Copper oxide is formed when copper reacts with oxygen. In chemistry experiments, this reaction can be sparked by heating copper with a burner, turning the original copper black.
The black copper oxide that is produced can be restored to original copper color through a reaction with hydrogen. Heated black copper oxide that is placed within a hydrogen atmosphere results in not only the original copper but water as well.
If the heated copper is removed from the hydrogen atmosphere, it turns black, forming copper oxide once again. This occurs because the copper is still warm enough to cause the original reaction between copper and oxygen.