Cold vs. hot bluing durability? Chemist in the house?

[Phil Thien]

I guess this question boils down to a chemistry or physics question.

I've done some cold bluing, using Oxpho-Blue. While most people typically wipe this product onto a gun (or whatever), my typical technique involves immersion of the part into the solution (wasteful of solution I suppose) long enough for the desired finish (black!). My parts are typically small screws, etc. I think doing a rifle barrel this way would be pretty expensive.

When I remove the part from the solution, while the part is still a bit wet with the Oxpho-Blue product, I've often observed what appears to be the continuation of red rust forming, and the quick conversion of that red rust to black rust (black oxide), until I dunk the part in oil to stop this process.

I've read (and keep reading) that hot bluing is far more durable than cold bluing. It appears to me that most hot bluing treatments create a thicker rust layer than what most people achieve with cold bluing and a cotton ball.

So I guess my question is, is the black coating left behind by cold and hot bluing the same stuff (black oxide) in both cases? Would there be a technical explanation why one would be more or less durable than the other if they are the same thickness?

BTW, I read that cold-bluing chemicals include copper sulfate, and a chemical that turns the copper black. That, the copper clings to the steel, and then something else turns the copper black. I'm pretty sure copper sulfate simply causes the steel to oxidize, but now I'm not sure.

[Jerry Bruette]

Not a chemist or a gunsmith, but my friend who's a gunsmith explains it this way. The cold blue is more of a coating on the steel using chemicals that color the metal. Hot bluing uses harsh caustic salts at closely regulated temperatures and ph levels to actually get into the metal. The mineral content of the water used can also affect the color and quality of the bluing job.

I've had guns hot blued and I've cold blued some small parts. I'll agree that cold bluing a whole gun probably wouldn't be economical, and I'm not so sure the quality and color would be satisfactory. BTW sending a barreled action to be hot blued isn't all that expensive versus buying and learning how to use the equipment needed.

Jerry

[Mac McQuinn]

It might be worth your time to investigate "Rust Bluing", For me this is the best looking bluing and from what I understand, is the most durable. Good luck.
Mac

[Phil Thien]

Not a chemist or a gunsmith, but my friend who's a gunsmith explains it this way. The cold blue is more of a coating on the steel using chemicals that color the metal. Hot bluing uses harsh caustic salts at closely regulated temperatures and ph levels to actually get into the metal. The mineral content of the water used can also affect the color and quality of the bluing job.

I've had guns hot blued and I've cold blued some small parts. I'll agree that cold bluing a whole gun probably wouldn't be economical, and I'm not so sure the quality and color would be satisfactory. BTW sending a barreled action to be hot blued isn't all that expensive versus buying and learning how to use the equipment needed.

Jerry

If I had to guess, I'm going to guess that the coloring from cold bluing is, in fact, iron oxide, same as the stuff created from hot salts (hot bluing). My guess is based on my observation of what I perceived as rust forming during the process.

I suspect the only reason industry doesn't use the cold bluing process is cost. I'm guessing the chemicals involved are quite a bit more expensive.

It would be great to hear what a chemist says.

But everyone is welcome to comment (obviously).

[Phil Thien]

It might be worth your time to investigate "Rust Bluing", For me this is the best looking bluing and from what I understand, is the most durable. Good luck.
Mac

I've done it.

This is a process by which you use an agent to rust the steel, and then you boil the rusted object in water.

It works, too. But I'm not convinced the later of rust left behind is any different than what is achieved by the other methods.

I'm beginning to think that they are all just different means to the same end. That is, the black oxide that is created is the same black oxide whether one uses the hot sales, the cold bluing, or the rust bluing.

But I may be nuts, and that is why I thought I'd ask.

[phil harold]

durability I think depends on how often you wipe them down with oil
Maintenance is what keeps tools in working order

I cold blue job site chisels...

[Mac McQuinn]

Phil,
Did you "Card" the material when doing the Rust Bluing?

Typically White Vinegar will remove most factory bluing, I wonder about Rust Bluing as it's involves a bit of a mechanical process also. Like anything else, preparation is everything. All the Rust Bluing jobs I've seen were much darker than Salts or liquid bluing and seemed to have a integrated appearance to the coloring.
Mac


I've done it.

This is a process by which you use an agent to rust the steel, and then you boil the rusted object in water.

It works, too. But I'm not convinced the later of rust left behind is any different than what is achieved by the other methods.

I'm beginning to think that they are all just different means to the same end. That is, the black oxide that is created is the same black oxide whether one uses the hot sales, the cold bluing, or the rust bluing.

But I may be nuts, and that is why I thought I'd ask.

[Phil Thien]

Phil,
Did you "Card" the material when doing the Rust Bluing?

Typically White Vinegar will remove most factory bluing, I wonder about Rust Bluing as it's involves a bit of a mechanical process also. Like anything else, preparation is everything. All the Rust Bluing jobs I've seen were much darker than Salts or liquid bluing and seemed to have a integrated appearance to the coloring.
Mac

I think the darkness of the job has a lot to do with the finish before treatment, and how thick the coating is. Highly polished steel tends to take a lighter, bluer finish, whereas less polished steel tends to get that very deep black appearance.

I think I'll do some samples each way and try to compare the durability.

[Mac McQuinn]

I would tend to agree although I have seen vintage, very expensive, collector grade shotguns which I believe to have the best metal preparation of their day and with Rust Bluing, appear very deep in color with no imperfections showing through the finish/coating.

That said I've also seen liquid applied bluing over very nicely finished barrels and it comes out very light, it seems the rougher the finish, the darker the results with liquid bluing. It seems this process requires a bit of "bite" to attach itself to and darken with increased film thickness.

I can't remark on Salts bluing either way, Your tests should prove interesting.
Mac


I think the darkness of the job has a lot to do with the finish before treatment, and how thick the coating is. Highly polished steel tends to take a lighter, bluer finish, whereas less polished steel tends to get that very deep black appearance.

I think I'll do some samples each way and try to compare the durability.

[Phil Thien]

I would tend to agree although I have seen vintage, very expensive, collector grade shotguns which I believe to have the best metal preparation of their day and with Rust Bluing, appear very deep in color with no imperfections showing through the finish/coating.

That said I've also seen liquid applied bluing over very nicely finished barrels and it comes out very light, it seems the rougher the finish, the darker the results with liquid bluing. It seems this process requires a bit of "bite" to attach itself to and darken with increased film thickness.

I can't remark on Salts bluing either way, Your tests should prove interesting.
Mac

I should also just add that I know the hardness and composition of the steel will also dictate how the black oxide finish appears, as well. Really hard stuff (like high speed steel) doesn't go very black, I don't think.

[Jerry Bruette]

I would tend to agree although I have seen vintage, very expensive, collector grade shotguns which I believe to have the best metal preparation of their day and with Rust Bluing, appear very deep in color with no imperfections showing through the finish/coating.

That said I've also seen liquid applied bluing over very nicely finished barrels and it comes out very light, it seems the rougher the finish, the darker the results with liquid bluing. It seems this process requires a bit of "bite" to attach itself to and darken with increased film thickness.

I can't remark on Salts bluing either way, Your tests should prove interesting.
Mac

The older vintage, collector grade double barrel shotguns were rust blued because the temperatures required for hot bluing would have melted the solder holding the barrels together. That would make for some very disgruntled customers.

The rust bluing precess is very time consuming. The barrelled action is hung in a box with a device which heats a liquid and a very thin coating of rust is formed on the metal after which is carded off using a very fine wire wheel. This process is repeated several times to obtain the level of bluing desired.

[Matt Marsh]

http://www.ahlmans.com/metalfinishing.html

These guys are the best in the business. There is a good explaination of their process at that link. I bet if you called them, they'd be happy to explain how their process compairs to the cold bluing process.

[Phil Thien]

Well I did a little experiment this even, and I've concluded that cold blue pretty much isn't iron-oxide based. It IS some sort of plating, something that probably deposits copper and turns that black.

My reason for say this is, it doesn't seem to come off in Evapo-Rust. Evapo-Rust DOES remove true black oxide (or red oxide), but it doesn't seem to go after the cold blue plating in the same way.

[Phil Thien]

Well I did a little experiment this even, and I've concluded that cold blue pretty much isn't iron-oxide based. It IS some sort of plating, something that probably deposits copper and turns that black.

My reason for say this is, it doesn't seem to come off in Evapo-Rust. Evapo-Rust DOES remove true black oxide (or red oxide), but it doesn't seem to go after the cold blue plating in the same way.

I took the part that had been cold-blued out of the Evapo-rust and scrubbed it a little w/ a nylon pad and was down to bare metal in little time. There is something like a black dye on the surface, but the Evapo-Rust seems to get under it and a little scrubbing is all that is necessary.

OTOH, I also took a piece that had been cold-blued and hit that w/ a nylon pad w/o first putting it in the Evapo-Rust. That came off pretty quick, too, LOL. Not as fast as if it has sat in the Evapo-Rust, though.

The entire thing is kind of interesting to me.

For my current projects, I decided to take the stuff to a place I've used before that does hot bluing. I still plan on perfecting a method of doing this work at home. I've previously done my own small pieces using boiling water on the stove, and using hydrogen peroxide to get the red rust to form. Basically, you clean the part, boil it in the water, dip in the peroxide, let sit in the air until rust forms, boil until the rust turns black, and start all over. I've got some pics I'll post later. I just need to try this on larger pieces is all.

Oh and I read the lye works pretty well to get rust forming. But the idea is to do this with materials that are fairly safe.

[Larry Frank]

As a metallurgist, I knew very little about this process. I had heard about it but never really thought about it. For others who would like to learn more about it, the description on Wikipedia on Bluing is very good. Basically the process is to produce Fe3O4 on the surface of the steel instead of the standard rust Fe2O3.

As some have mentioned, the composition of the steel will make a difference just as the different grades of steel show different rates of rusting. This would also apply to the surface finish of the metal.

[Phil Thien]

Apparently using a mildly acidic liquid with some copper dissolved in it (maybe just use copper sulfate, which is used in products intended to kill tree roots in sewers) will produce red rust quickly, and the red rust can be converted to black rust by boiling in water.

I might try that this weekend.

I'd previously used hydrogen peroxide and it took a long time to get sufficient rust.

I did find a picture of some previously blackened parts I did this way (with the hydrogen peroxide). I can't believe this was back in 2008 already. My gosh I'm getting old.