Charcoal is perhaps the greatest invention of the Paleolithic (Stone) Age. Charcoal is produced by partially burning organic matter (wood, bone, etc.) while limiting the supply of oxygen. One way of producing charcoal is to pile a large amount of wood, as shown in the figure, and covering it with soil to limit the amount of oxygen feeding the fire. During the burning process, considerable water is released, and at the completion of the burn, the wood is reduced to black brittle lumps of carbon (charcoal).
Charcoal played an important role throughout the Stone Age, the Bronze Age, and the Iron Age. Why? Very few elements (noble metals and copper in very limited quantities) occur naturally in their pure form. Elements usually occur bound with other elements forming compounds, and typically occur in a mixture with other compounds. Heat is usually applied to break the compounds or melt the element to produce the raw material needed for manufacturing, such as copper and iron.
The temperatures required depend on the compounds and elements involved and can vary considerably. The temperatures obtainable by fire depend on the fuel used and the supply of air. If wood is used as the fuel in an open fire, temperatures in the fire might range from 350 to 500° C. Charcoal, being a denser and drier fuel source, can provide temperatures up to 800 °C under similar conditions. If the fire is confined, such as in a kiln or a furnace, and air is forced into the fire, it is possible to obtain even higher temperatures. For charcoal, it is possible to reach temperatures above 1000 °C.
Later, in our lesson on metals, we will see that this temperature is insufficient to melt pure iron, which is why the processing of impure iron (iron plus carbon) was developed first. Impure iron has a much lower melting temperature than pure iron. We will see that an advanced design furnace coupled with a hotter burning fuel source (coke, a form of coal) was needed to obtain pure molten iron.