When we talk about civilization the first thing have to discuss is the history of Iron and Steel. We take many of the things for granted these days. Everything we build is dependant on these often used and perhaps, unappreciated, materials. The fact remains that these materials are so significant that we have named entire periods of human history after them. From the stone age to the bronze age, the information (space) age. They have all been made possible by the materials we have at our disposal.
Some of the earliest evidence of iron being used as material goes back as far as 3500 BC in Egypt. This is when beads of iron taken from a meteor were found. Meteoric iron was a highly prized material due to its heavenly association. Tutankhamun was buried with a dagger made of the material. But meteoric iron was the only naturally occurring source of iron at the time.
The history of Iron and Steel begins in Space
There’s no oxygen in space so meteors delivered this material to earth in a form humans could use without having the technology to extract it from it’s ore. It could be said that the Iron age began independently across the world.
However, it is likely that the technology for producing Iron spread from Egypt to the rest of the world. Production of Iron from Iron Ore begins in c1300BC in the Middle East. Active production being seen in Jordan around 1000BC. This then spread to Turkey in the near east, then to Greece and the rest of Europe. Reaching Britain and France by around 800BC. It did not reach Ireland until approx 500BC.
If we continued to define human history by the materials being mastered at that time, it could be argued that the Iron age lasted right up until a little over 150 years ago, when steel was first mass-produced. That’s almost 5500 years!
What’s the difference between Iron and Steel?
Aside from the fact that Iron is an element that occurs naturally and Steel is an alloy. The main difference between Iron and Steel is the amount of carbon they contain. Anything with a carbon content above 2% is cast iron. Higher carbon content typically results in a harder and less ductile material. Cast iron has a very high carbon content, which makes it very hard, but also very brittle.
Additives steel give in an incredible range of properties. From hardness to anti-corrosive properties found in the various stainless steels. But for the sake of simplicity, Steel is far more durable than Iron, it has better tension and compression properties.
The transition from Bronze to Iron to Steel?
Iron started to become popular and early bronze cannons were replaced with cast iron. Iron was cheap to manufacture and could be fired more often without being damaged. However, these material properties meant that cast iron cannons had a tendency to explode without warning. This made them dangerous to operate.
Cast Iron is not suited for structural use either. In fact, its use in bridges in the mid 19th century led to a series of bridge collapses. Later these bridges and future bridges were rebuilt using wrought iron. Wrought iron contains less than 0.08% carbon, which makes it a much better material for structural applications. This makes it ductile, allowing it to bend under loads without breaking. But it has a low carbon content, which makes it a lot softer than cast iron.
Steel is between the two with a carbon content between 0.2 and 2 percent. Giving it an ideal balance between hardness and ductility. The history of Iron is defined by our ability to control that carbon content.
Iron is the 4th most common metal on earth, just below aluminum. But it reacts with oxygen readily to form iron oxide ores. Rust is one form of iron oxide and preventing it is a constant struggle in structural maintenance. The ever rusting Eiffel tower has been painted 17 times since it’s construction to protect it from corrosion.
Every 7 years about 60 tonnes of paint is applied to the tower. And the color of the paint has changed over the years. The tower was originally a Venetian red and has changed a few times from a more yellowish-brown to a chestnut brown until the adoption of the current, especially mixed “Eiffel Tower Brown” in 1968. Strangely people still associate the color red with the tower. Perhaps because of that other metal structure, the Golden Gate Bridge.
How is Iron made?
Because Iron reacts so easily with oxygen to form iron oxide it does not exist on the surface of the planet in a usable form. The first step to process iron is to remove that oxygen. In the mid bronze age, the first signs of production of Iron are seen. Most of this early iron was smelted in these furnaces called bloomeries.
These bloomeries heat the iron ore using charcoal as a heat source. The burning of charcoal produces carbon monoxide, which reacts with the iron oxide in the ore to form carbon dioxide and iron. The bloomery is heated above the melting point of the impurities, but below the melting point of iron.
And so as the fire rages, material falls to the bottom of the bloomery and the heavier iron consolidates at the bottom, while the impurities form a molten pool called slag, which can be drained away.
When the iron is removed it is in the form of this porous mixture of impurities and iron. It needs to be worked with a hammer to consolidate the iron, while the waste material is beaten off. The material left over is wrought very small quantities of iron especially before the waterwheel was introduced to drive the bellows, which allowed the bloomery to grow in size while keeping the temperature high enough.
Industrial Iron Ore Smelting
Despite the small quantities, it produced the bloomery revolutionized human life, even beyond the obvious military advantages of iron weapons. Iron ore is much more common than the copper and tin that spurred the bronze age, allowing iron to be produced in many areas. These communities could make their own tools and weapons without having to import the material from abroad.
Iron plows were stronger and heavier allowing farmers to plow their land quicker and thus grow more food. Likewise, iron scythes could cut more hay. A single farmer could feed more people, allowing more people to dedicate their lives to different trades. Society was becoming more stratified and trade was increasing and things began to accelerate even more as we discovered better ways of extracting iron, like the blast furnace.
It’s the Blast Furnace
Blast furnaces increased the production of iron dramatically. These furnaces heat the iron above it’s melting point along with flux materials. Flux is a chemical that will combine with the impurities allowing them to be extracted easily, in this case, the iron ore is mixed with limestone and coke.
The furnace gets its name from the method that is used to heat it. Pre-heated air at about 1832°F is blasted into the furnace through nozzles near its base. The largest blast furnace in the US and the Western Hemisphere is Ispat Inland’s No. 7 blast furnace in Chicago, it produces 10,000 tons of steel per day.
Coke is a refined form of coal with very little impurities and it works similarly to the charcoal in the bloomeries by producing carbon monoxide when burned, which in turn reacts with the oxygen in the iron ore to remove it, as shown before.
This extraordinary metal, this iron, the soul of every manufacturer, and the mainspring perhaps of civilized societySamuel Smiles
Solving the problem with impurities
The heat from the process decomposes the limestone into calcium oxide and carbon dioxide. The calcium oxide then reacts with the silica impurities in the ore to form calcium silicate. This along with other impurities forms a liquid slag layer that floats on top of the heavy molten iron, which can be drained away.
This method allowed vast quantities of ore to be converted to iron quickly, but it has a drawback. At higher temperatures iron readily absorbs carbon. So the iron created in blast furnaces has a very high carbon content, making it cast iron. So an extra step is needed to decrease the carbon content to produce iron or steel.
This can be done in a number of ways. Refineries heat the iron back up to oxidize the carbon. It would then be beaten with a hammer to knock the oxidized carbon out of the material, to produce wrought iron once again.
There were methods of producing it, but the small yield and time needed making it expensive. One way, which small quantities were being produced by was to mix wrought iron and cast iron in a sealed crucible, which prevented atmospheric carbon from entering the material.
How Steel is produced
Though we think of steel as a modern invention there is proof that it was actually being made in Turkey by 1800BC, you can read more about that in our Guide to Steel.
The primary method for producing steel on an industrial scale came much later. It involved heating wrought iron with charcoal. It was then left for up to a week to allow it to absorb the carbon. The time and fuel needed to do this was far too expensive and not suitable for general industrial use.
Wrought iron was now being produced at an industrial scale, but a method for mass-producing steel was still not available.
With the expansion of the railroads in the early 19th century the pressure to develop a faster and cheaper method for producing steel was growing. All our modern rail tracks are made from high strength steel. Its superior hardness over wrought iron allows it to resist wear. Steel also has superior strength over wrought iron, allowing it to carry more load.
Mass Production of steel
So you can see why finding a method of mass production was so important. And this is where the British Metallurgist Sir Henry Bessemer came in. Bessemer created a converter where molten iron was poured from a blast furnace and hot air is passed through the bottom. This oxygen in the air oxidizes the impurities in the iron. The carbon reacts to form carbon monoxide which is expelled as a gas.
While the silicon and manganese, oxidize to form a layer of slag. This process was very fast. In fact, early on it was a victim of its own efficiency. As it removed too much carbon and left too much oxygen in the iron. To combat this another alloy, containing iron, carbon, and manganese called spiegeleisen was added.
The manganese would react with the oxygen to remove it and the carbon increased the carbon content as needed. But it had another problem in the early days. The process did not remove phosphorus from the iron and high concentrations of phosphorus make the steel brittle. So initially the Bessemer converter could only be used with iron obtained from ores with low phosphorus concentrations, which were scarce and expensive.
The end of the history of iron and steel?
This problem was later solved by Welshman Sidney Gilchrist Thomas. Thomas discovered that adding a material such as limestone to the process would draw the phosphorus into the slag.
This availability of cheap steel caused an explosion in growth in the rail industry. Steel is so vital to our daily lives, that it is often considered a measure of the economic success of a country. High production of steel means high demand for steel, high demand means your country is building infrastructure.
For example, the above graph shows China’s steel production from the 1990s to the present. It closely follows the rapid rise of China as a global superpower during its economic reform. Here in the US, we have had steady production for the past few decades. But the hope is that with a new surge in US manufacturing this will start to increase.
Without steel, our buildings could never have grown to the heights we see today. Bridges like the famous Golden Gate Bridge would not have been impossible. Futuristic materials are even giving way to steel. As it is being used to make a new generation of spacecraft taking us to the furthest reaches of our solar system and beyond.
Of course, Iron in both Cast and Wrought forms is still very much in use. Ask a chef and they’ll tell you that there’s nothing better than a heavy cast-iron skillet. The history of iron and steel is a fluid one and even after 5500 years, we are still finding uses for it.