Blow torching coins

My bedroom/office contains many distractions from work. As procrastination today, I blow torched a solid copper coin. After a few seconds, it went from oxidised brown to a beautiful peach colour. However, when the flame jet was removed, the surface immediately flashed through several colours before turning a dark blue/grey. The colours look very pretty indeed (like surface of oil, except uniform), but they last only a brief moment.

Do any of you know what's going on here or how these colourful states can be 'preserved'?

I have attempted to capture this and enclose a YouTube link, for your distractionary pleasure.

(By immediate dunking in water, I get some interesting effects, but not much in the way of fancy colours)

 

Log in or Sign up to hide all adverts.

We did that in metal shop. If you are careful, you can pick what color it becomes. In the end, my Dad gold plated the thing (a can opener) for me, and I got an A.

 

Log in or Sign up to hide all adverts.

Careful? Do you mean heat it gradually? Even if I was able to get it to a specific temperature it'd oxidise the moment I took the jet off it. Perhaps 'dunking' it in a bottle that contains no oxygen would do!

 

Log in or Sign up to hide all adverts.

You have to heat it evenly and then quench it. A coin is so small that it probably loses it's heat quickly.

 

Didn't your fingers get warm holding the tweezers with that heat hitting the tweezers? :shrug:

 

Ah - yes, as you can see from the video it cools pretty fast. 'Quenching' it causes horrible black and brown blistering though. What's really cool, is that a drop of water will not adhere to the hot surface, instead forming a spherical drop that rolls about on it.

The tweezers had a very low surface area of contact with the coin, but yes they were warm eventually. Muscular pain from squeezing them was greater!

 

The Leidenfrost effect.
There is a riveting essay about this by a real mad scientist - I would have loved to have this guy as a physics teacher!
Jearl Walker - Boiling and the Leidenfrost Effect

I haven't read this in 19 years, but it's stuck in my memory ever since, and I just knew it would be online somewhere!

 

That was a good read!

 

Unpolished copper is a vibrant pinky colour,... I remember in school cleaning up a copper coin to set in perspex, and used acid iirc to clean to the surface of it's oxidised layer.

I guess blowtorching reverses the oxidation, the gas being more reactive than the copper?

 

Maybe you could set up a blowtorch with some shielding gas, like they use for MIG welding?
You'll need a cylinder of a suitable gas (Carbon dioxide, helium, nitrogen, argon), a regulator, a nozzle, and a way of directing it onto the coin while you heat it. Some kind of mount for the coin would be handy as well... like a soldering stand:

Please Register or Log in to view the hidden image!

A helium-balloon rig would do the trick. Or a CO2 fire extinguisher.

Or... you could maybe make your own CO2 bath with vinegar and bi-carb. A tablespoon of vinegar and a tablespoon of bi-carb in a cup sitting in the bottom of a bucket should give you a bucket of CO2 in short order. But the problem will be keeping it in there once you start heating it with the blowtorch, I guess.

 

Titanium ring + gas oven + burger tongs = fun.