Disclaimer: Author is neither an expert in metallurgy nor Japanese swords. This is just a result of the author trying to educate himself.
Bronze Age China
The Bronze Age seems to be first apparent in China during Xia and Shang(approx 1,500-1,000BC) dynasties and possibly earlier(around 3,000 BC in western China). Archaeological work has revealed that other advanced Bronze Age cultures(in particular, the Sanxingdui culture), in modern China(Sichuan specifically), were contemporary to the Shang, Xia, and might be attested in earlier records as barbarian neighbors.
During the late Warring states period, the Qin, who were contemporary to Alexander the Great, still used bronze weapons, even when most of China were transitioning to Iron, and yet they conquered the rest of China. The Chinese Bronze Age seems to be unique in that it was later than, and lasted longer than the Mediterranean Bronze age, and had a long overlap with the Chinese Iron Age.
Some technologies such as the crossbow and differential bronze construction of swords seems to set the Chinese Bronze Age.(The Chinese seem to have also found tri-lobate arrows particularly useful against armor) My own theories regarding the length of the Bronze Age includes the fact that the Chinese didn’t suffer the collapse that the Mediterranean did, and given that they armed armies to the tens & hundreds of thousands, Bronze weapons were cheaper to construct.
An interesting development by the Qin seems to have been their meter long swords and chromium coating. I am not aware of bronze swords of similar length outside of the Qin and Hallstatt cultures.
The Iron Age in China
As previously mentioned, China stumbled onto iron a little later than Europe and the middle east(1-2 century BC), and had a long overlap between the adopting of Iron and the phasing out of bronze weapons. The Chinese ended up being really good at mass producing Cast Iron, which occurred as early as the 5th-6th century BC. Cast iron contains higher amounts of carbon than most steels used in weaponry, and thus was rather brittle. However, the presence of cast iron in China suggests that the Chinese were capable to producing fires hot enough to fully melt iron, something only achieved elsewhere in India in the pre-industrial world. This was probably achieved by harnessing the rivers to power bellows fanning fires to temperatures of 1420–1470 K. While early iron artifacts in China were likely cast, to make them usable as weapons, the cast iron had to be decarburized. By the time of the Spring and Autumn periods, weapons that were forged, rather than cast, started appearing. Techniques such as folding, in particular, “wrapping” or “bao-gang”(包鋼), were employed to optimize the hardness of the steel. These techniques were used through Chinese history into the Song dynasty.
With increased diplomatic ties between Japan and the Tang dynasty, Japanese started importing and recreating Chinese swords early in the middle ages, as attested by the existence of the chokuto. These swords, like their Chinese equivalent, were straight backed single edged swords of differential steel construction. However, it seems like sometime between the Heian and Kamakura periods, Japanese swords gained a curve. The curve resulted from the different speed of cooling of the different layers of steel during the quench; however, while the Chinese “corrected” this by straightening the sword afterwards, the Japanese eventually left it as is. These swords would eventually give birth to the tachi, katana, and the rest of the Japanese swords.
Japanese swordsmithing practiced today preserves many ancient Chinese metallurgical techniques. While the Japanese weren’t capable of fully melting iron like the Chinese, they achieved bloomery steel in the form of the Tatara furnace. The Japanese, not having access to good iron ores, used iron sand to feed their furnaces. What resulted from the Tatara was Tamahagane(jeweled steel), or pig iron, which was high in carbon and other impurities. It was up to the smith to sort out the pieces of steel that were low and high in carbon. These pieces were then assembled together, and folded several times to homogenize the steel. While there are a variety of ways Japanese swords were arranged, often, the hardened high carbon steel served as a “jacket” for the softer inner core.
The Katana in Context
The katana, when compared to swords around the world, stood out in that was that it was usually stiff and heavy for its size(a 4 foot longsword vs 3 foot katana usually weighted the same). This was because the katana was thicker and had little distal taper. Yet its secondary bevel meant that the katana couldn’t capitalize on using the width of the sword to maximize the thinness of the edge.
Differential Steel vs Mono Steel Construction
These techniques mirrored the use of high tin bronze in the edges and low tin bronze in the core during the bronze age. The Chinese, although technically capable of creating high quality mono-steel swords, didn’t do so. Rather than trying to achieve the production of high quality crucible steel like the Indians, the Chinese seems to have adopted a “brute force” technique of decarburizing steel and using steel of varying carbon content to create differential steel swords that were hard, but inflexible. This allowed for construction of stiff yet hard swords, which were very forgiving in cutting, but often stayed bent after a failed cut. This results as when a sword of differential steel construction hits a target, it has a harder edge and thus more cutting potential. But textiles, metal, and leather are rather cut resistant, so when a sword fails to cut, it forces to sword to flex. The differential constructions means that the different carbon content in the different layers of steel bends at a different rate, thus breaking the internal structure of the sword and staying bent. A Mono-steel sword, especially one of high carbon content and low impurities, would be able to hold a hard edge, yet flex when put under stress and be expected to return to its original shape.
East Asian Sword smithing compared to Pre-Industrial European
Metallurgical innovation in Europe seems to have been lead by the Celts, Iberians, and Germans. Differential steel construction in double edged swords seem to have been pioneered by the Celts, Germans, and Norse in the form of pattern welding since antiquity. Different strains of Iron seem to have dictated the quality of swords in the region. European societies near bogs and other marshy terrain could often find lumps of Bog iron, ore which had formed through the oxidization of the Iron in water. Particularly valuable ore seems to have been found in Spain and the Alps, thus the fame of Toledo and Noric steel. The quality seems to have primarily lay in the fact that these strains of iron had a specific alloy of metal the specifics of which are beyond me. The swords of medieval Europe transitioned to a mono-steel construction. The durability of these swords served them well as the increasing availability of maille, padded, and plate armor meant that swords took quite a beating in combat.
May the Real Super Sword Come Forward
If any swords could claim the place of the super sword of history, it would have to be the swords made of wootz and other crucible steel. wootz steel, also referred to as Ukku and Seric steel, originated in Southern India near Kerala. High quality high carbon steel had been reported by Greeks, Romans, Arabs, and Chinese since antiquity, and was sought after throughout the world. A rather famous case of exported wootz steel are the Ulfbert swords found throughout Northern Europe during the Migration period. These swords were expertly forged with imported Indian ingots of wootz, either by Frankish or Norse smiths. The quality of these swords were made famous by the crusaders, who took note of their opponents’ near mystical swords, giving rise to the term, Damascus steel, not to be confused with the modern misappropriated term for pattern welding. Similar steels were found in the Eurasian Steppe and Russia, often referred to as Bulat steel. Wootz however, was unique in the pattern that was produced after forging, often referred to as a “watered” pattern.
Magnetite ore was placed with carbon in a clay sealed crucible which would’ve been placed in furnaces with temperatures hot enough to fully melt the Iron, allowing for an even distribution of carbons and other inclusions, with most of the impurities rising to the top as slag.
Modern analysis of wootz has revealed the presence of carbon nanotubes, with the trace elements/impurities of vanadium, molybdenum, chromium etc contributing to its unique pattern. Wootz steel was unmatched in quality until the advent of the industrial steels, and as the specific vein of iron ran out, so did the watered pattern that made wootz famous.
Swords have been a symbol in cultural, political, and religious contexts. Often, they reach legendary status that blurs reality and fiction. While the katana is culturally significant for the Japanese, it is not the machine gun barrel slicing skull cleaver that many make it out to be. Its conservative design and peculiar cultural idiosyncrasies such as a lack of a significant distal taper and a differential steel construction might limit its abilities, but these characteristics serve as a reminder of a thousand years of Japanese sword smithing heritage and its value as a work of art.