Oxyacetylene torches reach temperatures up to 3480 degrees Celsius. This is more than hot enough to melt copper, but pure copper tends to oxidise and deteriorate when melting, so you need a few extra tools and tricks to keep your copper looking good and prevent cracking when it cools and hardens. If you want a clean cast, start with copper casting shot, available from jewellery and metal suppliers. This will be free of unknown impurities, unlike copper wire, coins and jewellery.
Prepare your melting area before you begin trying to melt your copper by filling a fire pit with sand.
Sink a metal-melting crucible into the sand and pack the sand around the lower 3/4 of the crucible. Tungsten or other crucible materials with melting points above 3480 degrees Celsius work best so you don't accidentally begin melting your crucible as you melt the copper.
Scrub your copper with a toothbrush under water to clean it if you are not using fresh casting shot. You can also apply a degreasing cleaner to old copper coins, but make sure you rinse them thoroughly in fresh water to remove all chemical traces before casting.
Paint a coating of copper flux onto each piece with a flux brush, and then set the copper pieces in the crucible. Copper flux may be found in the plumbing supply section of some home improvement stores.
Turn the acetylene gas on slightly for your torch.
Strike a spark with a flint striker to light the gas.
Put on safety goggles, gloves and protective clothing, such as a long-sleeved shirt, long trousers and closed-toe shoes.
Turn up the gas and oxygen until you have a bluish cone about 1/2 inch long at the base of your flame.
Circle the flame slowly around the inside of your crucible in steady sweeping motions, keeping the clear part of the flame just beyond the blue cone on your copper.
Move the torch constantly to keep the heat even. Add a small amount of lithium or calcium boride as the metal grows shinier, indicating it is about to melt. These agents will deoxidize the copper.
Keep moving the torch over the copper until it melts and flows, at approximately 1,083 degrees C.
Lift the crucible with your crucible tongs to pour the molten copper into your mould.
- Preheat your mould and then keep it warm as you melt the copper. This ensures a better flow of the molten metal.