As you all know we tried our hand at the two main traditional methods of producing charcoal: in a pit kiln and in a mound kiln.
While we didn't make a large amount of charcoal and came up against some problems in terms of firing and maintaining the pit we did learn a lot about the charcoal production process - especially in terms of wood stacking, firing, air control, topographical siting, and duration of charring / cooling.
This post will give a brief outline of the experiment but more detailed descriptions and discussion will follow along with lots of images and video.
The Pit Kiln
After this we stacked (and measured) our recently felled c.15 year old oak wood in the pit around an upright log which when taken out created our central chimney/ firing space.
On top of our wood pile we laid another layer of brushwood kindling and straw and then we covered this with our turf sods (grass side down). This was then in turn covered with evergreen vegetation and then a layer of soil.
After the pit was entirely stacked and covered we took out our upright log to create the chimney which we then filled with kindling and fired with hot coals and embers.
The Mound Kiln
We decided to have another go so we removed the vegetation and soil coverings and re-lit them, this time allowing a much bigger fire to develop before adding the sod covering.
The re-firing of the pit took place at 12 noon and the mound at 2pm. At several times during the day we had to open and close air vents in both kilns. The mound kiln was more susceptible to the wind when it picked up and it burned through the outer covering a couple of times which we had to repair. We closed off the pit at 10:30pm that night and the mound at 11pm (We needed to let them cool as we had run out of time and had to finish the next day) and opened them both the following morning c.12 hours later.
We retrieved little or no charcoal from the pit which was smoldering but obviously hadn't achieved a high enough temperature to create charcoal. We did retrieve a couple of buckets of charcoal from the mound – which when opened rather dangerously re-ignited! In fact when we were harvesting the charcoal one of our buckets of charcoal caught fire!!!
While we did not make as much charcoal as we would have liked, and certainly not enough to use in Brian's upcoming iron smelt we did learn a significant amount about the traditional methods for charcoal making that would have been employed by early medieval people.
Some lessons learned include the need to generate a large fire before covering the wood. Also, the cold weather and the fairly unseasoned oak and kindling probably contributed to our problems. The second firing of the mound was actually quite successful and would probably have generated a significant amount of charcoal if we had had time to let it burn for another day or two.
The entire charcoal production process is very labour intensive and it involved the investment of many hours and indeed days and so we were pretty disappointed we didn't get ore charcoal. However we can console ourselves in the knowledge that wood colliers in early medieval times also sometimes failed in their attempts to make charcoal!
We hope to take what we learned and carry out another charcoal burning at Brian’s Smelt 2010.