Black Oxide Test

[MrWhoopee]

I am determined to put together a hot salts bluing setup. Haven't got there yet, because I have no gas burners on hand from other hobbies. Still looking for a free one.

Until that day comes, I have had by far the best results from cold selenium bluing, particularly with Oshpo blue. It is much tougher than the Birchwood Casey equivalent, and much darker. A little bit of heat (just warm to the touch) will kick any chemical process into high gear, so that's a given.

I recently saw somebody advertising special formulation hot oxide salts that required a lower temperature. A fairly large container (IIRC, 5 or 10lbs) was about $90. Of course I forgot to bookmark it. Will post if I find it again.

 

[pontiac428]

Hey John, I keep an induction hot plate in the shop for heating up degreasers for the ultrasonic cleaner and I plan to use it to heat up the hot salts bluing mixtures. The first time I ever used it I was stunned at how quickly it boiled water. Like under two minutes to boil a few quarts of water.
Mine has a handy temp setting also so if I need a specific temp I can dial it in.

KNO3 melts at 633F, and NaNO3 at 586F. Can you use induction heat with a stainless pot? WIll it reach temp? I think the only limit would be the wattage of the appliance. It takes 467 kj/mol to melt the crystal at 586F, or about 250 watts per ounce. If a half hour is a reasonable amount of time to wait, it'll take 8 kw/h per pound. Even if I'm off by a factor of 5, it looks like it will take more energy than a hot plate would put out.

 

[pontiac428]

I recently saw somebody advertising special formulation hot oxide salts that required a lower temperature. A fairly large container (IIRC, 5 or 10lbs) was about $90. Of course I forgot to bookmark it. Will post if I find it again.

Ammonium nitrate has a melt point somewhere around the mid 300's Fahrenheit. I normally don't see the cation as an issue, but ammonium is special. Ammonium can be corrosive, so keep an eye on things.

 

[DavidR8]

KNO3 melts at 633F, and NaNO3 at 586F. Can you use induction heat with a stainless pot? WIll it reach temp? I think the only limit would be the wattage of the appliance. It takes 467 kj/mol to melt the crystal at 586F, or about 250 watts per ounce. If a half hour is a reasonable amount of time to wait, it'll take 8 kw/h per pound. Even if I'm off by a factor of 5, it looks like it will take more energy than a hot plate would put out.

It will heat stainless steel.
This is what I have: https://www.nuwavenow.com/shop/pic-gold

 

[MrWhoopee]

KNO3 melts at 633F, and NaNO3 at 586F. Can you use induction heat with a stainless pot? WIll it reach temp? I think the only limit would be the wattage of the appliance. It takes 467 kj/mol to melt the crystal at 586F, or about 250 watts per ounce. If a half hour is a reasonable amount of time to wait, it'll take 8 kw/h per pound. Even if I'm off by a factor of 5, it looks like it will take more energy than a hot plate would put out.

We ran commercial hot oxide at my shop back when. The oxide tank ran about 285-300°F. It was a highly concentrated (720g/l) aqueous solution of a product called Nickel Pentrate, which was mostly lye with some nitrates, a little nickel and other magic stuff.

https://www.chingfordtec.co.uk/pdf/nickel%20pentrate.pdf

The lye is present mostly to raise the boiling point to the necessary temperature for formation of black oxide. It requires much less energy than a molten salt bath.

 

[pontiac428]

It will heat stainless steel.
This is what I have: https://www.nuwavenow.com/shop/pic-gold

Forget the napkin math, these solutions aren't dry salt, they're aqueous. I thought a dry melt was one of the routes. If you can reach the 300F, it'll work. Just hold your breath and do it when the wife isn't home.

 

[WobblyHand]

I have trying to get stuff ready to try @benmychree 's formula for hot nitre bluing. I have both potassium and sodium nitrate and a small lead melting pot. Not sure what to use for the manganese oxide, as there seems to be quite a few forms of it. Wikipedia indicates there's quite a few forms.

• Manganese(II) oxide, MnO
• Manganese(II,III) oxide, Mn3O4
• Manganese(III) oxide, Mn2O3
• Manganese dioxide, MnO2
• Manganese(VI) oxide, MnO3
• Manganese(VII) oxide, Mn2O7

Other manganese oxides include Mn5O8, Mn7O12 and Mn7O13.

Anyone have an idea which manganese oxide is most appropriate?

Curiously, "solar salt" is 60% NaNO3, and 40%KNO3, and is liquid from about 260-550C (500-1022F). I think a eutectic mixture is formed at 45.67% NaNO3 & 54.33% KNO3, but not sure what the melting point is. The 50/50 mix above, is in between. It's apparently ok at 900F (482C).

 

[benmychree]

The manganese oxide that I bought and used is Mn02, I bought it online and it was so marked and worked fine. According to the info that I got from the sheet from American Machinist on niter bluing, the 50/50 mixture was the lowest melting mixture of the two salts. Once melted, it will still be slushy at less than 300 deg F. What bluing I have done was at 600 deg. F., for very deep gunmetal finish required by the British and US Government, temps up to 1000Deg F may be necessary, according to the article. In my experience, 600 -650 deg. F gave nice results. Also mentioned is not to leave the articles immersed too long; 5 min. may be sufficient, 10 minutes may be too long with a dirty grey as the result. Also cast iron may be blackened if highly polished, but perhaps 20 minutes may required for good results. This was done on Brown & Sharpe machine handles and was good for rust prevention. I have used the 50/50 mixture for drawing temper at about 1200 deg F, and it seems to be fine, but does a certain amount of fuming.

 

[RaisedByWolves]

I consider 600-650F hazardous. But that's me.

I handle 1800* parts from our heat treating oven at work without a second thought.

Hot sticky liquids, that's a hard pass.

 

[benmychree]

I handle 1800* parts from our heat treating oven at work without a second thought.

Hot sticky liquids, that's a hard pass.

No big deal, long sleeve shirt, gloves, face shield, about takes care of the potential hazard exposure. It is certainly hot, but not particularly sticky.