Copper melts at 1984°F, silver at 1763°F. Mapp gas torches reach that temperature, but when melting up at these temperatures, the power of the furnace is important if you are to succeed- long drawn out heating times with an undersized heat source introduces problems of gas absorption and oxide [or dross] formation. That loss of metal costs money!
I melted [and cast in DRY sand] copper in an iron ladle using charcoal as a boy in the fireplace with our vacuum cleaner on ‘blow’ providing the air blast. Don’t do this as it stinks out the house!
Any BBQ charcoal is fine, but I’d go for a good hardwood natural charcoal brand and not ‘briquettes’ or domestic coals, as you are then assured there is no sulphur in the fuel. This element, if absorbed into either Ag or Cu, will create havoc with any forging or other later work, making the metal brittle [ ‘hot’ or ‘cold-shortness’- as in cookies or pastry].
To get the temperatures required you need to blow air into the furnace with a blower and a tuyau [ let’s use the right words!] or pipe. Iron pipe or conduit is fine, stainless steel is better, as there is less likely-hood of melting the end off! A BBQ charcoal furnace with a blower will easily hit 3500°F, so take care or you can melt the firebricks. And believe me, a plastic vacuum cleaner pipe WILL catch fire!
‘The Furnace’ could be just some firebricks arranged to form a suitable hearth.
Both copper and silver absorb lots of oxygen and other gasses while molten, and these cause the metal to distort or ‘blister’ as the gasses come out of solution upon solidification. You need to melt quickly and then cast. [ Drag any dross or crud floating on the top of the melt off first]. Small amounts of blistering can be cold-forged out by hammering. If extensively cold-working these metals, an occasional anneal with a gas torch up to dull red-hot will make the piece more malleable and less prone to cracking. Let the piece air cool, or walk it with tongs to your water bucket for a quench- but see later paragraph.
Iron from melting-crucibles [ including stainless steel ] will be dissolved a little into the metals, probably not enough to bother you, but the electrical conductivity will be ruined by a very small amount of iron. Similarly, oxides of copper or silver in the cast piece will lower the conductivity and can also lower strength and other properties of the metals, hence the need for fast melt and cast.
If you are serious, there are www. sites which sell proper ceramic crucibles and tongs. Wear proper protective gear against burns [ cotton or leather, not man made ] and most important of all, as Norm says , wear Safety Glasses, because if you encounter ‘blister conditions’, the metals can spit. Do NOT cast into wet sand or damp plaster molds- that risks a steam explosion- in fact don’t have water anywhere near, except perhaps in a fire extinguisher. Working outside is the best plan. For ingots, 'chill casting' into a welded-up mild steel box-mold, with suitably tapered sides for release, would be ideal. Dust with talc or powdered graphite to aid knocking out.
Best of luck, and play safe!
Alan
[This message has been edited by Alan Belson (edited 11-18-2006).]
