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oil blackening

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Garry Coles29/04/2021 20:17:34
109 forum posts
100 photos

Hi, I'm at the cleaning, priming and painting stage now on my D & NY traction engine, and I would like to oil blacken all the cogs & gears but not sure about the heating temp to be used on cast iron. Will this be okay to do, and what sort of oil is used. I could just paint them matt black but was wondering about oil blackening. Any suggestions please.


bernard towers29/04/2021 22:12:35
687 forum posts
141 photos

+ 1 for chemical blacking

Emgee29/04/2021 22:15:43
2444 forum posts
291 photos


See this previous thread:

Blackening steel - Caswell Black-Ox kit



Edited By Emgee on 29/04/2021 22:16:01

Edited By Emgee on 29/04/2021 22:19:16

oldvelo29/04/2021 22:57:27
295 forum posts
56 photos

Wash clean the parts in petrol or Meths to remove any oil. Work in low light or deep shade.

Heat with LPG torch to dull red and drop into a container used used diesel engine oil, Metal preferably one with a lid.

Keep the torch well away from the gas cloud that forms it is highly flamable.

Be prepared for fire fighting if it catches fire from the hot metal by putting on the lid.

Works well on most steels and cast iron.


noel shelley29/04/2021 22:58:49
1436 forum posts
23 photos

Will it work on cast iron ? Noel

duncan webster29/04/2021 23:39:26
4107 forum posts
66 photos

I have a recipe somewhere using boiling caustic soda/sodium nitrate solution which all sounds very dangerous. If anyone wants to try it I will try to find it

Thor 🇳🇴30/04/2021 05:55:32
1658 forum posts
46 photos

Hi Garry,

I haven't tried oil blackening on Cast Iron, only small steel parts. I heat the work to about 270 deg.C to 300 and then apply linseed oil oil (burn the rag afterwards). You may need to reheat and apply oil more than once. Find a piece of scrap Cast Iron and try to get some experience and see if it gives the result you want.


Ray Lyons30/04/2021 07:07:25
200 forum posts
1 photos

I have a tin of used engine oil which works well. Heat the metal to just below red and then dip in using a wire. Sometimes takes a couple of dips to get the right colour and it lasts for ages without further treatment

Garry Coles30/04/2021 07:21:14
109 forum posts
100 photos

Thanks everyone for the useful info.

DiogenesII30/04/2021 07:50:07
587 forum posts
230 photos
Posted by noel shelley on 29/04/2021 22:58:49:

Will it work on cast iron ? Noel

I wondered the same thing..?

For steel, I'm not too keen on used engine oil, I find that organic (vegetable) oils give a noticeably 'blacker' and more even finish - linseed gives very good results..

noel shelley30/04/2021 09:36:01
1436 forum posts
23 photos

If machined to size and heated to red there may well a degree of stress relief - or distortion to contend with. Bear this in mind, it may look good but will it still fit ? Noel

Douglas Johnston30/04/2021 11:31:31
773 forum posts
34 photos

I think it is unwise to use engine oil that has been in an engine. I'm sure I read somewhere that there is a cancer risk with that approach. I use cheap supermarket rape seed oil and that works well for me.


Rod Renshaw30/04/2021 12:57:02
376 forum posts
2 photos

Oil blackening is a traditional process. It's in all the old books - "heat to red or nearly so and chuck it in old, used oil, the older the better." Lots of fumes so do it outside. If the parts are small and there is plenty of oil it won't (usually) catch fire, but be prepared that it might.

But times change, engine oils and petrol now contain additives, who knows what the fumes may contain, we are more aware of the risks, best avoided.

Vegetable oil works okay and won't be as potentially toxic, and many of us still harden/ toughen tooling in oil, so use that, but still do it outside - and the fire hazard is still there unless the parts are very small.

As has been said, any heating process may cause distortion.

Chemical blackening works, some makes are more durable than others. Some will only work on mild steel and not stainless, so you may need to check the one you choose will work on cast iron.

Any of these processes, heat or chemical, needs very good cleaning/ degreasing etc to get an even finish.


Martyn Edwards 230/04/2021 17:06:28
18 forum posts
34 photos

Back in the day it was Whale Oil.......Oh my!

Swarf, Mostly!30/04/2021 19:16:38
679 forum posts
78 photos

Hi there, all,

What if the item to be blacked is an assembly of three parts, silver soldered together?

Best regards,

Swarf, Mostly!

Pete.30/04/2021 19:25:50
815 forum posts
245 photos

Good cold black kits have 4 parts, a cleaner, an acidic etch, the black, the oil, they work very well on cast iron, providing it doesn't have decades of oil and grime worked into the surface.

The daisy wheel on this press I restored is cast iron, I skimmed 0.3mm off the face to assure a clean surface, all the black parts on this were cold blackened.

J&s press 1

Emgee30/04/2021 20:19:01
2444 forum posts
291 photos
Posted by Martyn Edwards 2 on 30/04/2021 17:06:28:

Back in the day it was Whale Oil.......Oh my!

I have some that was came with some tools including a kiln I bought from a retired toolmaker, I still use it for quenching gauge plate and silver steel tooling, perhaps the ss doesn't get as hard as water quenching but doesn't need tempering. Does leave a nice even black finish as in tools below.



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