The following contribution is by Drawing New York member Sharilyn Neidhardt.
Charcoal is traditionally made from burnt sticks, usually willow. Grape vines, as the name implies, are also used for vine charcoal in much the same way. Stick or vines can be set in a ceramic holder to keep them straight, and then are set in a VERY hot oven or even a fire pit. They need to smolder long enough to be thoroughly calcined, else the center is still woody/viney.
The dryness/greenness of the wood affects the quality of the charcoal, as does the hardness of the wood itself. Hardwoods are traditionally preferred for drawing AND cooking charcoals (mesquite being one of the hardest woods there is).
In creating charcoal, heat and timing need to be controlled – too much smolder renders merely ash and not the black stuff we like to use in this instance.