(Japanese) A pink alloy of 4% to 10% gold in copper, which goes black in air or under chemical treatment.

red copper. alloy of copper and gold.

Japanese alloy of copper with small amount of gold

An alloy of copper and gold that was used by the ancient Japanese in the decoration of swords. The main ingredient of this alloy is copper with 1/2% to 4% gold. This light copper colored alloy is easily oxidized to a deep purple/brown or black. Sometimes antimony or cobalt are added as hardening agents.

Shakudo (赤銅) is a billon of gold and copper (typically 4% Gold, 96% Copper), mostly designed for its beautiful dark blue-purple patina. It doesn't have a name in most languages other than Japanese. It was widely used in Japan for the manufacture of katana fittings such as tsuba and yatate.