Charcoal burning – the basics of a burn
Preparation of the kiln site
The ideal site on which to put a charcoal kiln has free draining sandy loam. Because of the high risk of fire never burn on peat – never! The site should be free from stumps, roots or rabbit holes. Turfs of grass and other vegetation should be cut and put to one side, giving a clean area of earth at least 1meter larger than the diameter of the kiln. Clay soils tend to cause drainage problems, as copious amounts of water are driven out of the wood in the early stages of the burn. Also when burning during the early part of the season, rain can gather around the kiln and into the kiln spoiling the bottom layer of charcoal. The prepared bare earth should be raked as level as possible.
Siting the kiln
Once the ground has been prepared, the kiln can be rolled into place. Carefully lower the ring centrally into place. If the kiln is old, don’t be too rough as they can distort. When satisfied that the kiln is level (spirit level ?) mark the position of the ports, which should be an equal distance apart. Some smaller kilns have 6 ports, most have 8. Lever up the kiln to position the ports one at a time. The chimney sockets should be about 3” from the kiln wall. Now fill the gap between the ports, either with sandy soil from the site – use spades or mattocks. If necessary import sand and mix it with soil from the site. Sand on its own can become very dry and light with the heat of the kiln and blow from the lid and near the bottom of the kiln.
Filling the kiln (the charge)
To lay the floor select 6 (or 8 with an 8 port kiln) good straight logs of c. 6” diameter and position these inside the kiln like the spokes of a wheel in between the ports. They should be long enough to almost reach the wall of the kiln but leave an 8” gap at the centre. Appoint one convenient port as the lighting port (mark it on the outside wall of the kiln), slightly open up the spokes either side of this port to ensure the lighting stick will be able to get a clear run through to the centre. Now construct a fireplace at the very centre, there should be sufficient room for the lighting stick to get underneath this. The fireplace is best made using partly charred wood from a previous burn, otherwise place dry tinder or kindling material which will readily light. Next, proceed to put the floor itself in place, start nearest the fireplace with logs that will span the two spokes that are either side of the lighting port. They should get progressively longer and thinner as you work towards the kiln wall. Now in a similar way fill the gaps of every other channel. At this stage the floor should look like the radiation symbol. Fill the remaining gaps in the same way. The floor is now complete and will provide space under the ‘charge’ for the air to flow in and the gases to flow out. Its worth taking care over putting the floor in place, if a log slips out of place it could restrict the flow of air and gases and prevent access for the lighting stick.
The kiln is now ready to accept the main charge. Ideally sound seasoned wood should be used. If you are using small stuff in the round (anything less than c. 6” diameter) follow the following procedure which should result in getting the maximum amount of wood in the kiln and achieving a good yield of charcoal. Logs bigger than 6”, should be split. Many charcoal producers use a firewood machine to fill their kilns.
With all the logs running in the same direction, place a mixture of thicker and thinner logs near the centre, careful not to disturb the par char fireplace. Continue filling the kiln this way, thicker logs towards the centre, thinner logs toward the outside. If large gaps appear these could be filled with chocks. Occasionally place several pieces of par char in the centre to help get up the temperature during the light-up phase. If you put too much par char in the centre and the wood is very green, the centre may just burn without actually connecting with the main body of charge. Remember the more compact the charge the more charcoal you will end up with. Fill the lower ring so it is mounded up in the middle by about 18 – 24”. If using a top ring as well now is the time to roll it along side, put it over onto the lower ring and slide it into position – definitely a job for two people. Apart from the obvious risk to your back during this operation, care also needs to be taken as getting a hand in the wrong place could result in severe injury! When filling a top ring it is much easier if one person is inside the kiln positioning the logs, and another passing the logs in. As for the single ring, mound the logs up in the centre.
Fitting the lid
Roll the lid along side about 3’ away from the base and rest it on the kiln. With a man either side lift the lid and slide it into position. With the charge mounded there should be a gap of c. 6” between the lid and the kiln, during the lighting phase the charge will drop and allow the lid to be fitted properly. If you find there is not much gap you may need to prop the lid open with 4 sticks to allow the charge to burn freely whilst it gets up to temperature. If the kiln is overfilled then some of your charge will simply be burned away during the light up. Some burners like to preheat the chimneys prior to the capping off by placing them on the lid. This creates a plug of warm air in the chimney which when put into the ports should rise an aid the circulation of the gases at the beginning of the burn.
Lighting up and capping off
Select a straight stick at least 4’ long and about 1” in thickness. Tie a rag around one end; then soak it with a little diesel. Ignite it and insert into the lighting hole placing the flame directly under the fireplace. If the kiln has been filled carefully the par char and / or kindling should catch quickly and smoke should start to drift from under the lid. If the wood is green this process may take some time but if the wood is seasoned the rate the fire takes hold can be quite surprising. Generally speaking it can take anything from half an hour to an hour before the temperature is sufficient to carry out the capping off procedure. Only experience will tell you when to cap off. As copious amounts of smoke are emitted during this stage it is easy to cap off too early when it has not reached a high enough temperature. At this stage the kiln is burning freely with the lid propped up and all the ports fully open. One test to check for temperature is to splash water on the lid and if it bounces off it is ready. Once operating temperature has been achieved the lid can go down fully, seal it with sandy soil, wearing gloves place the hot end of the chimney into the sealed ports. White smoke should immediately be rising from each chimney. Over the next half hour the amount of smoke should increase gradually, if it does not it may be necessary to remove the chimneys, open the ports fully and lift the lid. This needs to be done with caution as flame could escape from the ports and the lid. Make a note of the time when you cap off.
Changing the chimneys
Once the kiln has been capped off, the main attention the kiln needs is to periodically swap around the chimneys to produce an even burn; otherwise you may burn away the charge at the inlet ports. There is no absolute rule about when to change the chimneys, but for a 6’ single ring kiln which might be expected to burn for about 8 – 10 hours, changing them every 2 – 2.5 hours will be about right. The variation in time of a burn and consequently time between chimney changes will be caused by a number of factors:
- The moisture content of the wood
- The temperature when you cap off
- Soil conditions
- Atmospheric conditions
- Blockages in ports and chimneys
Finishing the burn
Towards the end of the burn tar begins to gather in the ports and chimneys. If these are left unchecked they become completely blocked, which in turn slows down the last stages. It is therefore necessary to remove the chimneys to clear any obstruction from ports and chimneys. As the burn reaches its final stages, tar deposits in the inlet ports begin to burn, producing a pink coloured residue. As each port ‘pinks out’ place a brick over the port to prevent any more air from being sucked in. Monitor the burn carefully at this stage until all the inlets reach this stage. There are other signs that the burn is complete, these are: smoke from the chimneys becomes thin and wispy. If you splash a line of water vertically up the kiln wall above each inlet port and the water immediately bounces off near the port then the burn could be complete at that port. These signs usually appear around the same time, but not necessarily. Only time and experience will allow you to read the signs accurately. If smoke from the chimney is completely blue then the finish has been overshot and you are burning charcoal. When you are sure the burn is finished evenly, remove chimneys and pour sandy soil into the ports. Starving the fire entirely of oxygen will kill the fire and stop the process. Take time to seal the kiln properly! It would be good practice to lightly water the ground around the kiln with a watering can to ensure no embers have been ejected and start a fire when you have gone. Most insurers will insist that you stay with the kiln for one hour after it has been put out.
Emptying the kiln
Ideally the kiln should be left to cool for 24 hours. Using a spade, scrape off all the sealing material around the lid, put the spade between the lid and the top rim of the kiln and lever the lid free, carefully slide the lid off, lowering it to the ground then roll it away. If you have used a top ring use a similar technique to remove this, best done with two people though. You are now ready to empty the charcoal. The best tool to use is an asphalt fork or a potato fork. Put the charcoal into wide necked plastic bags ready for grading. Try not to tread on the charcoal while you are in the kiln, crushing it and making it unsalable for the BBQ market. At the bottom of the kiln there will be about one wheel barrow’s worth of very small stuff to dispose of. The kiln should also produce a similar amount of par char which you will need to get the next burn going. The last job is to clean up all the ports and the angled seating rim for the next burn.
Credit: Modified by Maurice Pyle of Woodsmith Experience from an original script written by David Hutchinson of the Yorkshire Charcoal Company