Wood

Fuel
Bark can be used as a fuel in furnaces, smelting 1.5 items per block.

Smelting bark into charcoal is slightly better: each charcoal will smelt 8 items, but the charcoal itself needs to be smelted. The net items smelted therefore is 7. Also, the increased need to load and unload items will usually result in some loss of smelting time.