Oil Blacking

Just reading through Hopper’s thread on the lever tailstock build and noted that his method for oil blacking was to heat to red and dunk. I’ve always just gone up to tempering colours stopping at a deep blue and then dunked in clean oil which gives me a nice uniform black. Admittedly my parts are usually for clocks so are not subjected to wear or any sort of rough treatment. Have I missed something and does going to red heat give a more hard wearing surface colour and protection.?
I would be very interested in the comments of the rest of the clan.

regards Martin

Going to red heat produces the black iron oxide layer and quenching in oil fills this porous layer with carbon. Like blueing you should keep the finished surface oiled. I think it is called blacksmithing blacking. Many will go on about the oil having to be old whale oil etc.

I have used this method to get a good hard black finish on small steel parts down to 12 BA nuts and screws. The heat source was a domestic gas hob and the oil clean ISO32 from the lathe. The performance of the nuts and screws was not compromised.

Stopping at the temper colours have always given me the nice deep blue finish loved by clockmakers.

Happy New Year to one and all

JA

I have oil blacked many steel things. I heat the object till it is not enough to start the oil burning and keep hot till it stops then repeat until it looks black all over. This is not recomended if the heat is going to affect any heat treatment.

David

Edited By David George 1 on 30/12/2022 13:24:29

Well, heating up to tempering blue leaves a thin oxide layer that's mildly protective. Then the oil dip improves protection by depositing oil deep into the pores and microcracks, and I think it also brings out colour in the same way as clear varnish does to wood. A more advanced process increases layer thickness by baking the item in an oxidizer like Potassium Nitrate. Guns and decorative steel work are often elaborately blued or browned.

Dipping whilst red hot causes more dramatic chemical changes, releasing free carbon black inside pores and on the surface, where it attaches fairly firmly. Done quickly there is no oxide layer. I prefer red hot dipping because it needs less skill. With my primitive gear, it's hard to create an even blue oxide colour over an entire surface.

Neither method provides first class anti-corrosion protection like galvanising, but both help, can be done without fuss and look attractive. Full bluing is particularly attractive, but requires chemicals and is more complicated to do.

Dave

Edited By SillyOldDuffer on 30/12/2022 13:30:35

How is it protective without the black oxide layer? (Magnetite).

Lead held just on melting-point is at about blue-purple colour. I have used it for tempering small springs but it might also work for protective / decorative bluing.

Note that the steel, polished and de-greased first, does not change colour while submerged in the molten lead, only when raised into the air at that temperature.

Well that would make sense as it can only oxidise when exposed to atmospheric oxygen.

So as far as I can gather the oxidation by heating produces Fe3O4 Magnetite in a thin layer at tempering temperatures where the colour can be controlled by the layer thickness and halted by quenching (in oil) and somewhat thicker layers when taken to red heat where carbon from the oil adds to the colour.

If you keep the steel at red heat long you will get a thick black oxide layer. If you ever have the chance to visit a steel rolling mill you find thick bits of this black oxide layer all over the place. Once it was valuable for use in puddling furnaces.

JA

Dave refers to chemical blacking. Hot chemical blacking of steel usually involves a Caustic Soda solution at around 150C. This is very dangerous!

Edited By JA on 30/12/2022 17:11:09

FWIW ~ "Gun Blue" will produce a decent cold blackening on steel providing the surface is chemically clean first. I use it when I want to blacken m/s screws which need to be given thread-lock before insertion or parts I can't risk distorting with heat.
I found oil-blackening leaves a surface coating of oil embedded in the oxide layer which adversely affects thread-lock agents.
After the Gun Blue screws have been water-washed, then given the thread-lock & inserted, a rub over the exposed bolt head with black shoe polish maintains the black & stops (delays?) surface rust formation.

I've got a recipe using caustic soda, potassium nitrate and potassium nitrite to be used boiling at somewhere over 150C. I've never been brave enough to try it.

Well this is producing some interesting information, maybe we should cobble together a list of recipes/techniques from the simple to the downright dangerous.

regards Martin

Re. Martin K's posting above..............
As so often Wikipedia provides a succinct summary & bit of interesting history on the subject.
Wikipedia

DON'T.

I made enquiries some years ago to some companies that offered the process. Not one was interested. I don't think this was because I was a model engineer, more likely they DID NOT want to do it (not keeping websites up to date).

If anyone really wants some blacking done by this process I have since been told of a firm in east Bristol that is willing and does a good job.

JA

Hi again Martin. I have run a commercial blacking unit and it is a bit dangerous. Firstly you have to wear a full face helmet, a thick waterproof full apron, boots with protection that comes above bottom of apron and long gloves which are caustic proof. The caustic soda was heated by gas flame temperature controlled under a ceramic heat proof container it was about a meter by a meter and a half by half a meter deep. The top edge of the tank had grooves to hold the suspension bars which hold the components on wire loops and hooks. A frame lifts the bars and runs on a track above the tanks and lowers with a pully into each tank in order. The second tank contains water which dilutes and washes the caustic soda off which is checked and treated for caustic levels. The third tank contains lanolin oil ( from sheep fleece ) which drips back into tank when removed from tank. The lanolin prevents any corosion from water. It looked ike a bench for loading frames, followed by caustic tank, followed by lanolin tank followed by cleaning water tank and last bench with slope and drainage grooves back to lanolin tank.

David

Re the oil blacking in my lever tailstock thread, I did a few trials on the bits for the lever tailstock and before that on some other projects I made. I found that heating the bits up to lower temps, blue or even up to grey etc work ok-ish but the layer of black was a bit thin, inconsistent and tended to rub off with use.

On the lever tailstock bits, I heated them until they just started to go a slight dull red, then held them there for a bit to let the heat soak right through the job, then plunged them into the clean motor oil, jiggling them up and down as I moved them around in a circle so that pockets of oil vapour did not form and surround the job. I reckon that by the time I moved the job from the bench with the torch and hearth on it to the next bench with the oil on it, the job had pretty much lost its red colour so it does not seem critical. I did leave the job lying in the oil until it had completely cooled off. Then wiped them off and sprayed down with WD40 and let it soak in. This last seemed to help generate a nice shiny black finish.

The result of this was that seven months later the lever tailstock black bits still look great and have proven quite durable in finish.

I have also done the small sensitive drilling attachment this way and it looks very good too.

Obviously, if the job were made from silver steel or other higher carbon steel where red heat would affect temper etc I would use lower temps for blacking. But for mild steel this works well for me.

Edited By Hopper on 31/12/2022 02:58:20

My recipe is to use engine oil mixed with lamp black.

I did this as I'd just chucked away some used engine oil that I would have ordinarily used and wanted to make some fresh oil 'dirty'. Lamp Black is just the carbon deposits from burning oil or creosote so a pretty close approximation to the carbonisation of used engine oil.

I have been meaning to try used engine oil, as I usually have some kicking around waiting for disposal. But it seems every time I want to do oil blacking, I have just disposed of it! The fresh clean stuff seems to work quite well with enough heat, so very interested to see if old dirty oil full of vintage motorbike carbon makes any difference.

I used to save my old engine oil for my father to take to work at Singers sewing machines for oil blacking sewing machine parts they made. He was a tool room fitter at Kershaws in Leeds and worked on cameras for them, then marking out Centurion tank turrets at Rof Barnbow before sewing machines.

Never encouraged me with metalworking, I was an electrician, but I guess it’s in the genes.