Seem to remember there was a thread about this subject not too long ago. Hang in there,someone will be along soon.

One or more of the following may give some thoughts to start with and/or develop the discussion:

Help with cold steel blackening

Blackening mild steel

Blackening steel – Caswell Black-Ox kit

Am I blackening steel correctly?

Chemical blackening

Regards,

Phil

Edited By Weary on 28/11/2018 16:48:18

Same as Roderick, Blackgates or EKP supplies sell them,smallest kit under twenty quid. My kit still going strong after 5 years of regular use.

Much would depend on how you define "good" and "durable" for a finish – and how big the parts are.

As the link above to "Blackening mild steel" shows, many have found supermarket sunflower or rapeseed oil to be effective.
Heating to red and dropping in sunflower oil produces a nice black finish on steel every time for me. The oil protects the surface but. If you don't like the residue, clean off and apply black shoe polish to prevent rusting.

A cold method which is cheap and easy but very dependent on an excellent surface to begin with is "gun blue". (I've had variable results if the surface was not completely degreased first). It's quick to apply (wear disposable gloves) and wash off after. Do though coat with the aforementioned shoe polish or rusting occurs quickly. Put 'gun blue Bisley' into Google & you'll find YouTube vids on doing it and sellers. A small bottle from Bisley shop has lasted me for years.

+1 For gun blue, the results are instant, it can be re-applied for a deeper black and is a cold process. It can be “blochy” sometimes, but another application and a coat of oil usually fixes it. I use birchwood Casey “Super Blue” because its an extra strength concentrate solution that works significantly faster/better than the regular stuff. Its not suitable for stainless, aluminium or non ferous. It (according to the bottle) Works best on steel with a nickel or chrome alloys.
Ben.

Like I said,someone would be along soon !!!

Has anyone noticed the acronym of Frost Auto Restoration Techniques Ltd. ?

And yes that is their registered company name…

Neil

I agree that rapeseed oil should be a lot nicer than engine oil, but I wonder why people pay for blackening kits for mild steel?

Used cooking oil from the local chippy should be good as well.

Phil.

"Nearly as bad as the surname of the President of the USA, who for the benefit of non UK readers is a euphemism for flatulence. Seems appropriate somehow"
I mistyped his name recently and the spell checker came up with President Turnip, decided to leave it in as quite appropriate.

Not too sure I'd be trying ammonium nitrate, apart from being a reasonable fertiliser its a reasonable explosive.

Sound, published evidence that well-used, old engine oil is full of carcinogens.

I use the hot bath method. It runs at 135 to 140 C. So some care is needed. It works very well and I have never had any issues with it all. The downside is the time required to heat the bath, and the cleanliness of the parts. Some steels with high chrome content do not blacken very well with this process.

It is cheap to set up though and the basic formula is very simple. My current bath starts with 10 deionised water chlorine free. Chlorine free is important. 10 L of caustic soda pearl, or alternately 10 L of concentrated caustic soda solution, Mix water and pearl very slowly adding the pearl. Ready made solution does not need any extra pearl add. Then after it is close to room temp add 10 l of sodium Nitrite powder/crystals. Mix etc, use care as this is very caustic. Slowly heat to 135C or untill it just simmers. Stir often. The parts will take about 5mins for small sections (less than 25mm thick) to 15mins for 50mm sections) . Remove from the bath, let hang and drip off, then wash in hot water and I use vinegar water to neautralise the basic solution. Dry and oil. All done. This is really a more commercial set up, but does work very well. There are slight adjustments to the basic formula, like adding sodium Nitrate, and some add other stuff as well. If you have a set up in such a manner that you may drop and splash parts into the hot bath, then your set up is wrong. It needs to be arranged so that you can not make any splash of any kind , no matter the reason. You do not get a second chance with hot caustic things. But I am happy with with I have, have been since 09 when I started doing this commercially. Do not allow any non steel parts to contact the bath either.

Neil

The off the shelf blackening kits are ok for screws and threads thats about it. Goes on wafer thin and wont last 5 mins. its just meant as an immediate touch up.

The proper cold black you wont be able to obtain, its a closely guarded secret and would have to buy 20 gallon. Parts still need to be heated up to boiling and you paint the blacking on then wipe off building the layers up over 7 or 8 goes.

Hot blackings rather different around 150 – 155 degrees but no good for lead soldered parts. Easier to get hold of though would have to buy a bag of salts approxx £120 assuming they dont want to see your license.

Someone mentioned why buy cold black when oils free or words to that effect. Ones burnt on carbon the others chemical.

Other method to get colour in is just warm the part/s up from 230 to 285 degrees. OK for unhardened parts as it will do any tempering in ie whale oil burns around 150 degrees.

The practical difference between chemical blacking and oil blacking is that the chemical versions cause very little dimensional change.

Oil blacking is great if you don't need to be accurate to the last thou.

Neil