Charcoal

Charcoal can seem like a strange thing. Why do we want to burn something that has already been burnt? 

Well, charcoal is the result of burning, but a very specific and careful kind of burning. It might more accurately be considered refining. The original, raw, organic fuel found in nature (say, wood) is refined into a more effective fuel, charcoal.

The chemical reaction you really want when burning something organic for fuel is carbon and oxygen becoming carbon dioxide. Something like wood has a lot of carbon, but also a lot of other stuff that can get in the way of a good fire, like water. The slow way that wood is burnt into charcoal, mostly without oxygen, gets rid of most of that, leaving almost pure carbon.

Since charcoal is almost pure carbon, it: 

1. Has a lot more potential energy for it’s weight 
2. Burns hotter and 
3. Burns more cleanly with less smoke and less toxic smoke.

The higher heat obtainable is what made it most desirable through history because very high temperatures were required for working metal.