Rokushō
Usage. "Rokushō" is used to treat a number of metals, including raw natural copper, which holds impurities, purified copper, and copper alloy mixes with two to five metals, to produce "irogane" metals, including: "shakudō", an alloy of copper and gold, which becomes black to dark blue-violet; "shibuichi", an alloy of fine silver and copper (in a higher percentage than sterling), which turns grey to misty aquamarine or other shades of blue to green; "kuromido" which becomes dark coppery black. "Rokushō" was generally used to patinate all types of "mokume-gane" ("wood grain metal") as well. Although other patination  agents can be used on these metals, some artisans prefer the rich colors achieved with traditional "rokushō" in the "niiro" process. These metals are becoming increasingly popular in high-end artistic jewelry, especially in bi-metals (a layer of the alloy fused to another metal such as sterling). Because rokushō has a dramatically different effect on sterling silver than on the alloys typically fused to it in bi-metals, a common technique in art jewelry is to engrave through the alloy layer in a pattern to reveal the silver underneath prior to patination. This provides a rich contrast in color , highlighting the pattern. Formulation. The formulae for rokushō are not published widely or freely, but passed on in the Japanese craft tradition. However, some scholars have analysed samples of the material. Premixed rokushō can be purchased outside Japan through specialty jewelry suppliers. Additionally, several different formulas have been proposed to replicate the traditional product for those who prefer to make their own: Rokusho is not used alone, but mixed with one or more other chemicals. Further, metal to be processed is cleaned in advance of treatment, using a mild acid bath (oxalic or sulfuric acids are frequently used), scrubbing with  daikon radish or pumice, and/or a surface abrasive, and often treated after patination also.