Pattern welding

A high resolution image of a modern pattern welded knife blade, showing the dramatic patterning on the side below, and the layering of the steel in the spine above. Acid etching darkens the 1080 plain carbon steel more than it does the 15N20 low alloy nickel steel, producing alternating bands of light and dark on the surface.

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 brought out for cosmetic purposes by proper polishing or acid etching. Originally, pattern welding was 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.

History

Pattern-welded 19th century Moro (Philippine) barung sword

Close-up view of the blade of the same Moro barung

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. The bloom must then be heated and hammered to work out the impurities, resulting in the relatively soft wrought iron.

Iron is too soft to make a good cutting edge; a good edge requires the addition of carbon to make steel. Homogeneous steel, such as produced today in modern blast furnaces, uses carefully controlled amounts of carbon and other alloying elements to generate the ideal compromise of hardness and toughness, but that was not possible in primitive conditions with only the basic knowledge of metallurgy that smiths had. By heating thin iron rods in a carbon-rich forge, carbon could be added to the surface, making steel (see case hardening and carburization). Too much carbon, or too many of the wrong trace elements, and the resulting steel becomes too hard and brittle, which can result in a catastrophic failure of a sword. The ideal sword is one with a hard, sharp edge, and tough enough to bend, but not to shatter.

The compromise was to blend various alloys in various places in the blade, to provide hard, high carbon steel along the cutting surfaces, with soft, tough iron making up the remainder of the blade. This provided good edge holding, while providing a blade that, while it might bend and crack, would not shatter under impact. Blending a number of different pieces of iron and steel together also averaged out the properties, ensuring that one bad piece would not produce a total failure of the finished product. 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

The earliest known use of pattern welding in Europe is from an 8th century BCE sword found at Singen, Württemberg in Germany. By the 3rd century, 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 the 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, indicating that the pattern welding was primarily decorative rather than functional. 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 the Middle East 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.

Techniques

The term pattern welding is applied to a broad range of techniques which pursue different goals. Most modern pattern welded steel is done for purely cosmetic reasons using different types of high carbon steel or tool steel, both of which are well suited for the given purpose. Ancient techniques were more focused on blending certain qualities of the steels used to obtain better characteristics than was possible by only using one type of steel.

Altering carbon content

Steel made with the bloomery process, used throughout much of the ancient world, was of very poor quality for making knives and swords. The bloom itself consisted of sponge iron, a porous structure of nearly pure iron filled with pockets of slag. As the slag is worked out during forging, any traces of carbon is also burned from the metal, producing in the end wrought iron, with a very low carbon content. To achieve the desired carbon content for a blade (generally around 0.7 to 1.0%) carbon must be added back into the wrought iron, generally by means of carburization. Since carburization only penetrates the surface of the iron (providing, in essence, only case hardening) the thin veneer must be mixed back into the interior to form the desired high carbon steel. This is usually done by either stacking or flattening and folding the carburized bars. This creates the characteristic thin layers seen in pattern welded steel. Additional shapes can be made by differing how the bars were stacked or folded. Viking and Anglo-Saxon swords were often made with twisted bars, producing spiral patterns, and today many knifemakers use steel cable to produce complex, spiral-in-spiral patterns.

Conservation of high carbon steel

A Japanese katana. The outer skin is forged from alternating layers of steel with differing carbon content, and folded upon itself many times, producing a mokume (wood-eye) pattern.

Since producing high carbon steel from wrought iron was very difficult, pattern welding also served to conserve this difficult-to-make steel by using it only for the parts of the blades where it was needed. Many swords were made with the minimum possible amount of high carbon steel along the cutting edge, with the rest of the blade being made of low carbon steel or wrought iron. A thin strip of high carbon steel could be laminated between two layers of softer steel, or a core of soft steel could be wrapped in high carbon steel. The Japanese katana was often found with complex patterns of soft and hard steels; having 5 sections of differing hardness welded together to form the final blade was not uncommon. The end result would be a blade with a very high carbon edge (as much as 1.0%, equal to the highest carbon content found in low alloy steels in use today) and a softer spine. The very hard, but brittle, edge made the swords extremely sharp, while the spine gave the blade flexibility, so that it would bend rather than break. The problem with the laminate design was that if the brittle edge steel chipped down to the soft core (a near certainty if swords were brought into edge to edge contact), the sword was ruined; grinding out the chip would expose the soft core and result in an unsharpenable blade. Correct technique for these blades was to parry with the softer, stronger sides or back of the blade, to protect the more fragile cutting edge.

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.

Decorative finish

The ancient swordmakers also 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 centre 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 512 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

• Wootz steel, an Indian crucible steel
• Bulat steel, a Russian crucible steel
• Japanese sword construction, an intricate method of pattern welding.
• Mokume-gane, a similar technique, often involving precious metals, used to produce decorative pieces

References

• 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

External links

• Bladesmith Kevin Cashen's page on pattern welding
• Ancient carburisation of iron to steel: a comment
• Mediæval Sword Virtual Museum, which contains close-up images of Viking swords, showing the pattern welding structures.