It is known that men have known of iron for around 4000 years. However, for most of history Iron was very hard to extract from the ore. Only very small amounts of Iron could be obtained because the ore would be heated over a small fire. It was discovered that it would be necessary for the fire to be hotter to produce larger amounts of Iron. Eventually, a bellows system was created and the fire became hot enough to produce more iron per batch.
When iron ore was heated in these fires it would slowly change into a lump of iron that could then be hammered into the shape that was desired. When heated in these furnaces sometimes if not properly taken care of the iron would turn to a liquid state. This iron was then scrapped. Later in the 1500’s however, liquid iron was the goal.
As power from water wheels became more popular across Europe some iron workers developed a bellows that was powered by water. This bellows blew much more air and let the fire reach much hotter temperatures than the man powered counterparts. Using water power iron was easily turned to liquid form. It was found that iron ore always contained contaminates that would not allow the final product to be pure. Iron workers discovered that when limestone was added it would pull the contaminants out of the liquid iron and would form a lighter liquid which would then flow off of the liquid iron.
The pure liquid iron would then be poured into casts on the floor. These casts were long bars with shorter bars on right angles to the main chute. This shape reminded the iron workers of piglets suckling on their mother and deemed this type of cast iron pig iron.
The furnaces used in the production of cast iron were called blast furnaces. It was discovered that wood would not work for the production of cast iron and that charcoal would have to be used. Charcoal had a disadvantage however. It was very hard to produce at high quantities. The production of coal was then increased due to the push from the iron industry. Eventually, coal was put through a process similar to that of charcoal to produce coke. Coke is an even more efficient form of coal with less contaminants which would produce a better product.
Here is a very good source if you want to learn more
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Iron Slag: https://en.wikipedia.org/wiki/Slag#/media/File:Slag_from_iron_ore_melting.jpg
Pig iron: https://en.wikipedia.org/wiki/Pig_iron#/media/File:Casting_pig_iron,_Iroquois_smelter,_Chicago.jpg
Water wheel bellows: