“Shibuichi (四分一?) is a billon which can be patinated into a range of subtle muted shades of blue or green. 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”. An wide range of colours can be achieved using the whole range of alloy compositions, even above 50% silver. 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 materials microstructure while the silver rich regions are left largely untouched” (Wikipedia).
“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. Similar alloys have been used elsewhere but the use of shibuichi to achieve different colored patinas has remained nearly unknown outside Japan, despite recent interest from artisans in the West” (Wikipedia).