gun blue: food safe or not?

[emperorwang]

I'm going to the field for a couple weeks with my National Guard unit soon. I've decided I'll be taking a Mora 840MG with me into the field, carbon steel, but first I wish to force a patina on it, primarily to reduce reflectivity but also to increase corrosion resistance.

That being said, I'm curious about using gun blue on the blade. I have read directly contradictory claims about whether the knife will be food safe after bluing. Some say it won't be, others say it will. However, I'm unable to find any sources or links to substantiate either claim, and this is a concern as the knife will likely be used for food prep.

Any input would be much appreciated.

 

[Dewberry]

I would figure out what brand you want to use and call the manufacture, cause it might change based on who makes it.
just a thought

 

[Obsessed with Edges]

I've never used it, but all I've read suggests that so long as the steel is thoroughly cleaned after the bluing, it should be fine. Gun bluing works by oxidizing the surface of the steel with strong oxidizing chemicals. The blue/black oxide left behind is no different than any other 'patina', in the sense that it's permanently bonded to the steel and won't rub off without some abrasive action. But the chemicals themselves, if left on the steel, can be toxic and harmful to the steel as well, as they'll keep corroding it if not neutralized and cleaned up completely. I get the impression that if there's no chemical residue left to corrode the steel, there also won't be anything toxic left on it.

I'd take a very hard look at the manufacturer's instructions for cleaning up and neutralizing the chemicals, and follow them to the letter.

 

[eisman]

Blue the blade, and then boil the knife to remove any remaining salts (the blueing stuff). Boil it for a while. Then soak it in mineral oil for a couple hours. That should negate any of the poisons used to change the blade color.

Still, blueing is not a patina. Blueing wears off, a patina sinks in. I wouldn't blue anything I planned to prep food with, and never understood forcing a patina.

 

[Obsessed with Edges]

Seems to be a distinction too, between 'cold blue' products and a true 'hot' bluing treatment. Each use different chemicals and methods, and produce a different result.

The black iron oxide left by the 'true' (hot) bluing process is Fe[sub]3[/sub]O[sub]4[/sub], which happens to be the same thing (chemically) as the grey 'patina' formed on steel by other means. The bluing process just does it more aggressively, in shorter time. As with a natural patina, it affords some small protection against corrosion. It's true that it can be abraded/polished off, as with stropping a patinaed blade edge on leather. Being that it's the same thing, chemically, as a 'real' patina, I'd not worry about toxicity (assuming again, that the acids used to create the patina are fully cleaned off).

So-called 'cold bluing' products apply a coating of a chemically different compound (selenium dioxide), which rubs off more easily and is basically a cosmetic treatment only. No idea about the toxicity of the coating, if it rubs off into food or whatever. Look for 'selenium' or 'selenious acid' in the product's MSDS info. I wouldn't see much advantage in using this as a means of corrosion control; what I've read on it suggests it doesn't afford much protection.