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Rokushō (緑青) is a traditional Japanese compound for inducing patination in decorative non-ferrous metals, especially several alloys which are nearly unknown outside Japan. Shakudō, an alloy of copper and gold, becomes dark blue-violet; shibuichi, an alloy of fine silver and copper in a higher percentage than sterling, turns misty aquamarine; and kuromido dark coppery black. These alloys 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). Rokushō was generally used to patinate all types of mokume-gane as well. Although other patination agents can be used on these metals, some artisans prefer the rich colors achieved through traditional Rokushō.
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.
Premixed Rokushō can be purchased outside Japan through specialty jewelry suppliers. However, several different formulas have been proposed to replicate the traditional product for those who prefer to make their own:
- In a container made of glass, porcelain, or copper, dissolve 6g copper acetate, 2g calcium carbonate, and 2g sodium hydroxide in 150ml water. After a week, siphon or decant the clear liquid from the top; just before use, add another 2g copper sulfate.[clarification needed]
- Dissolve 4g copper acetate, 1g copper nitrate, 1g cupric chloride, and 4g copper sulfate in 1L distilled water.
- Dissolve 60g copper acetate and 60g copper sulfate in a 2L solution of white vinegar diluted 5-12% with water.
It is critical that metal be perfectly clean and free from any type of oil or grease before applying a patina such as Rokusho. This can be done through a mild acid bath (oxalic or sulfuric acids are frequently used), scrubbing with pumice, or using a surface abrasive.