Wrought iron

Wrought iron is a specific type of iron alloy defined by its very low carbon content. While cast iron contains between 2.1% and 4.5% carbon, wrought iron contains less than 0.05%. This small amount of carbon makes a massive difference in how the metal behaves. Wrought iron is highly malleable, which means it can be easily bent or shaped. In contrast, cast iron is brittle and will break if you try to shape it. Because it is easy to work, it was once the most common form of malleable iron in the world.

The manufacturing process for wrought iron is a method of refining high-carbon cast iron. This is often done through a process called puddling. In a puddling furnace, high-carbon cast iron is heated and melted in an open hearth. As the metal heats up, the excess carbon reacts with oxygen in a process called oxidation. To help this happen, a worker must stir the metal with a long rod. This stirring is called puddling. As the carbon leaves the iron, the melting point of the metal begins to rise. Eventually, the melting point becomes higher than the heat of the furnace can reach. Because of this, the wrought iron never becomes a fully liquid metal. Instead, it remains a semi-fused mass.

Wrought iron is not a perfectly uniform substance. It is a mixture of roughly 99.4% pure iron and up to 2% silicate slag. The slag consists of tiny, glass-like inclusions that are spread through the metal. When the iron is hot-rolled into long bars or rods, these inclusions form long, thin layers. This creates a unique, fibrous structure that looks very much like the grain in wood. This grain is visible if the metal is etched, rusted, or bent until it fails. Because these slag layers create planes of weakness, the metal is anisotropic. This means its strength is not the same in every direction; it varies depending on its orientation.

History shows that different cultures developed ways to produce this metal. In China, during the Han dynasty (202 BC – 220 AD), people used fining hearths. They used these to create wrought iron tools like the iron plough and multi-tube seed drills. In Europe, the puddling process was patented in 1784. This allowed for much larger scales of production. By 1876, the United Kingdom was producing over 4 million tons of puddled iron every year. The demand for wrought iron peaked in the 1860s. During this time, it was used heavily for railway use and ironclad warships.

There were many different shapes and types of wrought iron used in industry. Bar iron was a common form used as a raw material for further work. From bar iron, workers could create rod iron for making nails and spikes. They also made hoop iron, which was used for the hoops on barrels. For heavy construction, they produced plate iron for boilers and thinner blackplate. There were even specific types like voyage iron, which was sold in Africa for the Atlantic slave trade. The weight of these bars changed over time. In the 1660s, there were 70 bars per ton, but by 1731, there were nearly 92 bars per ton.

While wrought iron was incredibly useful, it had specific chemical requirements. To be considered good quality, it needed to have very low levels of impurities like sulfur, phosphorus, silicon, and manganese. If the sulfur content was too high, the iron became "redshort" or "hot short." This meant it would crack when worked at a red heat. If it had too much phosphorus, it became "cold short." This made the metal very brittle when it was cold, though it could still be worked at high temperatures. Interestingly, some high-phosphorus iron was used to make antique music wire because it could be easily drawn into wire.

The use of wrought iron declined as technology improved. The development of the Bessemer process and the Siemens–Martin process made steel much cheaper to produce. Modern mild steel, or low-carbon steel, is now the functional equivalent of wrought iron. While many people call garden furniture or gates "wrought iron," they are usually made of mild steel. They are only called that because they are made to look like the decorative items once worked by hand by blacksmiths. Today, wrought iron is no longer produced on a commercial scale, but its history remains a vital part of how we learned to shape the world.