On the sloping edge of the kiln just prior to completely covering the pit with grass turfs and soil we inserted three metal pipes to act as air flows/ smoke outlets in the kiln. Of course back in early medieval times there was no such thing as steel pipes!! A substitute for these three metal pipes would be to simply use three very green branches and once the wood was stacked around these they could be pulled out in order to create a cylindrical space that could be used as an air flow/ smoke outlet. Likewise instead of the hollowed out upright timbers a simple gap (chimney) in the outer covering of the pit would suffice.

When making charcoal it is very important to create an oxygen limited environment but it is even more important to get an air convection flowing within the kiln to aid carbonisation and this was done through the use of air flows/ smoke outlets.

After the initial firing quite a lot of noxious dirty yellow smoke (gases) and steam were released from the kiln and this eventually slowed to a steady release of steam and smoke from the kiln over the next few hours. This release of steam was an indicator that carbonisation was taking place within the kiln i.e. that moisture was being slowly driven off the oak timber by the heat of the kiln.

Sunday Night

After 16 hours of burning we decided to shut the kiln down at midnight on the Sunday so that the charcoal would have enough time to cool down for extraction on the Monday. Ideally we should have left the kiln for another few hours. We could tell from the steam and smoke outlet and from the shrinkage/ collapse of the pit covering that one half of the pit had burned well while the other half had not burned so well. To combat this at around 21:00 (circa 3 hours before we shut the kiln down) we opened another air flow hole on the less active side of the kiln by simply punching a hole in the cover with a shovel handle.

Interestingly the fire and heat scorched patterns on our reconstructed charcoal pit were very similar to the scorch patterns on the excavated charcoal pit at Russagh, albeit the original excavated pit was much more fire reddened.  

ALL IN ALL OUR EXPERIMENT WAS A SUCCESS...WE MANAGED TO RECONSTRUCT AN EARLY MEDIEVAL CHARCOAL PRODUCTION PIT AND MAKE CHARCOAL IN IT THROUGH TRADITIONAL METHODS!


Niall Kenny

This is a time-lapse video of Niall Kenny and Eoin Donnelly's experimental charcoal production pit at Smelt 2010 being opened and emptied. The charcoal burn went well and a substantial amount of charcoal was made. The charcoal burn was featured in the Naked Archaeology Podcast which can be downloaded here or listened to online here.

More updates on the charcoal experiments will be put up asap and you can have a look at some video from the smelt here. 

Brian

A very interesting series of postcards of charcoal making in the black forest in the early twentieth century from Wikimedia Commons.

Hi All just a quick update on how our charcoal making experiment went...

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.

We based our pit kiln on a charcoal production pit excavated (by Ellen O’ Carroll) at Russagh 4 Co. Offaly on the N6 road scheme. The pit we dug measured 1.4m x 0.91m x 0.51m and it was a little bit deeper than the excavated example (which measured 0.21m) so as to allow for the topsoil depth and truncation. We kept the soil and sod aside for later use. Once the pit was dug, we laid a covering of straw and oak brushwood kindling on the base of the pit.

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.

We decided to build and fire a mound kiln with the remaining oak wood. We picked out a flat area adjacent to the charcoal production pit and de-sodded an area to erect the mound kiln. We then levelled the de-sodded area and built a central flue/ chimney out of split lengths of oak wood. Once this was built we carefully stacked the remaining oak wood around it. We then filled the chimney space with kindling and covered the mound of stacked timber with straw, then with evergreen vegetation, de-sodded turf and lastly soil. Once the mound was covered we again fired the kiln in the centre with hot embers.

We had problems maintaining the fire in both the pit and mound kilns. We had fired the pit at 3pm and the mound at 5pm but by 8am the next morning after a cold night both the pit and mound had gone out.

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.

We produced this handout for people who came along to the experiment. It details the aims, methodology etc for the experiment and some of the archaeological and ethnographic background.

Dear colleagues and experimental archaeology enthusiasts,

Myself (Niall Kenny) and Brian Dolan plan to dabble in experimental archaeology and produce charcoal in the traditional way just like it would have been made in early medieval times (except with the possible use of a chainsaw). We plan to dig, fire and (hopefully) control a charcoal production pit kiln (and possibly a charcoal production mound kiln).

We will be basing our experiment on a pit kiln excavated on an archaeology site in recent years. We have shortlisted a number of excavated kilns and will single one out for our upcoming experiment. Essentially we plan to dig a pit in the ground, stack it with oak wood, cover it with vegetation and soil and then ignite it. However it is not as easy as it sounds and we will have to be very careful in stacking the wood and creating a firing space as well as controlling air flow into the kiln throughout the process. This may prove tricky and the experiment could easily end up in tears and flames! However fingers are crossed and hopefully all shall go well!!

As part of the experiment we have also felled 15 oak trees aged between 15-20 years. Oak was the preferable wood species used for charcoal production (associated with metal working activities) in the past as it did not crumble as easy as softwood charcoals and because of its high calorific values and its longer and higher heat burning properties.

I have been carrying out some research on charcoal making in early and late medieval Ireland and while a good deal of information can be gleaned from traditional charcoal making practices and from the ethnographic evidence we want to gain a better understanding of the actual process, in its entirety, through experimental archaeology. In particular we want to investigate how charcoal kilns were constructed and controlled but also we want to scrutinise the various nuances associated with the charcoal production process. Brian is currently undertaking a PhD on early medieval ironworking in UCD and so if all goes well and we successfully produce a charcoal yield then Brian will use this charcoal in an experimental iron smelt he plans to carry out in Ferrycarrig (Wexford) in March.

We are basically indulging in and flirted with pyromania :-) and we whole-heartedly encourage anybody interested to join us!

Niall Kenny