Shibuichi (四分一) is a historically Japanesecopper alloy, a member of theirogane class, which ispatinated into a range of subtle greys and muted shades of blue, green, and brown, through the use ofniiro processes, involving therokushō compound.
Shibuichi means "four plus one" inJapanese, and indicates the standard formulation of one partsilver to four partscopper, though this may vary considerably according to the desired effect.Several major variants of the alloy have specific names, as detailed below. In addition, the metal in general, and especially the paler shades, may be namedrogin.
Aside from the basic 25% silver to 75% copper mix, combinations as divergent as 5% silver to 95% copper are also marketed as "shibuichi".[1] A wide range of colours can be achieved using the whole range of alloy compositions, even above 50% silver, e.g. 90% copper and 10% silver for a dark grey and down to 70% copper and 30% silver for lighter greys.[2]
| name (JA) | Ag : Cu, +Au[4] | Notementioned colors are after patination |
|---|---|---|
| Shibuichi | 25 : 75 | Dark grey, has a trace of gold |
| Shiro-shibuichi (kin-ichibusashi) | 60 : 40, +1 | Shiro = "white" in Japanese; lighter grey, harder, lower melting temperature |
| Ue-shibuichi (kin-ichibusashi) | 40 : 60, +1 | Ue = "upper" in Japanese; grey, harder |
| Nami-shibuichi uchi-sanbu (kin-ichibusashi) | 30 : 70, +1 | Nami = "regular" in Japanese; lighter than basic shibuichi |
| Nami-shibuichi soto-sanbu (kin-ichibusashi) | 23 : 77, +1 | Darker than basic shibuichi |
Kuro is black in Japanese and kuro-shibuichi is different from other variants in the table, being a mixture of shibuichi (c. 40%) andshakudō (c. 60%) with around 1% of gold (proportions could be, for example, 9.9% silver, 87.3% copper, and 2.8% gold). Kuro-shibuichi will develop a black patina which is different from the black patina of shakudo.
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.[citation needed]
The first official mention of the material is from the early 18th century, in documents from the State Mint, though it is believed to have existed before that. For most of its history, shibuichi was mostly used to ornament various fittings forJapanese swords until theMeiji reforms, when most swordmakers began to make purely decorative objects instead. The material is often used inmokume-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.
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