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Pattern welding is the practice in sword and knife making of forming a blade of several metal pieces of differing composition that are forge-welded together and twisted and manipulated to form a pattern. Often called Damascus steel, blades forged in this manner often display bands of slightly different patterning along their entire length. These bands can be highlighted for cosmetic purposes by proper polishing or acid etching. Pattern welding was an outgrowth of laminated or piled steel, a similar technique used to combine steels of different carbon contents, providing a desired mix of hardness and toughness. Although modern steelmaking processes negate the need to blend different steels, pattern welded steel is still used by custom knifemakers for the cosmetic effects it produces.
Pattern welding developed out of the necessarily complex process of making blades that were both hard and tough from the erratic and unsuitable output from early iron smelting in bloomeries. The bloomery does not generate temperatures high enough to melt iron and steel, but instead reduces the iron oxide ore into particles of pure iron, which then weld into a mass of sponge iron, consisting of lumps of impurities in a matrix of relatively pure iron, which is too soft to make a good blade. Carburizing thin iron bars or plates forms a layer of harder, high carbon steel on the surface, and early bladesmiths would forge these bars or plates together to form relatively homogeneous bars of steel. This laminating different types of steels together produces patterns that can be seen in the surface of the finished blade, and this forms the basis for pattern welding.
Pattern welding in Europe 
By the 2nd and 3rd century AD, the Celts were commonly using pattern welding for decoration in addition to structural reasons. Alternating layers of steel would be forged into rods, which would then be twisted to form complex patterns when forged into a blade. By the 6th and 7th centuries, pattern welding had reached a level where thin layers of patterned steel were being overlayed onto a soft iron core, making the swords far better as the iron gave them a flexible and springy core that would take any shock from sword blows to stop the blade bending or snapping. By the end of the Viking era, pattern welding fell out of use in Europe
During the Middle ages, Damascus steel was being produced in India and brought back to Europe. The similarities in the markings led many to believe it was the same process being used, and pattern welding was revived by European smiths who were attempting to duplicate the Damascene steel. While the methods used by Damascene smiths to produce their blades was lost, recent efforts by metallurgists and bladesmiths (such as Verhoeven and Pendray) to reproduce steel with identical characteristics have yielded a process that does not involve pattern welding. However even these attempts have not been a huge success.
A similar technique was also employed by Scandinavian Medieval swordsmiths. The Mora knife is today manufactured with a similar technique. Today the traditional crucible steel is seldom used, but the high carbon steel is usually tool steel or stainless steel.
Modern decorative use 
The ancient swordmakers exploited the aesthetic qualities of pattern welded steel. The Vikings in particular were fond of twisting bars of steel around each other, welding the bars together by hammering and then repeating the process with the resulting bars, to create complex patterns in the final steel bar. Two bars twisted in opposite directions created the common chevron pattern. Often, the center of the blade was a core of soft steel, and the edges were solid high carbon steel, similar to the laminates of the Japanese.
The American Bladesmith Society's Master Smith test, for example, requires a 300 layer blade to be forged. Large numbers of layers are generally produced by folding, where a small number of layers are welded together, then the blank is cut in half, stacked, and welded again, with each operation doubling the number of layers. Starting with just two layers, eight folding operations will yield 256 layers in the blank. A blade ground from such a blank will show a grain much like an object cut from a block of wood, with similar random variations in pattern. Some manufactured objects can be re-purposed into pattern welded blanks. "Cable Damascus", forged from high carbon multi-strand cable, is a popular item for bladesmiths to produce, producing a finely grained, twisted pattern, while chainsaw chains produce a pattern of randomly positioned blobs of color.
Some modern bladesmiths have taken pattern welding to new heights, with elaborate applications of traditional pattern welding techniques, as well as with new technology. A layered billet of steel rods with the blade blank cut perpendicular to the layers can also produce some spectacular patterns, including mosaics or even writing. Powder metallurgy allows alloys that would not normally be compatible to be combined into solid bars. Different treatments of the steel after it is ground and polished, such as bluing, etching, or various other chemical surface treatments that react differently to the different metals used can create bright, high-contrast finishes on the steel. Some master smiths go as far as to use techniques such as electrical discharge machining to cut interlocking patterns out of different steels, fit them together, then weld the resulting assembly into a solid block of steel.
See also 
- Damascus Steel, a steel used in swordmaking during the Medieval Period.
- Wootz steel, an Indian crucible steel
- Bulat steel, a Russian crucible steel
- Tamahagane, steel made from intricate methods of pattern welding used in Japanese sword construction.
- Mokume-gane, a similar technique, often involving precious metals, used to produce decorative pieces
- John D. Verhoeven (2002). Materials Technology. steel research 73 no. 8 http://www.mse.iastate.edu/fileadmin/www.mse.iastate.edu/static/files/verhoeven/steelresearchsize2.pdf
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- Ian G. Peirce, Ewart Oakeshott. Swords of the Viking Age. p. 145.
- Goddard, Wayne (2000). The Wonder of Knifemaking. Krause. pp. 107–120. ISBN 978-0-87341-798-3.
- Ed Caffery. "Damascus Pictoral".
- Ed Caffery. "Bits of Steel".
- Don Fogg. "Damascus".
- Dona Meilach, Decorative and Sculptural Ironwork, 1st ed. 1984 (ISBN 0517527316), 2nd ed. 1999 (ISBN 0764307908)
- Ian Peirce, Ewart Oakeshott (Introduction), Swords of the Viking Age, 2004, ISBN 0-85115-914-1