The Home Machinist!

Has anyone actually melted copper before? I see a couple questions about it in the past,but no real success stories...

I decided to see if my little electric furnace could get hot enough to melt copper today, figured if it could I would make some pigs from the 15-20lbs or so of scrap wire Ive been collecting. I wasn;t hopeful as the furnace tops out just shy of 2000F... but I figured I would give it a go. At about 1850F on the pyro, sure enough the copper wire went to liquid, but a small steady stream of white smoke started coming out of the furnace. When I pulled the crucible a large stream of white smoke came out as soon as it hit the air. Anybody know what this might be?

I've read up and melting copper and I expected porosity and poor pouring, but the intention was to just testing and consolidation of my scrap if it actually worked. I didn;t expect the smoke. It really looked like zinc oxide fumes, but this should be essentially pure electrical grade copper.

The stupid GFCI kept tripping as I went to reheat... so that ended my experiments for the day, but I ended up with a pretty good plug of copper. The top half of the crucible had a greenish coating, which I expected, the bottom half was left with a heavy layer of crusty white material. Is this the cause of the smoke? I have used the crucible for 15 or so melts of aluminum previously, but it was scraped clean after the last use. The best I can figure, aluminum or maybe flux absorbed into the crucible previously and then dissolved into the copper as it melted, then oxidized as it hit the air. So maybe it was aluminum oxide or salt fumes?


Adrian

Yes, several times. However, my purpose was strictly to pour ingots for use in the recovery of silver from solution. I got mixed results, likely due to the varied operating conditions of the furnace, which was gas fired. Some of the ingots had minor porosity, but were suited to the purpose for which they were cast.


Everything I've read about casting copper indicates it's not easy to pour. If memory serves, traces of phosphorus help, to lower free oxygen content.

No idea on the fumes you spoke of, although I know it's not a good idea to use a crucible for more than one alloy or metal.

If you solve the riddle, I'd enjoy hearing more.

Harold

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Wise people talk because they have something to say. Fools talk because they have to say something.

I have melted copper several times but with a glass cover over the charge. Copper is dificult to keep pure in small equipment.

Anything that exits a furnace at high temps, does so as smoke. I suspect you are correct about it being aluminum products, or the coppper itself casting an oxidation product.

I do not share the same experience you describe.

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Big Dave

Be Alert! The world needs more Lerts.

Electrical copper is essentialy pure copper it will cast very poorly. the pure metal will draw oxygen into the metal itself and then oxidize at the crystal boundries. to cast pure copper you have to have an Oxygen free atmosphere, preferably under vacuum. also when the metal cools it releases some of the air absorbed into the metal and that results in porosity. the result is a very brittle and porous chunk of metal. Small amounts of additives can make it much less prone to do this but they also destroy the conductivity of copper. No problem if you are not using it for electrical purposes.

Well.. I did a second melt last night, about 7.5lbs in the crucible by the end. I scrapped the white residue out before starting and didn;t have nearly as much white smoke this time. A small steady stream as the copper got up to ~1800F and the copper started bubbling away, but not the big plume like before.

Ended up with 8 little ingots ready for remelting when I get a use for it... far less space than the half a box of scrap wire it replaces. I could only get the temp up to about 1850F or so, poured alright but really could have been a couple hundred degrees hotter (yet 7lbs of 1800F copper is really hot!). It all poured out of the crucible, but copper seems to have a much higher surface tension than aluminum, wanted to flow in round blobs. I think some phosphor bronze would have helped.

The ingots came out black with an reddish-brown tinge. After hitting one with the brass wheel, it quickly cleans up as a bright copper bar. No hint of brittleness yet, but it is really hard. I smacked one repeatedly with a 4lb sledge and it barely dented. Milled one open and the inside was clean. The surface is irregular where it cooled quickly against the form though, ended up layered and separated like sheets of paper in a couple places.

It's apparent your pyrometer is not correct, for copper melts @ 1981° F. You may find literature that claims a slightly higher melting point (just a few degrees). Unless you're melting an alloy, you're achieving a much higher temperature than you assume.


A little trick that will help with surface appearance is to dump the solidified ingot in to cold water while it's still red hot. That will remove the vast majority of oxide coating. I used to do that with the ones I cast. A fast and easy way to clean them.


If you're troubled by that, coat your mold with mold dressing (or soot from an acetylene torch), then preheat the mold somewhat. That should eliminate the cold shuts you're achieving. Super heating the copper a little will help, too, but if you're using an electric furnace, you're already running it near it's top end I would think.

Well done.

Harold

_________________
Wise people talk because they have something to say. Fools talk because they have to say something.

Yep, hence why I thought I wouldn;t be able to melt it to begin with. Not sure which is wrong at the moment as I don;t have a second meter that will go that high. My furnace is just an old ceramics Satellite J100 kiln I picked up to try heat treating some parts, so it wasn't really meant for getting this hot. This is also just a plain graphite crucible from Budget, so that maxs out at 2000F as well.

As I recall, when I first got it, I tested out on a piece of steel, at full power I would describe the color as "white-orange", not quite yellow, which ought to put it in roughly the 1800-1900F range. Pouring the copper, the color was initially a bright orange in the furnace (though it went dull orange as soon as I got it into the air), but pouring quickly I did see a bit of yellow towards the bottom.