How to Make a Sword (Part One of Two) by The Mirror
This column was written by Katherine Derbyshire, Shobu Aikido of Boston
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First of two parts
In the modern world, steel is a commodity. Steel cars are in every garage, steel knives in every kitchen, steel tools no further away than a trip to the hardware store. This abundance is largely a result of the Industrial Revolution. At the dawn of the Iron Age, the only way to get steel was to make it yourself. For millennia after that, it remained a rare and expensive material. This article explains how iron ore is transformed into steel, particularly swords. Part Two discusses what to do with a sword once you have one.
Steel is an alloy of iron, carbon, and perhaps trace metals like nickel or trace impurities like sulfur. To make it in a modern blast furnace, you start with iron ore and carbon, usually in the form of coal.
Fe2O3 + 3CO -> 2 Fe + 3CO2
That is, the coal extracts the oxygen from the iron ore as it burns, creating an alloy of iron with 3-5% carbon. In a modern blast furnace, smelting happens at a temperature of several thousand degrees Fahrenheit. The iron melts into a liquid, while a catalyst like crushed limestone draws out impurities in the form of slag. The slag floats; the iron sinks and is drawn out as pig iron. A second furnace uses oxygen to burn off the excess carbon - most modern steels have less than 1% -- and adds alloying elements as desired. The molten steel from this furnace is cast into ingots, ready for further processing to make the desired end product.
Early steelmakers didn't have blast furnaces. Nor did they have motorized cranes and other equipment to move giant ladles full of molten metal. The first steel ingot was cast in 1742; the first blast furnaces were built in the 15th century. Before that, small hearths refined small amounts of ore to make low quality pig iron, which often contained slag inclusions. A modern replica of such a furnace is described at http://www.bradford.ac.uk/acad/archsci/depart/resgrp/amrg/Rievaulx02/Rievaulx.htm.
Available ores would have included recycled scraps of metal, iron-bearing minerals like hematite and magnetite, and meteorites. Iron meteorites, which are relatively pure and contain high concentrations of nickel, would have been especially valuable. Broken pieces of weapons and armor would have been valuable to battlefield scavengers as metal, even if damaged beyond use.
To make steel from pig iron, you have to control the carbon content. With no carbon at all, iron is very soft. With carbon concentrations above 3% or so, it becomes hard and brittle. To carburize pig iron without modern equipment, steelmakers wrapped iron bars in charcoal and heated them to dissolve carbon into the metal. The process was hard to control, could only treat relatively small pieces, and took several hours or even days.
Slag inclusions-impurities left over from the smelting process-- are also undesirable in steel. They can create brittle spots and other defects that would weaken the finished product.
To achieve consistent carbon concentrations and reduce slag content, many ancient smiths used a series of hammering and folding steps. Heat a metal bar until soft, hammer it smooth, fold over, and repeat. These steps helped mechanically remove impurities and maintained a consistent chemistry throughout the piece. Charcoal fires provided both heat and carbon.
All this hammering and folding had another effect. It created hundreds of microscopic layers within the steel. These layers, which alternate hard and soft forms of steel, make the metal strong, but ductile enough to bend without breaking. This laminar structure is found in traditional Japanese katanas, in Damascus steel (which was actually made in India), in the Indonesian kris, and in blades throughout the ancient world.
The hardness of steel depends on both the carbon content and the heat treatment. Heat a steel bar to red hot and allow it to cool slowly in air, or even in the furnace, and it will reach a steady state in which the entire bar forms a soft structure called austenite. Take the same red hot bar and drop it into ice water, and you'll get a hard, but brittle structure called martensite. In between, other structures can form at different temperatures and cooling rates.
For a sword, neither extreme is desirable. Austenite is too soft to hold an edge. Martensite is too hard to absorb the impact of combat. Ancient sword smiths used a variety of methods to control the balance between hardness and softness. They used precise (and highly secret) sequences of hammering and folding steps, with esoteric (and supposedly magical) quenching baths with ingredients such as urine, blood (human or otherwise), clay, or brine. Damascus steel smiths were rumored to quench their swords in the bodies of prisoners.
The methods used by classical Japanese sword smiths are still followed today, and as a result are among the most well-documented. There's a photo essay at http://budogu.com/html/smith_1.html.
A katana was, and is, a composite structure. The blade is roughly wedge-shaped, tapering from the back of the blade to the edge, and from the tang to the point. The core of the wedge is a softer, low-carbon steel. The outer part of the wedge is a high-carbon, laminar steel, folded around the core and welded with heat and hammering. Then the whole is shaped (without modern machine tools) to form the blade. This step controls the balance of the blade, and also ensures that it is straight, rather than warped or lopsided. While the hard work of hammering and folding the steel might have been left to apprentices, shaping the blade requires the master swordsmith's personal attention.
Next, the smith coats the blade in clay, ash, and other materials, and heats it. The combination of the clay coating and the variable thickness of the blade ensure that the edge cools more rapidly than the back of the blade. The result is a hard edge, backed by a softer, resilient core, but not yet a usable sword. Finally, the sword polisher, a second expert craftsman, polishes the blade with successively finer grits of sand to achieve the katana's famous mirror finish and razor edge.
Fine swords have never been cheap. They were given as gifts to loyal and heroic officers, handed down from father to son, buried with the family jewelry to protect them from invaders. Because swords were so valuable, many of the people on ancient battlefields didn't have them. They used bows and slings, spears or pikes, even clubs. Commoners in particular were unlikely to be able to afford swords of their own, or to be trusted with them by their officers. Though the Japanese sword arts are rooted in battlefield experience, they evolved into their modern form in a far different environment. The establishment of the Tokogawa Shogunate after Battle of Sekigahara in 1600 essentially ended battlefield combat in Japan. Still, as Part Two discusses, the basic principles of sword use hold in any environment.
© 2004 Katherine Derbyshire.
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