History of metallurgy in China
|History of science and
technology in China
Bronze and its Components
The use of copper in ancient China dates to at least back to 2000 BC., though it had to have been known earlier, because bronze artifacts that are older have been discovered, and bronze cannot be made without copper. The reason for the lack of earlier evidence of copper is that it was both highly valuable and reusable, so early copper could have been recast as early bronze.
Although bronze artifacts were exhumed in the archeological sites of Majiayao culture (2300 BC to 2700 BC), it is still commonly accepted that China's Bronze Age began from around 2100 BC during the Xia dynasty. The Erlitou culture, Shang Dynasty and Sanxingdui culture of early China used bronze vessels for rituals as well as farming implements and weapons. By 1500 BC, excellent bronzes were being made in China in large quantities, partly as a display of status, and as many as 200 large pieces were buried with their owner for use in the afterlife, as in the Tomb of Fu Hao, a Shang queen.
At the tomb of the first Qin Emperor and multiple Warring States period tombs, extremely sharp swords and other weapons were found which were coated with chromium oxide, which made the weapons rust resistant. Chromium only came to the attention of westerners in the 18th century. The alloys of tin and copper enabled weapons such as bronze knives and swords to avoid rust and remain sharp in spite of 2000 years of degrading conditions. The layer of chromium oxide used on steel swords was 10 to 15 micrometers and left them in pristine condition to this day.
Cast-iron artifacts are found in China that date as early as the Zhou dynasty of the 6th century BC. An Iron Age culture of the Tibetan Plateau has tentatively been associated with the Zhang Zhung culture described in early Tibetan writings. In 1972, near the city of Gaocheng (藁城) in Shijiazhuang (now Hebei province), an iron-bladed bronze hatchet (铁刃青铜钺) dating back to the 14th century BC was excavated. After a scientific examination, the iron was shown to be made from meteoric iron.
Around 500 BC, metalworkers in the southern state of Wu developed an iron smelting technology that would not be practiced in Europe until late medieval times. In Wu, iron smelters achieved a temperature of 1130°C, hot enough to be considered a blast furnace. At this temperature, iron combines with 4.3% carbon and melts. As a liquid, iron can be cast into molds, a method far less laborious than individually forging each piece of iron from a bloom.
If iron ores are heated with carbon to 1420–1470 K, a molten liquid is formed, an alloy of about 96.5% iron and 3.5% carbon. This product is strong, can be cast into intricate shapes, but is too brittle to be worked, unless the product is decarburized to remove most of the carbon. The vast majority of Chinese iron manufacture, from the Zhou dynasty onward, was of cast iron. Iron, however, remained a pedestrian product, used by farmers for hundreds of years, and did not really affect the nobility of China until the Qin dynasty (ca 22 ad).
Cast iron is rather brittle and unsuitable for striking implements. It can, however, be decarburized to steel or wrought iron by heating it in air for several days. In China, these ironworking methods spread northward, and by 300 BC, iron was the material of choice throughout China for most tools and weapons. A mass grave in Hebei province, dated to the early third century BC, contains several soldiers buried with their weapons and other equipment. The artifacts recovered from this grave are variously made of wrought iron, cast iron, malleabilized cast iron, and quench-hardened steel, with only a few, probably ornamental, bronze weapons.
During the Han Dynasty (202 BC–AD 220), Chinese ironworking achieved a scale and sophistication not reached in the West until the eighteenth century. In the first century, the Han government established ironworking as a state monopoly and built a series of large blast furnaces in Henan province, each capable of producing several tons of iron per day. By this time, Chinese metallurgists had discovered how to puddle molten pig iron, stirring it in the open air until it lost its carbon and became wrought iron. (In Chinese, the process was called chao, literally, stir frying.) Metal casting was spread westwards to the Dayuan by Han deserters (Shiji, 123).
Also during this time, Chinese metallurgists had found that wrought iron and cast iron could be melted together to yield an alloy of intermediate carbon content, that is, steel. According to legend, the sword of Liu Bang, the first Han emperor, was made in this fashion. Some texts of the era mention "harmonizing the hard and the soft" in the context of ironworking; the phrase may refer to this process.
In the Korean Peninsula, iron objects were introduced through trade just before the Western Han Dynasty began (c. 300 BC). Iron ingots became an important mortuary item in Proto-historic Korea. Iron production quickly followed in the 2nd century BC, and iron implements came to be used by many farmers by the 1st century AD in Southern Korea.
Shen Kuo's written work of 1088 contains, among other early descriptions of inventions, a method of repeated forging of cast iron under a cold blast similar to the modern Bessemer process
Chinese metallurgy was widely practiced during the Middle Ages; during the 11th century, the growth of the iron industry caused vast deforestation due to the use of charcoal in the smelting process. To remedy the problem of deforestation, the Song Chinese discovered how to produce coke from bituminous coal as a substitute for charcoal. Although hydraulic-powered bellows for heating the blast furnace had been written about since Du Shi's (d. 38) invention of them in the 1st century CE, the first known illustration of a bellows in operation is found in a book written in 1313 by Wang Zhen (fl. 1290–1333).
Gold and Silver
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