Iron is more plentiful than bronze but was not generally used until about the 2nd millennium B.C. Iron does not melt until about 1,537 degrees Celsius. Iron has a very high melting point, since most early furnaces could not reach this melting point so that the impurities in iron could be separated; the iron that was produced was full of impurities. However, the early metal workers could produce a purer form of iron by making bloom iron. Bloom iron was made by early metal workers, who would heat iron until it became a mushy substance, then the workers would hammer the slag, the unwanted materials, out of the iron.
Iron replaced the widely used bronze in the second millennium B.C. There is archaeological evidence that iron was used for a variety of purposes by various cultures before this time period, but bronze was used more often because bronze is harder than iron. Why early people shifted from using the metal bronze to iron, has to do with a discovery that the Hittites made in the second millennium B.C. When making iron bloom, the iron has to be kept between 1200-800 degrees Celsius in order to be able to hammer the iron. During the continual process of heating the iron, carbon from the fire and from the air would attach to the surface of the iron. When iron and carbon mix together, steeled iron was formed. Through a long process of trial and error the Hittites figured out how to make steeled iron a stronger material than bronze.
Steeled iron had a major impact on the ancient world. Imagine soldiers carrying steeled iron swords and shields facing an enemy carrying bronze weapons. The steeled iron carrying soldiers would easily cut through the bronze weapons of their enemies. The discovery of steeled iron enabled the Hittites to dominant central Asia for some time. After the discovery of steeled iron early metal workers also discovered that if they were to dip the heated metal into cold water they could also increase the hardness of the material, this is called quenching. Quenching also made steeled iron brittle, but early people were able to figure out how to overcome this obstacle. By briefly reheating the metal, they could make the metal less brittle. This reheating process is known as tempering. These metal working techniques slowly spread from Central Asia to the rest of the world.
The Chinese learned these iron-working techniques around 1,000 B.C. The Chinese using their knowledge of bronze metalworking were able to melt iron, an achievement that other civilizations would not be able to do for a millennium. When the Chinese were still using bronze they had made developed furnaces that could achieve high temperatures, high enough temperatures to melt iron. The Chinese could make objects out of cast iron. Being able to make objects out of cast iron made mass production possible. The Chinese could pour cast iron into clay molds and easily make many identical tools very quickly. Having abundant and available tools led to major accomplishments in China’s agricultural and irrigational system. Cast iron was harder than bronze but was still brittle. The Chinese used it for a variety of purposes, from crossbows to plows. They figured out how to make wrought iron, so that their iron tools were less brittle. The Chinese somehow figured out how to expose iron to air, which exposed iron to copper, making wrought iron. They then learned that if they took wrought iron, re-melted the wrought iron, stirred it, and exposed it to air so that they could make iron-steeled objects.
Written by Alex Boyce, 2003