Shibuichi (四分一) is an alloy which can be patinated into a range of subtle muted shades of blue or green, through the use of rokushō treatments. Its name means "one-fourth" in Japanese and indicates the standard formulation of one part silver to three parts copper, though this may be varied according to the desired effect. A 5% silver / 95% copper alloy is also marketed as "shibuichi". A wide range of colours can be achieved using the whole range of alloy compositions, even above 50% silver. 90% Copper and 10% Silver for a dark grey and down to 70% Copper and 30% Silver for lighter greys.
It is a common misconception that both copper and silver oxides form, but in fact a detailed study has shown that only copper oxides are formed on the copper rich regions of the material's microstructure, while the silver rich regions are left largely untouched.
For most of its history, shibuichi was mostly used to ornament various fittings for katana until the Meiji reforms, when most swordmakers began to make purely decorative objects instead. The material is often used in mokume-gane combinations. Similar alloys have been used elsewhere but the use of shibuichi to achieve different colored patinas appears to have remained nearly unknown outside Japan, until recent interest from artisans in the West.
Kuro is black in Japanese. Kuro-Shibuichi is different from others in the table. Kuro-Shibuichi is mixture of Shibuichi (40%) and Shakudō(60%) with additional 1% of Gold. Roughly proportion of Silver 9.9％, Copper 87.3％, Gold 2.8％. Kuro-Shibuich will develop black patina which is different from the black patina of Shakudo.