Getting Dirty: What Charcoal Can tell us

Most of us are familiar with charcoal from sketching with it at school, or using charcoal bricks for a barbecue. You will have noticed that it got your hands dirty, that it is brittle, and that it is quite light—at least, lighter than an equivalent piece of uncharred wood. You may also have associated the black residues left after a bonfire with charcoal. If you have been to an area where the vegetation has been destroyed by wildfire, you may have also noticed black residues of charcoal on the ground that make a crunching sound beneath your feet. Our first two examples of charcoal are both products of human manufacture. The bonfire charcoal is a naturally formed material, but still the link with wildfire may not be made. When we see images of burning vegetation it is natural to imagine that all the plant material is consumed by the flames. Yet, as I came to realize on my visit to the site of the Hayman Fire, there is often a significant quantity of unburned material, and charcoal residues as well. Why are we left with charcoal after a fire? Charcoal is produced by heating plant material (most commonly wood, but not exclusively so) in the absence of oxygen. So it isn’t a product of the fire itself, but of the intense heat from the fire. Wood is essentially made up of two organic compounds: cellulose and lignin. Both compounds consist of carbon, hydrogen, and oxygen, but they differ in structure and therefore in properties. In cellulose, the carbon atoms are arranged in straight lines (it is an example of an aliphatic compound). It is the material from which paper is made. In lignin, on the other hand, the carbons are arranged in rings (it is an aromatic compound), and it is this structure that gives wood its toughness and strength. Industrial charcoal is used for a variety of metallurgical processes, and as adsorbents and food additives, as well as for barbecues and artists’ materials, so its formation has been carefully studied.