hot black oxidation

I made a couple of clamps and gave it a black oxidized finish, similar to those made on firearms.

https://youtu.be/AQegF40xbWU

I hope you enjoy the video

Greetings from Brazil

Celso Ari

Very nice! How critical is the temperature?

These were done with Blackit

@Sonic: The typical range is between 135-145°C (275-295F). Above that you start getting a red "rusty" oxide, below that the coating is thin or non-existent. The rusty coating can often be rubbed clean with oil, but it usually leaves a matte finish and messes up the bath.

For simple batch processing you run the bath at very slow boil, this gives a very constant (but rising temperature). Start with a slight excess of water, and run the bath until you're done or it gets to warm.

Adding water to a hot bath is very dangerous. The heat causes the water to flash boil, spraying hot salt everywhere. If you must do so, wear a face mask and try to add it very gentle, for instance with a ladle held at the bath level. And don't blame me if you get burned.

@Fulmen,

Exactly, your explanation was perfect.

You can't just add water to a simple hot black bath. It requires sodium Hydroxide as well to get the boiling point higher. The water level can drop, but the temp needs to stay in the range for that material. Re heating a pot needs to be done slowly and the whole process takes a lot of care. I did hot black for a few years with no real issues. Made black mostly A2 tool steel parts. I used an electric top initially, then went to small gas burner. The chemicals only last so long and then it needs dumping and starting again. Fortunately, at the time, they needed a strong alkaline mix to neutralize and acid process, so instead of buying sodium hydroxide to essentially tip down the drain, they used the old blackening mix. It worked well for the time. Now they don't do that process at all.

For the most part, the cold black is good enough and some of the newer etch paints are really good too. Recently learnt that letting the bits rust, then treating with a rust converter is what some are doing, and so far it seems to work well.

The boiling point of the salt bath discussed in this thread rises as water is boiled off, hence the bath will get hotter with time. Adding water is dangerous, because the water can/will boil explosively. However, significant water loss shouldn't be a problem, unless you're doing a long run of components.

The idea of treating a rusted surface with 'converters' may work OK. As far as I know, these converters (mostly?) contain tannates, which 'convert' the red rust oxide to blue-black tannates. But why bother with the expense? If you lightly wire brush the rusty surface, to get all the friable, loose surface off, and then boil the part in water, the red/brown oxide will convert to black oxide(s). Re-doing the rust, scratch, boil cycle a number of times, until the colour is satisfactory, is the centuries-old, traditional gunsmith's colouring technique. Oil, and perhaps wax, afterwards, to inhibit further (red) rusting. However, it's a slow job... The rusting can be speeded up by suspending the parts over a saucer of strong acid, in an improvised polythene tent.

Can any passing chemist please explain why acid 'fumes' cause oxide formation, rather than acid salt formation, as the liquid acid would?