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Because of its ridiculous prerequisites, namely having both iron and coal within a city radius, the Iron Works small wonder is notorious for being the worst one in the game with many players having never had the opportunity to build it.
Civilopedia Entry
As mankind’s dependence on metals increased through the ages, it became very important to cheaply and effectively separate the base metal from impurities. The process of smelting, through the use of a structure called a blast furnace, achieved just that. Using carbon dioxide created from oxidized coke to absorb the oxygen from the molten iron, it leaves the metal ‘slag’ to sink to the bottom of the furnace. This separation allows for a pure iron to be derived, which is then used for numerous purposes, including the creation of steel.