Röda jorden


Extracting iron from iron ore


In the 12th century, men started to mine iron ore from rock. They learnt how to recognize rock that contained iron ore and broke off pieces of rock by lighting fires against it, to make it brittle. The work was done from the surface down, excavating the rock in opencast mines and following the veins of ore. Eventually, the work changed from opencast mining to underground mining, creating galleries and tunnels. Once they had worked out how to pump water out of the mines, they were able to work deeper below ground.


The process of manufacturing iron advanced with the extraction of iron from the ore. First, the ore was crushed into small pieces, which were then roasted to concentrate the iron content. Thereafter, the roasted ore went into the blast furnace for smelting. This involved introducing a heavy charge of charcoal, together with some limestone, through the top of the furnace and smelting it at a high temperature. Because the charge goes in at the top, and the smelted iron runs down into the bottom of the furnace, a high yield of iron is obtained, as hardly any of it ends up in the slag or waste, which floats in the form of a protective layer above the molten iron.


In common with the pit furnace, the blast furnace has a nozzle (tuyère) which blows oxygen-rich air in near the bottom of the shaft, but in an arrangement that enables the molten iron and slag to be tapped off at the side whilst the furnace is operating. This is done several times a day. The process is therefore a continuous one and can run for weeks or months. The charge (ore, limestone, etc.) is loaded at the top and the molten iron and slag are discharged at the bottom of the furnace.


The molten iron was tapped and left to cool in moulds known as pigs. This pig iron was a brittle cast iron containing a high percentage of carbon (2–4%), which made it unsuitable for malleable or wrought iron. The pig iron therefore had to be re-smelted in special furnaces in the forge. This was know as the refining process, in which surplus carbon was burnt off, leaving the iron pure, soft and malleable or wrought.


If the amount of carbon remaining in the iron is very small, the iron will be truly soft. If the carbon concentration is higher, 0.5–1%, the product will be steel suitable for hardening.