I was curious what happens to the purity of copper when you melt it. I don't
know alot about melting, but I want to make some parts for something I'm
going to be trying to sell. I can get very pure copper, but I'm going to
need to melt it. What I want to know is will the "pure" copper remain pure
after I melt it, or will I have try to purify it after I melt it. I have one
other question if anyone has time to answer it. If I stick a copper rod into
the form that I have then pour the melted copper will it cause the two to
fuse or will the copper cool down to quickly to melt the existing copper?
Any help would be greatly appreciated. Sorry for my ignorance, but when I
checked goolge for this topic I was unable to find a definitive answer.
Thank you again for your help.
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.
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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
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.
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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.
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.
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