It has been very warm in Pop’s Wood this week with glorious sunny weather and the temperature reaching 23°C – 25°C each day. Ideal for standing next to a very hot kiln. Pop’s Wood is oriented approximately east west along its length with a downward slope across its width from the top which is south facing and the bottom which faces north. The charcoal retort is located at the top of the wood and is in a small clearing adjacent to a small copse of beech trees on one side and a very overgrown blackthorn on the other. This means that the retort itself is in full sun or dappled shade for most of the day which makes it a very pleasant spot to sit and monitor the progress of each burn/try to do the crossword. During the very first burn I was very much focused on ensuring that I was on hand to observe the whole process really closely and so found myself not doing anything else for the whole day. That’s fine but one of the thought processes behind having the charcoal retort was that it could be an “in the meanwhile” activity efficiently converting the smaller branches into a more valuable resource whilst I got with other things.
I am now on the fourth burn and beginning to get the hang of the whole process. If you don’t want to read all this guff go straight to the animation link below.
- The first phase, which begins at the point that the lid has been put on and the fire is lit, is pretty hands on. The important task is to keep the fire stoked up to ensure that the retort reaches and stays at a high temperature as soon as practicable. The best way of stoking the fire is to drop pieces of firewood down the chimney, this way they land directly on top of the existing flames and ignite and burn in the chimney itself and roar away. This vigorous burn helps create a strong updraught and acts to “pull” the steam, that is being driven off the logs in the retort, down through the smaller inner exhaust tube and back into the flames and away up the chimney. This phase lasts for about 2 hours
- The second phase is less easy to define but involves the wood in the retort continuing to dry out and release the water vapour in its cells and gradually enter the next phase where the rest of the volatile compounds that make up the wood turn to gas. Basically this is everything else apart from the pure carbon (charcoal) that will be left. These gases are driven off right from the start but are diluted by the steam in the early stages. It is not until they reach sufficient concentrations in the exhaust flow that they splutter into life. You can hear this start with the occasional extra roar as the gases briefly ignite and then go out again. This phase lasts for approximately 1 hour.
- The third phase is where the gases being driven off reach such a volume/concentration that they start to burn continuously and at this stage the whole process accelerates. The gas flame roars up the chimney and heats up the wood in the retort further; this drives off more gas and this burns so making the retort hotter driving of more gas etc. At this stage there is no need to provide any more firewood as the gas is providing all the heat and is self sustaining. This section can go on for between 45 mins and 1 hour and provides an ideal time to get on with something else.
- The fourth phase marks the point when the gas flame begins to falter as the last of the volatile gases are evaporated from the wood. There is a noticeable drop in the roaring noise in the chimney and is a clear signal that more firewood is needed to maintain the oven temperature to complete the cooking process. I guess you could simply stop at the end of the gas phase and leave the oven to burn out and cool down. However, the result would be an incomplete burn and a lot of the wood would be left as brown ends and not charcoal. To try and maximise the production of charcoal and minimise the brown ends I have been carrying on heating the oven for another hour or so making the burning phase approximately 5 – 6 hours long.
- Finally the oven is left to cool down overnight. This ensures that the residual heat continues to completely cook the wood. Critically the retort is left with the lid tightly shut so that air doesn’t get into the oven and allow the charcoal to ignite. This would leave just a pile of ash in the morning!
Here is an animation of all those words……….just click on the link and then play the presentation as a slide show.
Trying to be more efficient and multi-task whilst the burn is in progress is becoming easier. During the last session I had time to prepare all the logs for the next burn and create a small sunny glade next to the retort site.
This involved taking down a very spindly sycamore that had started to rot off at its tip and thin out an adjoining blackthorn. Really encouraging to see the size and number of sloe berries already formed on the tree. Bodes well for a bumper sloe gin season later in the autumn. Traditional to pick after the first frost but may not be able to wait that long.
I plan to erect the solar log drying kiln on this spot over the next couple of weeks. The delay has been largely as a result of the pieces being too big to fit safely into the van! Not a great result but should be remediable and then we will be able to start that project.
Enough for one week so I will pick up the nettle story another time.
Tim Burr 