This post is a summary of Dr. Lee A. Jones' paper entitled, Blade Patterns Intrinsic to Steel Edged Weapons. Most of the text below were quoted verbatim, so as not to distort what Dr. Jones said.
When we look at modern industrially produced iron and steel, little or no texture is readily apparent without magnification, even when the objects have been corroded. Earlier iron and steel artifacts will frequently show a pronounced grain structure, often due to impurities banished from or tightly regulated in modern materials. In the case of antique edged weapons, the smiths frequently manipulated the grain properties and joined together dissimilar materials to achieve desired performance and/or appearance. Such patterns often yield clues to how such items were made. Wrought iron - Wrought iron contains silica strands as a result of the manufacturing process. In the processes of fabricating the wrought iron into the semifinished forms supplied by the manufacturer, whether by hammering or rolling, a grain structure arises as the impurities move along with the metal. Etching or corrosion will disclose this grain (see "A" above). Wootz steel (true Damascus) - These blades are directly forged from a small cake of heterogeneous steely iron which traditionally was produced in India. The steel, which is called wootz, was produced by heating iron ore, charcoal, and vegetable matter in a crucible for a prolonged period of time. This would produce a fairly high carbon steel, indeed sufficiently high carbon that special handling was necessary in forging the blade if fractures were to be avoided. The above close-up of such a blade ("B" above) shows a wavy pattern of shiny and dark steel. These patterns are made up of networks of steel showing different metallographic structures. Coarsely laminated steel (e.g., in keris) - Laminated steel pattern is achieved by using contrasting materials welded together in sandwich fashion and then folded back over upon themselves a few (six in the example given by Solyom (1978), pp. 9-10) times to yield a few thousand layers. This is very similar to the fabrication of the Japanese sword, except that the number of layers is smaller in the keris, yielding a bolder pattern ("C" above). A similar effect can be seen in an Indian tulwar ("D" above). Finely laminated steel - Japanese sword blades show a variable grain structure ("E" above) which arises as a result of folding the billet from which the sword is being formed back upon itself many times (up to, but usually fewer than twenty, which would give just over a million layers). The concept here is simple: when dealing with steel of varying composition, maximum strength may be achieved by averaging weak and strong areas by forming a laminated structure. Finely laminated steel with differential heat treatment - The above photograph ("F") is a detail of a Japanese dagger (tanto) blade of the 17th century. In the upper half of the picture we again see the wood grain pattern which results from the many layers of steel making up the blade. Additionally, a wandering misty pattern of increased brightness (nioi) may be seen about one-third of the way back from the edge which represents the "tempering line" (hamon). In Japanese swords, the differential tempering is achieved by a technique in which the swordsmith applies varying thicknesses of different types of clay to the surface of the blade before it is quenched. Thicker and more insulating clay over the back and body of the blade slows cooling and results in less brittle and more malleable iron there, while the edge cools more quickly and is harder, but also more brittle. In this way, a hard sharp edge may be part of a sword without excessively increasing the brittleness of the entire blade. Piled blade structures - The wide blade of the above Philippine barong shows several bands of contrasting patination ("G") which converge at the tip and towards the hilt where the blade narrows. Such piled structures most likely naturally evolved from the need of smiths to join together multiple pieces of steel to form a larger fabrication such as a blade. The piled structure also has the potential to bring significant advantages in durability and localization of desired characteristics (hardness at the edge and fracture resistance at the back and center). The other example ("H") is that from an Indian dao sword. Mechanical damascus (pattern welding) - The above photograph ("I") is a detail from the blade of a yataghan and is characteristic of Turkey and the areas surrounding it. In addition to laminated (layered) steel on its edge and back, the blade shows three bands of patterned steel running the length of the blade. The pattern, created from a twisted rod incorporated into the blade for each band, repeats itself along each band with slight changes as curved patterns merge into a zone where the appearance is more that of diagonal lines. Similar patterns may also be seen in a keris. The twisted rod technique by which this pattern has been achieved is called pattern-welding and is essentially identical to that used in medieval Europe during the Migration Period and Viking Age.
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