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Example of Mokume-gane patterns in gold and silver alloys
Mokume-gane brass/copper billet

Mokume-gane ( 木目金 ) is a mixed-metal laminate with distinctive layered patterns. Literally translating as "wood eye (i.e., burl) metal", the name was borrowed from one type of pattern created in the forging of swords and other edged weapons.

First made in 17th-century Japan, the mixed-metal was used only for sword fittings until the Meiji era, when the decline of the katana industry forced artisans to create purely decorative items instead. The inventor, Denbei Shoami (1651-1728), initially called his product "guri bori" for its simplest form's resemblance to "guri", a type of carved lacquerwork with alternating layers of red and black. Other historical names for it were kasumi-uchi (cloud metal), itame-gane (wood-grain metal), and yosefuki. (Pijanowski & Pijanowski, 2001)

The traditional components were relatively soft metallic elements and alloys - gold, copper, silver, shakudo, shibuichi, and kuromido - which would form liquid phase diffusion bonds with one another without completely melting. After the original metal sheets were stacked and carefully heated, the solid billet of simple stripes could be forged and carved to increase the pattern's complexity. To achieve a successful lamination using the traditional process required a highly skilled smith with a great deal of experience.

The modernized process typically uses a controlled atmosphere in a temperature controlled furnace. Mechanical aids such as a hydraulic press or torque plates (bolted clamps) are also typically used to apply compressive force on the billet during lamination and provide for the implementation of lower temperature solid-state diffusion between the interleaved layers, allowing the inclusion of many nontraditional components such as titanium, platinum, iron, bronze, brass, nickel silver, and various colors of karat gold including yellow, white, sage, and rose hues as well as sterling silver. (Binnion & Chaix, 2002)

[edit] Coloring

Katana, showing mokume-gane from the forged in layers of steel with differing carbon content.

To increase the contrast between the laminate layers many mokume-gane items are colored by the application of a patina (a controlled corrosion layer) to accentuate or even totally change the colors of the metal's surface. One example of a traditional Japanese patination is the use of rokusho a complex copper verdigris compound produced specifically for use as a patina.

To color the shakudo and gold, submerse the piece in boiling rokusho, and hold there - agitating constantly - until it reaches the desired color. Rokusho colors shakudo a black-purple. The more gold is in the alloy, the more purple it turns.

Rokusho is produced in small batches in a traditional process and is somewhat difficult to acquire outside Japan. There are some proposed substitute formulas see the rokusho article

Traditionally a paste of ground daikon radish is also used to prepare the work for the patina.

The paste is applied immediately before the piece is boiled in the rokusho to protect the surface against tarnish and uneven coloring.

(Pijanowski & Pijanowski, 2001)

Shakudo can also be darkened by adding salt with ammonia in a plastic bag. The warmer the solution, the faster it will darken the metal.

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