This is a reprint of an article published in Australasian Survivor Magazine, back in the early '80s
Bill Tarplee has passed on to me some comments from a reader who apparently managed to melt the backside out of two iron crucibles in the process of melting some copper. I say ‘apparently’ because it is more likely he has burnt the backside out of his crucibles, unless his furnace is an extremely hot one.
Unalloyed copper is notoriously difficult to cast. It will melt readily enough, but absorbs oxygen from the furnace atmosphere to such an extent that a significant percentage of the copper will be lost as black copper oxide slag, and the resulting castings will be well and truly ‘gassed’ – full of bubbles – unless precautions are taken.
Of course it is not only the final castings which are affected. The molten copper combining with the absorbed oxygen acts as a very efficient oxidising agent: it eats its away at an iron crucible in much the same way as a flame burns a piece of steel wool.
The moral of the story is don’t use iron crucible for melting copper. Theoretically, at least, you could use an iron crucible lined with approximately 1” to 1 ½” of fireclay (depending on the size of the crucible). This would have to be fired in the furnace before use, of course. However iron/fireclay crucibles tend to give a slow and inefficient melt.
In casting metal, as in any other work, the best results are obtained by using the tools designed for the job. An iron crucible should work well enough for melting tin, lead and even aluminium, although in the last cast the crucible should be given a refractory “wash” (zirconium etc.) to prevent direct contact with the molten aluminium.
For melting copper and its alloys, there is no substitute for the old clay/graphite, or if you’ve got the pennies, silicon carbide crucible.
I’ve already pointed out that pure copper castings tend to be gassy. There are basically two solutions to this problem. The ideal, and expensive, solution is to use a high frequency reduction furnace, which means the oxygen in the furnace atmosphere is not constantly being replenished as it is in a conventional fuel-fired furnace. The alternative is to remove the oxygen from the metal before it is cast. To do this we can use a deoxidising flux.
There are many different kinds of fluxes. Some introduce oxygen into the melt for the purpose of removing oxidisable impurities. Some remove oxygen. Others simply form a slag over the melt to prevent the furnace atmosphere coming into contact with it etc, etc.
The simplest flux for copper, which works well for smaller (say up to 20-30 pound) melts, is an inch or two of fingernail sized charcoal pieces spread on top of the copper in the crucible.
Calcium boride is an excellent deoxidiser, and can be obtained from most chemical supply firms. An ounce or so per 10 pound should be sufficient. Finally, a handful of “phos. copper”, ie phosphorus copper alloy in the form of shot, is also excellent, but because the phosphorus forms an alloy with the copper it is not suitable for pure copper castings.
For other articles in this series, or other series of articles about backyard foundrywork by the same author, check out here