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Oil Blackening and other Oil Issues

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Chris TickTock09/01/2021 10:16:36
622 forum posts
46 photos

Hi Guys,

I recently made a steel friction pin for a clock. It would have been nice to have protected it from rust and even better to blacken it. The steel was hardened and could not be tempered.

I have heard that you can oil blacken could anyone clarify how and does this temper or not in the process.

Second question relating to oil quenching and hydrocarbons. Petroleum type oil gives off a lethal smoke when quenching metal in confined spaces but is rapeseed oil safe?

can anyone help with advice on these points?



SillyOldDuffer09/01/2021 10:56:54
8892 forum posts
1998 photos

On a small scale I wouldn't worry about lethal smoke when quenching in oil. The odd job done in a well ventilated space won't do any harm unless you have a pre-existing medical condition like Asthma. Fire is a much more likely hazard. Unpleasant fumes and the risk of a flash fire (when hot oil vaporises and then the whole cloud ignites) make it unwise to quench in a confined space. Avoid. The risk rockets with size; a red-hot hammer head is far more dangerous than a tiny clock part.

Not tried it, but blackening with oil requires the metal to be hot enough to carbonise the oil. That's enough to draw the temper, so best not to do it with parts that must be hard.

For the best blackening, dirty old engine oil is often recommended. It's partly disintegrated and already has free carbon in it. Nasty stuff, it's a known carcinogen, so don't spend a lot of time messing with it! Clean oil is less effective for blackening. I don't think Rapeseed Oil is any more or less dangerous than similar weight oils.

Light oils like petrol and paraffin should be avoided because of the fire and explosion hazard. Also specialist oils, most famously old formulation transformer oil. They contain additives that react badly to extreme heat; old transformer oil can generate Dioxins - very toxic.

Gunsmiths use a chemical blue, applied cold. Not as protective as thick heat blue, but considerably better than bare metal, and safe!

Another way it might be done is in a temperature controlled oven or with a torch and steady hand. At about 450° a thin blue oxide layer appears on steel. Difficult to get an even layer - the commercial process sprays steam on steel objects in a carefully controlled oven. Unfortunately the amount of heat needed is likely to reduce hardness.


Edited By SillyOldDuffer on 09/01/2021 10:57:36

Oily Rag09/01/2021 11:02:33
540 forum posts
184 photos

+1 for chemical Gun Bluing - imparts an oxide protective layer which will resist further corrosion.

Chris TickTock09/01/2021 11:11:28
622 forum posts
46 photos

Thanks for posts thus far. Anyone that can with the use of rapeseed oil as opposed to petrol type oil would be appreciated.


Iain Downs09/01/2021 11:17:44
860 forum posts
756 photos

For blueing check out various clips by Clickspring on youtube.


roy entwistle09/01/2021 11:25:15
1551 forum posts

Small clock parts can be blued by submerging them in a bed of fine brass filings in an old table spoon and heating from underneath.

For best results part should be clean and polished

Edited By roy entwistle on 09/01/2021 11:41:40

Ramon Wilson09/01/2021 11:32:07
1399 forum posts
447 photos

It won't matter what type of oil you chose, if you do not want to lose the hardness the part you have hardened then your only option is to chemically black it. The temperature required to black with oil will temper your part to a high degree - almost if not all the way, back to it's pre hardened state

Played around a lot with 'Cold Black' at work and home at times but that can rust if not liberally oiled, not ideal for a clock I would have thought - I believe I'm right in saying though it is not a rust 'preventative' as such

Though I have not used it Gun Bluing would appear to be your best bet from answers to similar questions seen over the years


Swarf, Mostly!09/01/2021 11:34:27
682 forum posts
78 photos

Hi there, all,

Some time ago I was given an assortment of (threading) taps. They were rusty so I treated them to a couple of days in an Evaporust bath.

After rinsing and drying, I was surprised to see that some of them were bright and shiny (presumably HSS) while the others had developed a blackish colour (presumably carbon steel).

In my opinion, the blackish colour would be an acceptable long-term finish for some components but I have not yet had the opportunity to test its durability.

Best regards,

Swarf, Mostly!

Emgee09/01/2021 11:39:10
2445 forum posts
291 photos

Many years ago I bought a 1100C Kiln and included with it some whale oil to quench parts which also gives a durable black finish. The other evening there was a part quenched in linseed oil on the Restoration TV program, looked like a good result.

tools below quenched in whale oil.



Dave Wootton09/01/2021 11:52:36
319 forum posts
69 photos

I use the Birchwood Casey gun blue a lot for small parts, and if parts are thoroughly degreased before application it lasts well and looks good. Not as hard wearing as proper chemical blacking but a lot less faff for parts not subjected to constant handling.

I apply it with very fine wire wool and treat it with ACF50 after application which seems to prevent corrosion.


Terry B09/01/2021 12:23:31
16 forum posts
5 photos

I use Mild Steel Blackener purchased from Although it cost approx £25 for a litre bottle it can be brushed on neat or soaked for 30 or so minutes in a 25% mixture and can be reused several times.

The steel needs to be completely free from oil, this can be achieved with any domestic degreasers rather than the stuff sold by the manufacturer and washed in water afterwards. I ignore the 3 or 4 stage process advocated and it still works OK

JA09/01/2021 12:25:35
1402 forum posts
81 photos

I have successfully oil blacked small steel screws and nuts down to the size of 10BA. Just heat to red heat and drop into clean ISO32 oil. After you have blacked them just keep the parts oiled. There is no magic, such as very old black horrible oil, in the process. The black is an iron oxide and carbon. If thick enough it would called mill scale.

Blacked small steel fixings


The iron oxide is porous and holds the oil and carbon from the quench.

Edited By JA on 09/01/2021 12:29:20

Thor 🇳🇴09/01/2021 13:11:42
1660 forum posts
46 photos

I use linseed oil to blacken small steel items. Heat the steel to about 280 to 300 deg.C and apply the oil with a small piece of rag. I burn the rag afterwards, it may self ignite.


Pete.09/01/2021 14:38:35
815 forum posts
245 photos

If you search for the thread I posted on a cold blackening kit, you'll find the info for a UK supplier of a 4 part kit, it worked very well, all the bare metal on this J&S press I restored were done with the kit.


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