Purity of copper when you melt it

While brass and bronze have low melting elements that change the properties and lower the melting point, copper is pure copper and melting it should make no difference unless you melt it in a crucible that yields crud. I am not sure how you would purify it after melting anyway. You should (he said hopefully) be able to pour molten copper around a copper rod in a mold and have them merge. If you have any doubts, aim a torch at the rod and preheat it before pouring.

Thanks for the help. I'm definitely not going to do any casting anytime soon. I was mostly looking in to casting to see if it was a good idea to pursue casting further or simply to try something else. I think that I will go ahead and try casting the copper. Though not until I've read a lot more. Thanks for the help, at least now I have a direction to go towards. Thanks again

-Zipper

One thing I have read is that melted copper never melts. That is, like the bottom of the Charles River on the way up, it just gets less vicous and less pasty, but must be very hot to become what we'd call a pourable fluid, something that pours in a stream rather than in chunks.

Yours,

Doug Goncz (at aol dot com) Replikon Research

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The metallic portion of the melt stays the same, assuming you are melting in a crucible made of silicon carbide, or clay-graphite. The problem is the copper will oxidize and you are forming CuO (Copper Oxide) and other forms of Copper Oxides. In additon molten coppers may also pick up other gasses from the atmosphere and from the combustion products, such as carbon monoxide, and hydrogen. Normally all these are in small amounts. The big problem is the CuO.

To answer that I'd need to know a little more about your application. For example, if you are making a welding electrode, then the oxides could cause you some problems it they are entrained into the molten metal. Usually when melting copper the foundry will work to some purity specification. That is so a deoxidizer can be added to the metal. A popular deoxidizer is Phos-Copper (an alloy of Phosphorous and Copper). This material is supplied in small shot form. It often has a greenish patina on it. A small amount can be plunged into the melt, and the phosphorous readily oxidizes, meaning it attracts all the oxygen to form phosphorous oxides, thereby deoxidizing the molten metal. Some residual phosphorous remains in the metal. Now the purity level of the copper will determine it's electrical conductivity and it's thermal conductivity. But, if you have gas hiles in your part and you want good conductivity then it's better to deoxidize.

Some people will also deoxidize with very small amounts of aluminum or zinc. They do the same thing as the phos copper. When placed into the molten copper bath, there is a favorable chemical reaction process that takes place, i.e. the aluminum or zinc will quickly and readily oxidize at the temperature that molten copper exisits at. What a metallurgist would say is a more favorable thermodynamic reaction of these elements (aluminum, zinc, etc.) exisits compared to the thermodynamic reaction of copper oxidizing.

The answer is a little more complex. It will depend a lot on 1) how clean is the copper rod that you have placed into the form (mold). Cleaner copper would have a better possibility of fusing 2) what is the relative mass or density of the portion of the rod that will be in the molten copper in comparison to the molten copper itself? If the relative denistity is great, then there is a better chance of fusion. Example: Place 12" of a clean,

1/4" diameter copper rod into a mold. The mold is large enough to hold 20 pounds of molten copper in the form of a cube. Most likely you will get good fusion. On the other hand, if the mold contains only 1//2 pound of metal, and the 1/4" rod is in the center of a 1/2" hole, the rod will "chill" the melt (like an ice cube) so quickly that a good fusion will not happen 3) a lot depends on how good the fusion must be. If you are looking for a good electrical connection then thats one thing. If your'e looking for simply a good mechanical connection that's another. I would recommend casting your part, and then drilling and tapping the copper, and also threading the copper rod if you really want it to work well.

The only bad question anyone has is the one they don't ask.

Mark Fields

hey, Just what are you trying to make Zippy? Are these objects jewelry size or basketball size? Casting copper is not for a beginners. It melts HOT, almost 2000F. It flows poorly into molds, and is subject to oxidation, which will make porous, crappy looking castings. I think unless you are an experienced caster you are setting yourself up for a LOT of frustration. Maybe fabricating is a better idea. Even in the early days of metalworking, copper was rarely cast into a finished shape, for the same reasons mentioned above and by others. Ingots of copper were cast and then the metal was beaten, sheared, sawn, stamped or otherwise shaped (either hot or cold) into finished products. Makes sense I think. Basic hand tools are cheap compared to casting equipment & supplies. Then again you could probably look for a commercial caster who might be willing to cast your molds. I suspect most will avoid it like a plague or charge trough the nose.

Glen G.