Oil Blackening 12L14

There is such a lot of stuff to know about having the metal arrive at a colour, but not so much about the actual properties. "Cold blue" solution as favoured by Quinn Dunkie (Blondihacks), for example. Is it all simply decorative?

As I understand it, blueing and blackening processes that involve generating a magnetite layer on steel are the ones that provide corrosion protection, basically because the oxidation layer reaches a thickness where further rust-type oxidation becomes self-limiting. The layer will absorb oil, so oiling afterwards gives considerable protection.

Pretty colours seen during heat processes, that straw colour indicating a tempering state, the blues and violets, are all about the thickness of oxide layers so thin, they are transparent, but produce the colours in the same way as happens in soap bubbles. This is by wavelength interference of incoming light with in-layer reflected light. The stainless steel flue over my woodburner has a great display of repeated rainbow spectrum colour rings which travel upwards every year, to re-start the rainbow sequence, as the oxide layer thickens from heat through each successive quarter-wavelength.

I don't know the detail on what mechanism traps black when quenching in oil, but I am not thinking it's carbon. It seems to be a different sort of oxidation happening on the surface while heating before quenching, and we can't see it because it is glowing red.

On steel, interference type colours on surface oxide are too thin have much anti-rust properties. All the recipes I see for "gun blueing" and the like do seem to depend on getting up an oiled layer of something durable. Whether it was applied from someone's proprietary paste, or by boiling in alkali after quenching, the aim seems to be getting up a nice colour, blue or black, and rubbing oil into it.

[Edit: Hmm - good ole' Wikipedia says quite a lot! --> https://en.wikipedia.org/wiki/Bluing_(steel) ]
 
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Since bluing is a surface oxidation related thing, I started wondering about using something that's the reverse of electrolysis-based rust removal. A little online searching found this. There was some discussion about adding iron filings to the solution but I don't that will make a bit of difference.

It looks to me like the downside is the formation of red-colored rust on the surface that has to be removed to reveal the black oxide. The heat-based approach wouldn't require that.
 
At 600F, lead is all liquid; at 1200F its all vapor. The blue temper color starts to fade to bright at 600F. That's right on the edge for losing lead to the atmosphere. I'd just do the work outdoors in the open air and not heat past peak blue.
 
Nitre blueing (heated salts and oil) (the only process that gives a real blue color in small thin parts) and the quick bluing aka cold bluing/tool blackening provide little rust protection and is more or appearances only.

Hot bluing provides some protection.

Rust bluing (which is a really deep black) and parkerizing provides significant rust protection as structure retains a significant amount of oil to do so.

Rust bluing is not hard to do at home, just takes time.
 
Ferric chloride, the same stuff electronics hobbyists use to etch printed circuit boards, is reputed to quickly blacken steel. Some add vinegar to it to stretch the FeCl3. It looks like it's used at room temperature.

I know from personal experience that it stains hands and clothes . I'v only used it to etch copper so I can't vouch for the results on steel.

It will eat through most any kind of metal container so you'd want to store it in plastic, and use a plastic container when bluing/blackening with it. Don't dump used solution down the drain, it will eat copper pipes in addition to circuit boards.

I may have some on a shelf somewhere. If I find it I'll have to try this.
 
FeCl3 (traditionally known as vitriol) will blacken iron if it's not bound by an alloy matrix. Alloys with lots of chromium/vanadium won't get black, but good ol' high carbon steel will. Vitriol is a tougher black than selenide, that's for sure.
 
Haven't tried it on 12L14, but rust (cold) bluing 1018 with peroxide, salt and boiling water is pretty easy. Made a post on it. I blued the tee-nuts for my rotary table. Had I done a better job on carding, they probably would have looked nicer, but they are fine for my intended use. If you want the bluing to look nicer, then you have to put more work into it, like most things. Didn't take very long to do, maybe an hour at most. Next time would take half that time, since I wouldn't be as hesitant as I was the first time I tried bluing.
 
I'm a total neophyte at bluing/blackening steel. A friend, a gun person, suggested Birchwood's Super Blue. Directions (condensed) are polish the steel, apply this stuff, several coats, rinse, scarify (steel wool or scotchbright) between coats. Rinse and repeat.

Edit: Steel used, 12L14.

I put on four coats, overnight it had a slight rust color to it. I put another three or four coats on it, let it dry thoroughly and oiled it. Today it has a dusty dark brown look to it. (rusty). Did I do it wrong? I was hoping for blue, none showed up, just flat black, now this. It's for my new lathe in the Netherlands, to mound the four jaw chuck on. It won't matter, but I probably won't use it again...
 
T Bredehoft said:
I'm a total neophyte at bluing/blackening steel. A friend, a gun person, suggested Birchwood's Super Blue. Directions (condensed) are polish the steel, apply this stuff, several coats, rinse, scarify (steel wool or scotchbright) between coats. Rinse and repeat.

Edit: Steel used, 12L14.

I put on four coats, overnight it had a slight rust color to it. I put another three or four coats on it, let it dry thoroughly and oiled it. Today it has a dusty dark brown look to it. (rusty). Did I do it wrong? I was hoping for blue, none showed up, just flat black, now this. It's for my new lathe in the Netherlands, to mound the four jaw chuck on. It won't matter, but I probably won't use it again...
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I don’t think you suppose to let it dry and stay on the part. I use birch wood products and all the direction at wipe on, spread, and rinse. I’ve heated the part which it’s absorbs better I think but it will make the paste or liquid dry quicker. I just keep rubbing with more products and it evens out the color. I usually do 3-4 coats. Cleaning in between and degreasing.
As with any coating your surface finish has a lot to do with the end result.
 
Thanks, Cadillac, I've just retyped the instructions from the bottle, and now understand the procedure.

I left out some critical steps. I've printed the instructions so that i can read them without magnifiers, and will be able to do a better job next time.
 
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