The Chinese have been using blast furnaces to smelt iron since the 4th century BC. This advanced step was made possible due to several factors. China has a very good type of refractory clay for building blast furnaces. The Chinese also discovered how to lower the melting temperature of iron. They introduced a substance into the blast furnace that they called “black earth”, which contained a high amount of iron phosphate. By adding 6% phosphorus to the iron mixture, the normal melting temperature of 1,130 degrees Celsius could be reduced to 950 degrees. This technique was utilized in early China but was eventually abandoned as improvements in blast furnace technology made it unnecessary for steel production.
Coal for higher temperature was used as fuel for blast furnaces at least since the 4th century AD. One method of smelting iron involved placing iron ore in long tubular structures, covering it with coal, and igniting it. This technique also had the advantage of eliminating sulfur from the smelting process.
The widespread use of cast iron in ancient China led to many significant consequences, such as the adoption of cast iron plows in agriculture along with other iron tools like rakes. During that time, there were also iron knives, scissors, axes, saws, and awls. Food was cooked in cast iron pots, and even toys were made of cast iron. In Han dynasty tombs from the 2nd century BC to the 2nd century AD, small cast iron figurines shaped like animals were found. Additionally, cast iron molds dating back to the 4th century BC have been discovered, likely used for casting hoes and axes made of bronze or iron.
The art of fine cast iron casting allowed for the production of pots and pans with very thin walls, something other iron forging techniques could not achieve. These thin-walled pots became essential for mass-producing table salt by evaporating brine, which led the Chinese to exploit natural gas through deep drilling methods to harness energy for salt production. This industry expanded significantly when the Han dynasty nationalized it along with the iron industry in 119 BC. Thus, the salt production and natural gas extraction industries could not have emerged without the iron smelting trade.
In the 3rd century BC, the Chinese discovered that they could make cast iron more malleable by annealing (which means holding the iron at a high temperature for about a week). Annealed cast iron would not be overly brittle, able to withstand impacts, even strong ones. This meant that plow blades would not break when hitting large rocks. Annealed cast iron has a malleability close to that of wrought iron but is much more durable and harder due to its casting process. It is nearly as good as steel.
Some achievements in ancient Chinese cast iron technology reached extraordinary levels, almost unbelievable even when seen with the naked eye. For example, the Yuqing Pagoda in Dangyang (Hubei) is entirely made of cast iron and was constructed in 1601, standing 13 meters tall. In 659 AD, Empress Wu Zetian commissioned an octagonal cast iron column, known as the Great Zhou Column, commemorating the virtues of the Zhou dynasty among vassal states. The column rests on a 6-meter high cast iron base with a circumference of 51 meters. The column itself has a diameter of 3.6 meters and stands 32 meters tall. At the top of the column is a 3-meter high “cloud canopy” with a circumference of 9 meters, supporting four bronze dragons, each 3.6 meters tall, each holding a gilded pearl. Today, it is known that the total weight of the metal used for this project is approximately 1,345 tons.
The largest single-piece cast iron object (the pagodas are not solid pieces) was commissioned by Emperor Shizong of the Later Zhou dynasty (951 – 960 AD) to commemorate the victory over the Xiongnu in 954 AD. This extraordinary statue, known as the Great Iron Master in Shangzhou (Hebei), stands 6 meters tall and has a wall thickness ranging from 4 to 20 centimeters.
