What we did
The pit we dug was exactly the same as the last one and it measured circa 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 dug and stacked the pit on Saturday afternoon (after strenuously weighing all our fuel wood). This time we decided to create two flues or chimneys at the sloping edge of the kiln. To create one flue we simply laid two lengths of timber parallel to each other, c.20cm apart, down the sloping side of the kiln leading from the top of the kiln right down to the base.
We packed the base of the kiln and the two flues with dry hazel kindling and straw.
Across the two flues (four lengths of timber) we stacked our oak wood along the long axis of the rectangular pit. On top of the oak wood we laid a layer of dry bracken. On the opposite side of the kiln directly across from the flues we placed two hollowed out lengths of timber (old wooden land drains) set upright in the kiln to act as smoke outlets for the kiln.
We fired the pit kiln early the next morning (Sunday) at 08:00. We lit two fires on the sloping edge of the kiln with hazel and oak kindling just above the two flues/ chimneys spaces so that the fire would then spread down through the kindling packed chimneys to the base of the pit. We got the fire going really well within the pit and then started to gradually cover the kiln by placing grass turfs (grass side down) over the layer of bracken and around our two upright chimneys.
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.
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.
Monday Lunch time
After allowing the kiln to cool down for 13 hours (it was a particularly cold night - minus 3 degrees celcius) at 13:00 we decided it was time to open the pit and extract our charcoal...if we did in fact have any charcoal!!
AND WE DID! WE MANAGED TO MAKE CHARCOAL!!!
We cleared off the soil and grass turf covering (as you can see in the time-lapse video) and extracted our charcoal yield. The least effective half of the kiln contained many brands (partially charred timbers) and almost fully charred timbers as well as charcoal throughout while the other half contained much charcoal. We sifted through the kiln separating our oak charcoal from charred bark and kindling. After allowing it to cool further we were satisfied that it would not reignite so we bagged the extracted charcoal. In parts of the kiln many timbers were thickly coated with a black shiny tar. In total we managed to collect 14kgs of pure oak charcoal (a charcoal: fuelwood ratio of 1:12) so it wasn’t the most efficient charcoal burn. But then again this was not a very efficient way of making charcoal! Owing to the fact that we shut the kiln down early and half the contents of the pit was composed of brands a pit of this nature would probably at best be able to produce an output of 30kgs of oak charcoal – roughly the amount used in Brian’s experimental smelt!