In Japanese metal arts and crafts, some copper alloys are colored by a traditional surface treatment called <i>nikomi-chakushoku* 1 (coloring by heating in solution). These colored alloys are called * 2irogane</i> (colored metals). Since copper is one of the important metals for <i>irogane</i> it was experimentally colored by the traditional method, and investigated for its colored layer. Scanning electron microscopy, X-ray diffractometry, infra-red and X-ray photo-electron spectroscopy were applied and the layer was proven to consist of Cu₂0 (cuprite). The distribution of copper, oxygen, and other elements in the layer was also investigated through secondary ion mass spectroscopy. -- AATA