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Pattern welding is a smithing practice of folding and/or twisting metal, possibly multiple pieces (which may have differing compositions, or be completely different types of metal) that are forge welding. This results in differing layers in a pattern, hence the name. This process was independently discovered by many ironworking societies. Often wrongly called Damascus steel, blades in this manner 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. Pattern welding also, more importantly, reduces impurities and, most importantly, homogenizes the steel. However modern steelmaking processes negate the need to blend different steels, reduce impurities, or homogenize the steel, pattern welded steel is still used by custom knifemakers for the cosmetic effects it produces. It is also used with non-steel metals, for its aesthetic properties, such as with mokume-gane.
In medieval swords, pattern welding was more prevalent than commonly thought. However, the presence of rust makes detection difficult without repolishing.
During the Middle Ages, Wootz steel was produced in India and exported globally, including 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 Damascus steel. The methods used by Indian smiths to produce their Wootz steel was lost over the centuries.
The ancient swordmakers exploited the aesthetic qualities of pattern welded steel. The , 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.
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 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.
Blacksmiths will sometimes apply Wite-Out, Liquid Paper, or other types of correction fluid to metal that they do not want to weld together, as the titanium dioxide in the correction fluid forms a barrier between the metal it is applied-to and any other pieces of metal. For example, when creating pattern-welded steel by filling a steel canister with pieces of metal and powdered steel and forging it together into a single mass (" canister damascus steel,") smiths frequently coat the inside of the canister with correction fluid and let it dry before adding their materials. Thus, when the canister is forge welding, the material on the inside of the correction fluid is forged together, but it does not forge to the canister, allowing the pattern created by forging the different materials together to be seen in the finished piece because it is not covered by the homologous steel of the canister.
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