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-welded. 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 forged 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.
History
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 process, in which different types of steel together produce patterns that can be seen in the surface of the finished blade, forms the basis for pattern welding.
Pattern welding in Europe Pattern welding dates to the first millennium BC, with Celtic, and later Germanic swords exhibiting the technique, with the Romans describing the blade patternation. By the 2nd and 3rd century AD, the Celts commonly used pattern welding for decoration in addition to structural reasons. The technique involves folding and forging alternating layers of steel into rods, then twisting the steel 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 overlaid 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 Age, pattern welding fell out of use in Europe. 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 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.
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