Niiro, Jizai-okimono

A dramatic color change is induced by dissolving copper sulfate and patina in water and then boiling the metal in the solution. This process is called "Niiro". With the exception of gold and silver, which do not change color, metals change color due to this treatment: red-copper changes into black, copper-silver alloy (shibuichi) to gray, and crude copper to a red-brown.