Shakudō

Shakudō (赤銅) is a Japanese billon of gold and copper (typically 4–10% gold, 96–90% copper), one of the irogane class of colored metals, which can be treated to develop a black, or sometimes indigo, patina, resembling lacquer. Unpatinated shakudō visually resembles bronze; the dark color is induced by the niiro artificial patination process, involving boiling in a solution, generally including rokushō.

Naming

The characters in the name shaku-dō mean "red" and "copper" but combined they represent this material which begins with a darkened coppery-bronze color and is then modified to black or near-black.

History

Shakudō was historically used in Japan to decorate katana elements such as the tsuba (guard). When it was introduced to the West in the mid- 19th century, it was considered unknown outside Asia, but recent studies have suggested similarities with alloys used in Egypt, Greece, and Rome. Modern jewelry artisans have helped revive the use of this alloy as a decorative element, especially for the mokume-gane technique.

Early uses

The word "shakudō" first appears in records of the Japanese Nara period (710-794 AD), but it is not clear to what it referred (it could have been some form of copper, or a form of the now-known material). There are actual pieces known from the 12th century onwards. Shakudō was historically used to construct or decorate Japanese sword ("nihonto") fittings such as tsuba, menuki, and kozuka, as well as other small ornaments, sliding door catches, and small boxes.

Introduction to the West

The recipe was kept strictly secret for many centuries and only reached Europe in the 19th century. Shakudō is characterized by its dark brown to black-violet color, which is only achieved by etching the metal surface in a hot salt solution. The name red copper, despite its dark brown, almost black patina, is explained by the color name赤銅色 shakudōiro, meaning “bronze-colored” and “black-brown”. Due to its intense and variable color, Shakudō is a frequently used alloy in the Japanese Mokume-Gane forging technique. The traditional use in the manufacture/decoration of the Tsuba of the Japanese Katana sword was already discussed in the Western world by Gustav Jacoby (1905) and Henri Louis Joly (1912) at the beginning of the 20th century.

Possible origins

Materials like shakudo were historically thought to be specific to the Chinese and Japanese, and perhaps other Asian, milieu, but recent studies have noted close similarities to certain decorative alloys used in ancient Egypt, Greece, and Rome.

Production

The origins of shakudo date back to a period when Japan was still importing significant techniques and materials from Korea and China, but accounts of production all derive from much later, and little is known of their evolution. By the Meiji period, the initial production process entailed the heating of copper, addition of fine gold, and some addition of shirome, a by-product of copper production containing iron, arsenic and other elements. In the Edo period, it appears that the process may have used nigurome rather than copper; nigurome being itself a pre-made mix of copper and shirome. The resulting alloy was then allowed to rest in ingot moulds in heated water, before being shaped, and annealed at around 650 °C. In cooled form, the metal was then surface-finished using the niiro process. The modern process tends to omit the shirome, working with copper and gold, and other additives directly if needed.

The alloy usually consists of copper with 2–7% gold. Other metals are rarely added in small quantities (<1%). It is used in Japanese crafts such as inlay work. When colored, it turns a bluish-purple black. The color used in expressions such as "copper skin" means that it is close to black.

The coloring process is called "niiro-shiage," "iro-age," or "niiro-age," and the color is produced by artificially applying an oxide film (rust) to the surface by boiling the piece in a liquid containing a mixture of verdigris (copper carbonate or copper acetate), aluminium (copper(II) sulfate pentahydrate), and alum (potassium aluminum sulfate dodecahydrate) (niiro-shiage or niiro-age).

The niiro (煮色) technique for achieving a durable purple-black color involves passivating the metal surface with rokushō (緑青), a mixture composed primarily of copper carbonate, chalcanthite, and alum. What gives patinated shakudō its distinctive color is a layer of copper oxide a few micrometers thick, in which gold particles about 10 nm thick are dispersed. The gold particles reflect light in all directions, lowering the apparent brightness. Light passing through the gold turns blue (impervious to wavelengths other than blue), and light reflected by the copper oxide turns red, forming the characteristic coppery, black, and purple color. The artist adapts the composition of the alloy and the rokushō to achieve the desired color.

The reason why shakudo looks black is because a coating of cuprous oxide several micrometers thick forms on the surface of the boiled copper. In this coating of cuprous oxide, 5-10 nm gold particles are dispersed, and these gold particles absorb and attenuate the incident light by multiple reflections, lowering the brightness and making it appear black. Also, light that passes through gold turns blue (it does not pass light of wavelengths other than blue). The reflected light of cuprous oxide is originally reddish brown, and the combination of these two creates a slightly bluish black color known as shakudo.

Copper-gold content and coloring

Ichibusashi composition, 99% Cu + 1% Au, pure black

Sanbusashi composition, 97% Cu + 3% Au, bluish-black color

Gobusashi composition, 95% Cu + 5% Au, bluish-black color.

Hachibusashi composition, 92% Cu + 8 % Au (tolerance values of 90–93% Cu and thus 7–10% Au apply, but the name Hachi clearly refers to an 8-part gold content), black-violet color.

Use

Due to the expensive gold content, shakudō was normally limited to accents or small items such as tsuba. Larger historical objects (such as vases) that are described as shakudō may be mislabeled, especially if the glossy blue-black color is not evident. Unpatinated or repolished shakudō will not spontaneously patinate in air.

Modern artisans have revived the use of shakudō as a striking design element, in the making of jewelry, vessels, and for the technique of mokume-gane.

Shakudō is sometimes inaccurately used as a general term for damascened decorative metal inlays of Japanese origin. These were widely known in the West as Amita damascene, from the name of a 20th-century manufacturer of such items for export. Amita damascene included shakudo, shibuichi, gold, silver, and bronze for inlays.

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