Garry,

See this previous thread:

Blackening steel – Caswell Black-Ox kit

https://www.model-engineer.co.uk/forums/postings.asp?th=137955

Emgee

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

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

Will it work on cast iron ? Noel

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.

Thor

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

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.

Rod

Hi there, all,

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

Best regards,

Swarf, Mostly!

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.

Emgee