Charcoal was made as a fuel to supply the heat for many of the industrial processes in Viking Age Britain. It is also a 'clean' fuel that contains little or no acidic and tarry products as they have been sweated and roasted out of the wood. Without charcoal, the work of the glassworker as an example couldn't have happened, the precious metal smith would also have severe problems with any fuel other than charcoal. The process is at least a 24 hour task, much longer if you include the cutting of the timber to supply the clamp. The Charcoal maker didn't use the primary wood, that is to say the main limbs of the trees that were felled.
They all went to supply the demand for straight timbers (or bent ones if needs be) to build houses, bridges etc. All structures at this time included timber as their main component. The only exception to this is stone, and this only affected the more rocky and therefore largely tree poor zones around the coast.
The branches and secondary limbs of the tree were the main ingredient in the clamp, leaving the smallest trimmings as the tinder to start the process. The only other part that was removed possibly prior to the timbers inclusion in a clamp or as building timber, was the bark. This was destined to become part of the thick liquor in the tanning pits for leather, as Oak trees in particular have high levels of tannin in the bark.
The Charcoal burners life was to a certain extent itinerant, as he followed in the footsteps of the wood cutters who were either directed or who themselves decided which of the various trees to cut down. He had to be on hand at all times, not necessarily on his own either, to watch that the clamp did not race and burn rather than roast the wood inside. A change in the wind overnight could do this, and as more air than was required was forced through the footings of the clamp, the charcoal burner had to try and screen off the up-wind side of the clamp with withy screens. These would have been made by the burner on site, and as they got too old and brittle to use, they too went into the clamp as tinder.
Charcoal is best made from 'hardwood' rather than 'softwood', because conifers make a fuel that is far too crumbly, which doesn't reach the temperatures that a forge requires. This begs a question as to how lands that are above the latitudes where deciduous trees grow abundantly fared with iron working. Perhaps trade was their only solution rather than to import charcoal to burn.
It takes little imagination to see the woods then being wrapped in the writhing smoke of these acrid clamps.