"Roger Hull" wrote in message
s.net...
I have zero foundry experience. I want to melt a few pounds of Copper
and cast it into backup plates for welding. What should I make the
crucible and moulds out of?

Well first of all, read up on foundry practice obviously... ABYMC.com and
backyardmetalcasting.com are good starts.

You *can* make your own crucibles, but the thing with pottery is it has to
be fired slowly (over at least 5 hours, much longer if you need a more
stable product), and any typical formulation is about as good a conductor as
your average 3000 degree rated castable refractory, not to mention the
expansion is greater. Try this: toss a coffee mug on a fire. Watch it snap
into pieces. Thermal shock in action.

Even Vince Gingery in his crucible book goes only as far as grog and clay
with a little feldspar, as I recall. Something like 77% 20-80 mesh grog,
20% fireclay and 3% spodumene (lithium feldspar, a flux). Now fireclay
alone has a high silica content, which - when not tied up as something
else - tends to form quartz or crystobalite. Both of these have dramatic
changes in density at specific low temperatures; the "quartz inversion"
occurs at 573C (1% change, IIRC) and cristobalite at 180C (0.5 to 1%, I
forget exactly). Not only that, but the high grog content, coarse grog no
less, means high porosity - holes that insulate, just what you don't need!
The only thing it's really got going for it is the high melting point - up
around cone 25, at least (2800-3000°F I'd say).

OR, is it possible to buy a crucible somewhere?

Well, yeah...

LA Graphite is the cheapest, IIRC. Clay-graphite takes more care than SiC
or steel crucibles, but can also handle iron if you desire to try that.
Don't use steel, it'll dissolve and not only will your alloy be askew, but
the crucible might burn through as well.

Once you make a furnace and get a crucible, you need fuel... go with
propane, it's just easier. You can use wood, coal or charcoal (stay away
from briquettes), but melting something hot like copper isn't the easiest
goal with it. Not to mention, the uneven and often all-or-nothing heat
control is sure to crack any but the best crucible.

You asked what for mold, sand mold is fine. Read for yourself, I don't have
to say another word on this!

As for melting and pouring copper... use a slight reducing atmosphere. The
posters above who say pipe just kinda sorta collapses royally screwed
something up. Copper is a noble metal and as such, its oxide is very easy
to reduce to pure metal. Not to say there won't be any trouble: oxygen
(gas, not oxide!) is soluble in copper, just like CO2 in water. When it
freezes, it comes out and gives you trouble. The common fix is borax plus
either soda (baking soda, washing soda, etc.) or boric acid, with crushed
charcoal (any kind) on top to absorb the oxygen. If you don't mind
alloying, you can add 1-5% zinc (pennies) and that'll combine with the
oxygen, keeping it clean. There's still the matter of hydrogen, but oxygen
is probably your #1 concern.

I personally have recast scrap wire as ingots for storage. Works fine,
though it did come out all bubbly!

Beware of shrinkage defects in the casting BTW: copper - being pure - has a
steep melting/solidifying point, so it'll want to suck in metal all at once.
Use big gates and even bigger risers. Pour around 2200°F.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms