of the passive black surface rust.
I've found also some old formulas that could be used :
brown- black color: ferrous chloride 15 grams, distilled water 0.1 liter ( ferrous chloride is used in electronics plate making and could be obtained easily )
velvet black: ferrous chloride 15grams, ferric sulfate 30 g, copper nitrate 12g, distilled water 1 liter, ethyl alcohol 5 ml
antimony black: antimony trichloride 1 part, distilled water 3 parts
There were more recipes, some of them included selenic acid and copper, and guess what, commercially "cold gun blue" solutions contain selenic acid too.But guess what also wikipedia says about that :
"Cold black oxide is applied at room temperature. It is not an oxide conversion coating, but rather a deposited copper selenium compound. This coating produces a similar color, but tends to rub off easily and offers no corrosion protection."
So, "cold bluing" from trade solutions is not chemically equal to the "cold" rust bluing (as opposed to hot caustic bluing) and will NOT produce passive magnetite rust surface coating.
In my experiment I used the "velvet black" solution, whatever that means because it didnt look much like black velvet, maybe a little like red-brown rust velvet, but I dont want red-brown look.
I've come up with some changed procedure regarding the rust bluing;
First I've cleaned the surface of the metal pipe with sandpaper, than with some solvent
Second I applied the solution, left it outside to rust and next day used steel wire wool to remove the rust.Because I' didn't wanted to screw around with boiling (I don't have so big pot for the pipe) I just applied the solution again.
I repeated the process about 5 times and at the end I took the pipe with rust on it (no carding) and my idea was to now boil it in water till it turns black iron oxide, but again no pots (I did that on some smaller part), so I choose to first coat the surface with used old motor oil and than scorched it till it evaporates with propane blowtorch in attempt to blacken the surface.
Also I read in some book that blackening of the blued surface could be done with coating of linseed oil and burning it with blowtorch on 400 degrees celsius.
After that I used the steel wool on burned surface to remove the black rust layer, which presented partially brown-black surface and than again coated it with mot. oil and burned it again.
In the end I cleaned the pipe with soap for heavy stains on hands (like mechanics use) apparently it removes some of the rust surface or whatever because the water was brown when I rubbed the soap (the soap has some particles in it that rub the surface of your hands or anything else for better cleaning)
The result is a nice black surface, it's a bit grainy when looked from near distance but I'm satisfied with it, of course there could be improvements in the process and materials used, and for the first time of testing it, it turned out ok.
I'll put pics in next post.
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Joined: January 04 2010 Posts: 393
Posted: July 18 2010 at 11:38am | IP Logged
Due to photographing the color is not 100% the same as in the reality.
Surface after 2 coatings of acid solution and after carding.Note that small part on left end is intact and in shiny metal state for comparision.
Final rust on surface (not carded) that was coated with used motor oil and burned with propane torch, show in the state after the process, not yet carded
Look of the surface after the burned black rust was removed with wire.Now its mostly black with some brown spots.Another coating and burning is required
Final look of the surface that is now coated with wd40.Before it was again burned and after that washed with soap.
On the far left side you can see intacted part that is still bare metal (altough turned brown), but it is smooth and do not contain passive magnetite rust coating.
x 1 x 1
Joined: January 04 2010 Posts: 393
Posted: July 18 2010 at 8:12pm | IP Logged
I've find old book from 1914. that covers some of the processes of metal conversion coatings, also there are some recipes for controlled metal rusting solutions :
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