Blueing salts

[Eric The Viking]

Looking at a chemical datasheet, it seems like blueing salts are potassium nitrate.

I also understand the process (for doing clock hands, at least), but I wondered if anyone knows about what the chemical reaction is, and at what temperature it occurs?

Thanks for any info - trying to do the recessed area of a casting, if I can, but it may be impractical.

E.

 

[AES]

No idea, sorry EtV. Suggest you try searching at the Model Engineer/Model Engineer Workshop web site (sticky at the top of this section) or data such as Model Engineer Handbook if you have/can borrow a copy, 'cos that's full of "miscellaneous" data.

Otherwise, any clock making or gun making sites (both do bluing).

I've just double checked my Zeus data tables, but as expected, it isn't there.

AES

 

[monkeybiter]

I don't know if this is of any use, interesting though,https://youtu.be/qM_Z1Tax2k0
I've previously used bluing paste and solution successfully at room temps.
https://www.google.co.uk/search?saf......0j0i22i30k1j33i21k1j33i160k1.0.RSns_IX67VI

 

[novocaine]

https://www.youtube.com/watch?v=NhjiIPohUyw

 

[Brandlin]

Blueing is a process of oxidising the outer layer of the steel, to Fe3O4 to prevent it from forming rust Fe2O3

You need around 150C (a quick google suggest 135-155C)

the depth and consistency of the colour is determined by the availability of the Potassium Nitrate but more critically by the consistency of the temperature.

I've blue'd small things but never part of a larger thing like a casting. and it's been many many years. I suspect that heat disipation is going to be your biggest challenge in blueing part of a casting. I figure you'll need to heat the whole casting and accurately apply the Potassium Nitrate as a mask? In my limited experience its not a precision process.

 

[Eric The Viking]

Blueing is a process of oxidising the outer layer of the steel, to Fe3O4 to prevent it from forming rust Fe2O3

You need around 150C (a quick google suggest 135-155C)

the depth and consistency of the colour is determined by the availability of the Potassium Nitrate but more critically by the consistency of the temperature.

I've blue'd small things but never part of a larger thing like a casting. and it's been many many years. I suspect that heat disipation is going to be your biggest challenge in blueing part of a casting. I figure you'll need to heat the whole casting and accurately apply the Potassium Nitrate as a mask? In my limited experience its not a precision process.

Thank you - that is very helpful.

It's a #73 shoulder plane. I bought one in need of a lot of resto and cleaned it up, but that was largely polishing rust off. It's on its way to Steve Maskery at the moment, who picks up the baton from me of completing it. The work needed on that one is to do with the mechanics rather than appearance though (up to him if he wants to blue bits).

I've since been given a much nicer #73 as a delightful and unexpected Christmas present. It too needs a little TLC (but hardly anything). But I was surprised to find that on the second one, the two adjustment wheels, the iron extension, and the one that causes the handle to clamp down, and the penny dropped about the recesses in the casting: they were originally blued too (well, black really).

I'm fairly certain that copies of Preston/Record original shoulder plane designs by modern makers have those recesses blued (well, chemically blackened somehow).

So the target bits are small-ish recesses that can be kept horizontal, and the temperature is achievable in the domestic oven (on a day when "nobody's looking", that is).

Many, many years ago, as a teenager I worked in a light engineering factory as a holiday job. Our toolmaker used to blue things in an electric oven, so I think it can be done fairly easily.

Your info was very helpful.

E.

 

[Sheffield Tony]

I don't believe that temperature. Both to melt potassium nitrate, and to get a blue oxide on steel requires more like 300-350C, so more than an oven.

I'd be very careful with molten potassium nitrate. It is a strong oxidiser, and any organic matter, oil, etc will cause a vicious conflagration !

I like Clickspring's way of doing it. Wish I had his skills.

 

[Eric The Viking]

If it's that high, the temp wouldn't be good for the casting, either - tricky to both heat up and cool slowly afterwards, never mind the potassium nitrate - it doesn't sound like fun.

I agree about Clickspring - found that doing earlier research on this.

I think my toolmaker colleague years back used an oil quench rather than creating an oxide layer, which might explain why he could achieve it using an oven. They were small parts too, and steel.

 

[sunnybob]

There must be a gun shop still in bristol, what about newavon arms?

If all else fails

https://www.amazon.co.uk/Birchwood-Case ... lueing+kit

 

[Brandlin]

I don't believe that temperature. Both to melt potassium nitrate, and to get a blue oxide on steel requires more like 300-350C, so more than an oven.

I'd be very careful with molten potassium nitrate. It is a strong oxidiser, and any organic matter, oil, etc will cause a vicious conflagration !

I like Clickspring's way of doing it. Wish I had his skills.

The melting point for KNO3 is around 334C. I understand that clickspring does this with the salts applied neat. However the more common process (especially for DIY) is to suspend the parts in a solution of Potassium Nitrate and the heat is a catalyst for the reaction. 150C is adequate.

Its not hard to find the information. https://en.wikipedia.org/wiki/Bluing_(steel)

I agree entirely with the warning of using neat molten salts. You need to be rigorous about preparation and cleanliness of equipment.

 

[Keith 66]

Bluing steel can be done several ways. Temper colouring, which is useful for small parts. Modern production firearms are blued with a caustic (alkali solution) used at high temperature. Then there is the old fashioned Browning & bluing which is an older but similar process.
Browning is a controlled slow rusting process where the degreased steel is wiped over with a horribly poisonous concoction usually containing mercuric chloride or ferric chloride & often other chemicals too. The item is left for a few hours then carded (the loose rust is brushed away with a soft wire brush) The process is repeated to deepen the colour. This is a room temperature process & will produce a uniform brown colour.
The rust coating is basically ferrous oxide. If you boil the item between passes the oxide layer is converted to ferric oxide which is blue!
A slow rust blue is used on high quality shotguns where the barrels are often soft soldered together. It also is more lustrous & durable.
The last process is a cold bluing cream that can be bought from gunshops. Its ok for small parts but tends to be streaky if used on large items. It is not very durable.

 

[Eric The Viking]

I thought you could oil-quench too, to achieve some sort of blueing. I appreciate that might give only the colour but not any rust-resistance, but I was wondering if it was possible.

As I said, I thought that was what our toolmaker used to do (no salts involved). Do I remember it incorrectly?

 

[novocaine]

I thought you could oil-quench too, to achieve some sort of blueing. I appreciate that might give only the colour but not any rust-resistance, but I was wondering if it was possible.

As I said, I thought that was what our toolmaker used to do (no salts involved). Do I remember it incorrectly?

thats oil blueing/blacking or carborising amongst many of it's names and it's something I do all the time on smaller parts, it forms magnatite on the surface. it's not great for rust prevention but better than nothing, I'll usually go through the motions about 4-6 times to get something fairly dark. Not a chance I'd do it on a casting though, to many cycles of heat to cold. works best with old engine oil but I've found that 2 stroke works ok too.

tempering is what clickspring is doing, no salts added.

 

[Eric The Viking]

This has been really informative and helpful - thanks everyone.

I'm probably going to pass, and not attempt to "blue" the casting areas, but there are a couple of thumbwheels that that seem to have been oil-blued, and I might freshen those up a bit (have some 2-stroke ).

 

[Sideways]

I'm reminded of one detail from reading up about parkerising - another protective finish - it's not the chemical conversion alone that provides the protection from rust, it's the oil / grease that is then appllied over the top and is absorbed into the finish layer.
Routinely wiping tools over with a rag I keep dampened in camelia oil seems to help me without being too offensive in a wood shop. I would recommend something like this whatever finish you choose.

 

[Eric The Viking]

AndyT kindly donated some Camelia oil last time he was here - it's poised and ready

 

[AndyT]

AndyT kindly donated some Camelia oil last time he was here - it's poised and ready

No, that was some Shield brand rust preventer - far more expensive than your lowly flower oil!

 

[Eric The Viking]

Oops!

I've been treating it very reverentially though!