People have always loved metals. From the fortuitous finding of meteoric iron and native gold and silver by our ancient ancestors, to the highly refined and versatile array of modern alloys, they have all captured our imagination and been key to our technological and cultural progress.
Our first contact with metal was in the finding of gold and silver. These metals occur natively on our wonderful planet, that is to say, in the metallic form that we know and love, and have been treasured and worked into jewellery for many thousands of years. Most metals are only found as ores – rocks containing minerals of metals combined with other chemical elements – which must be processed to extract the useful metal.
Native gold. Image by: http://www.photomacrography.net/
The melting points of lead and tin (327°C and 232°C respectively) are lower than the heat inside a decent wood-fuelled campfire. This meant that, perhaps by accident, our prehistoric ancestors discovered the art of smelting – the extraction of metals from their ores by heating them in a fire. What actually happens, where an ore contains an oxide of the metal, is that carbon in the fuel combines with the oxygen in the ore to create carbon monoxide gas and release the metal from its ore. Brilliant! Where ores contain sulphides of the metal, like the common lead and copper ores; the ore must first be roasted. Roasting, which is simply exposing the ore to very hot air, causes the sulphur in the ore to combine with oxygen in the air to create sulphur dioxide gas, leaving an oxide of the metal behind. The resulting oxide can then be smelted to extract the metal. In the early days both of these processes occurred in the same fire.
And for all those wonderful people who love chemistry, here are the balanced equations:
Roasting of Galena (lead sulphide ore) to produce lead oxide:
Lead Sulphide (2PbS) + Oxygen (3O2) → Lead Oxide (2PbO) + Sulphur Dioxide (2SO2)
Smelting of lead oxide to produce lead:
Lead Oxide (2PbO) + Carbon (C) → Lead (2Pb) + Carbon Dioxide (CO2)
There is evidence of the smelting of both lead and tin in the Near East and even the casting of lead beads in Çatal Höyük, Anatolia (modern Turkey) dating back as far as 6500 BCE! As far as I can tell, that is the very beginning of metallurgy.
By 5000 BCE the Vinča culture in Belovode in modern Serbia had smelted copper. With a melting point of 1085°C, the extraction of copper from its ore required superior technology to a wood burning fire. By 4500 BCE we can see evidence of such an advance; in furnace design in Pločnik (again in modern Serbia and again, achieved by the Vinča culture). The archaeologists who excavated the site at Pločnik said,
“the sophisticated furnace and smelter featured earthen pipe-like air vents with hundreds of tiny holes in them and a chimney to ensure air goes into the furnace to feed the fire and smoke comes out away from the workers”.
Not only did they come up with a new furnace design, but, around the same time, they also discovered that by adding some tin (roughly 12%) to their copper, they could produce a substance of superior material properties. They had created bronze and discovered the wonders of alloying! In 4500 BCE!
The next two thousand years saw the extremely slow spread of the technologies, and perhaps their independent rediscoveries, in the Middle East and China. Archaeologists have also discovered examples of worked meteoric iron, which dates from this period. The meteoric examples are not relevant to extractive metallurgy though, because the metal was simply picked up from the ground.
Next the Sumerians of Tall al-Asmar, Mesopotamia (in modern Afghanistan) enter the story in 2700 BCE, with their achievement of the smelting of iron in shallow pits using charcoal as fuel. It must have been very difficult to achieve the necessary temperature to smelt iron in a shallow pit, hence the invention of the bloomery furnace, which seems to have occurred shortly afterwards and then spread across to China. The bloomery remained the main method of smelting iron across the Middle East and Europe until the (re)invention of the blast furnace in Dürstel in Switzerland in 1205.
A medieval bloomery furnace. Image: http://shropshirehistory.com
Bloomery furnace cross-section. Image: http://www.wpi.edu
The temperature in a bloomery is not hot enough to completely melt iron, instead a spongy mass of iron (known as a bloom) riddled with impurities (known as slag) sinks to the bottom of the furnace. The bloom is extracted from the furnace, then worked by repeatedly beating and reheating it to physically drive out the slag. We then have wrought iron, which has a very low carbon content (less than 0.08%) and is tough, malleable and ductile.
By 400 BCE the Chinese Zhou dynasty could produce cast iron tools, ploughshares and pots, as well as weapons and structural ironwork for pagodas. Cast iron, which has a very high carbon content (2.1% to 4.3%) is brittle, resistant to deformation and wear and has a relatively low melting point of about 1150 °C compared with pure iron’s melting point of 1538°C. Cast iron did not become available in the West until the 15th century! That’s over a thousand years later!
In yet another leap of brilliance, the master Chinese metallurgists realised, sometime during the first century BCE, that by combining known amounts of cast iron (high carbon content) and wrought iron (low carbon content), they could reliably create an iron alloy with intermediate carbon content (about 2%) that had the most superb qualities! It was steel! In the first century BCE!
By the year 100, the Han dynasty (the Chinese really were millennia ahead of us Europeans) had invented both the blast furnace and the finery process. The blast furnace needs fuel, iron-ore and limestone (which is used as a flux to help form the impurities into liquid slag) continuously poured into the top of the furnace. Simultaneously, air is mechanically blasted into its heart using bellows. This creates a huge amount of heat, enough to melt pure iron, and allows greater quantities of the metal to be extracted, as the process runs continuously.
Fining reduces the carbon-content of the iron. The chemical process is called decarburisation. By stirring the molten cast iron while exposed to air, carbon in the iron combines with oxygen in the air to form carbon monoxide and leave behind wrought iron.
Chinese open air fining c. 1600. Image: https://commons.wikimedia.org/wiki/File:Chinese_fining.png
Quite incredibly, there were no further new technological breakthroughs in metallurgy until 1709, when Abraham Darby pioneered the use of coke instead of charcoal as the fuel for iron smelting, but that’s another story!