Scrap silk and copper bars are very expensive. Copper bars are strong, heavy and low in loss. Pure copper is probably the best. Pure copper, also called red copper or copper, is also the most expensive. The second time, it was brass. Brass with a third of zinc is closer to gold in color. It is also called alloy copper. There is another kind of alloy copper called bronze. When 8% to 10% tin is added to pure copper, bronze is called bronze. These three kinds of copper are pure copper.