Irogane (色金 "coloured metals")[1][2][3] is the term for a set of Japanese metals – forms of copper (with natural impurities), and copper alloys – treated in niiro patination processes,[4][5] traditionally used in sword-making, catches for sliding doors, and luxury highlights on larger objects, and in modern times, in jewellery.
Some scholars believe that methods similar to those involved in irogane production may also have been used in ancient Egypt and the Roman world, as well as China (wu tan) and Tibet (dzne-ksim).[6]: p.
[10] The single most common historical usage was on the guard known as a tsuba that is fitted between the grip and the blade of the swords carried by samurai and the shōgun, and the banning of ceremonial sword-wearing under the Meiji Restoration had a material impact on production of irogane metals.
The better known irogane metals include: According to William Gowland, a professor and senior member of the Japan Society in London (who was for a time Director of the Imperial Japanese Mint), aside from copper with natural impurities, the Japanese craftsmen would sometimes add a "pseudo-speiss" (a mix including some arsenic and / or antimony) to copper ore to make a base for production alloys.
[9] The colouring and finishing of irogane surfaces generally involves polishing and cleaning, and chemical patination, and might include some final waxing or other treatment.