Mkng-Charcoal-art - 1/2/10
"Making Charcoal" by Viscount Sir Corin Anderson (KSCA, OP).
This article was submitted to me by the author for inclusion in this set of files, called Stefan's Florilegium.
These files are available on the Internet at: http://www.florilegium.org
Copyright to the contents of this file remains with the author or translator.
While the author will likely give permission for this work to be reprinted in SCA type publications, please check with the author first or check for any permissions granted at the end of this file.
Mark S. Harris...AKA:..Stefan li Rous
stefan at florilegium.org
This article was first published in issue number 30 of "Cockatrice", the A&S newsletter of the Kingdom of Lochac.
by Viscount Sir Corin Anderson (KSCA, OP)
Ever since I joined the SCA I've been fascinated by blacksmithing. I promised myself early on that I'd eventually have a go. So, nearly 20 years later I piled up some bricks into a rough square trough, filled it up with wood and set fire to it. I knew that you needed to supply extra air but, lacking a set of bellows; I used an electric airbed blower.
Well, it sort of worked. My steel bar did eventually get hot enough to shape on the anvil but it took much longer than I'd expected. I did notice after a number of trials that as the fire was reduced to charcoal it seemed to burn hotter and the steel bar achieved that satisfying yellow glow in less time.
After a little web surfing I discovered that it was, in fact, traditional to fuel a forge with charcoal rather than raw wood and learnt why this was a good idea.
The next problem was that I didn't have any charcoal and was having a bit of difficulty locating a supply. Back to the web and I found some articles on how to make charcoal. It didn't seem too difficult so, what the hell, I thought I'd give it a go.
What follows is a bit of background on what charcoal is, why it is preferable to raw wood in some situations and how it was made in period.
Then there's the tale of my own, in some cases amusing and disturbing, efforts to make some.
The Wikipedia, that great but potentially dubious resource defines charcoal thus:
Charcoal is the blackish residue consisting of impure carbon obtained by removing water and other volatile compounds from animal and vegetable substances. It is usually produced by heating wood in the absence of oxygen, but sugar charcoal, bone charcoal and others can be produced as well. The light, black, porous material is 85% to 98% carbon, and resembles coal.
Wood is made up of four principle components:
Combustion starts with the application of heat to wood in the presence of oxygen. Heat causes several reactions.
First, it raises the temperature on the wood's surface and for some depth into it. As the wood's surface temperature approaches 100 degrees C, the water in the wood begins to boil and then evaporates. As long as water remains in the timber, its boiling and evaporation rob energy from the fire. The water must be driven off before combustion can begin, so wood with a high water content is harder to ignite and will burn with less heat until it has dried out.
After the water has been driven from the wood and the applied heat raises the temperature above 150 degrees C, the lightest of the volatile compounds begin to evaporate. At about 280 degrees these light gaseous compounds ignite. The heat produced by the burning of these lighter gases begins a chain of reactions where heavier and heavier volatile gases also begin to evaporate and then ignite.
The visible smoke a fire produces is largely composed of these unburnt gaseous, volatile, compounds. As the volatile compounds burn they combine with oxygen to form invisible carbon dioxide and water vapour.
As the wood in a fire dries out it stops losing energy to water evaporation and burns with greater heat. The increased heat causes a greater proportion of the volatile compounds to ignite. This is why the smoke of a fire clears as its timbers dry out and become hotter.
The volatile gases contain up to half of the potential heat energy of the wood. Because the gasses are able to mix freely with oxygen in the air they burn at a much faster rate than the carbon in the solid, non-volatile portion of the wood. The envelope of burning gasses that surround a wood fire is what we perceive as flames. Once all of the volatile gasses have been driven from the wood the flames themselves die and what is left is the burning charcoal.
At some point during combustion the heat produced by the burning volatile compounds ignites the carbon in the chains of cellulose and lignin molecules. In the presence of enough heat the carbon combines with oxygen to make carbon monoxide and carbon dioxide, releasing more heat. Unlike the volatile gasses, which can mix freely with oxygen in the air, solid charcoal's comparatively low surface area limits the oxygen available for combustion. This means that in relatively still air charcoal burns longer with a lower rate of heat output.
Because charcoal has lost up 50% of its weight in water, and doesn't lose energy to evaporation, it produces twice the heat energy of an equivalent weight of raw wood. This is an important factor when considering the logistics of transporting fuel to an urban area. Raw wood can be turned into charcoal with only a moderate effort when compared with the effort of cutting the timber. At the least, a given weight of charcoal can cook twice as many meals as the same weight of timber. If transport were a significant part of the cost of production you would expect charcoal to be more profitable.
Because all of the water and volatile organic compounds have been removed from charcoal it burns with little smoke. In tightly packed urban areas where buildings often lacked a hearth with a chimney, the ability to cook without smoke would have seemed advantageous, further adding to the appeal of charcoal.
The flames of a typical campfire fire burn at a temperature of 600 - 700 degrees C. This temperature is far too hot for any kind of cooking except boiling water; food placed in the flames would most likely be burnt. Anyone with experience with campfire cooking knows that it is best to wait until a fire is reduced to coals before putting over spits or placing on pots. Because it contains no volatile organic compounds, charcoal burns slower and more evenly than raw wood, and burns without flames, making it a better fuel for cooking. Starting to cook with charcoal has the added advantage of not having to wait for a fire to be reduced to coals i.e. it saves time.
As charcoal burns, oxygen atoms are combined with carbon to produce carbon monoxide and carbon dioxide. This results in the air immediately around the charcoal's surface being lower in free oxygen that the surrounding air. Free oxygen does mix in from the surrounding air but in still air this happens slowly enough to limit the rate at which charcoal can burn.
When bellows force air quickly over burning charcoal they replace the oxygen depleted air with oxygen rich air. This allows the charcoal to burn much hotter than it would in still air (this is also the reason why bushfires are far more violent in windy conditions).
The difference between the temperature of charcoal burning in still air and charcoal burning with forced air can be as much as 50%. Charcoal can burn at about 800 degrees C in still air. With forced air the temperature can reach 1600 degrees C.
This is important in the refining and working of metals. Copper requires a temperature of about 1200 degrees C to smelt while iron melts at a temperature of about 1500 degrees C.
The use of charcoal, along with the invention of the bellows, has been instrumental in the development of the Bronze and Iron ages.
The simplest way to make charcoal is to light a wood fire and then extinguish it with water when it has been reduced to coals. The problem with this method is that much of the solid carbon is also burnt resulting in a lower charcoal yield.
A more efficient method is to burn the wood in some kind of kiln. Once the volatile organic compounds and water have been removed, as indicated by the clearing smoke, the kiln can be given an airtight seal to prevent further combustion. The kiln also serves to concentrate the fires heat so that the water and volatile compounds are driven from the wood faster, and with less combustion of the solid carbon, than in an open fire.
Rather than bring wood to a permanent kiln, it was more common to create temporary kilns at the site where the wood was cut and then transport the charcoal to where it was required. Two common methods of creating these temporary kilns are known as the pit method and the clamp method.
In the pit method a rectangular pit is dug on the side of a hill so that the base of the pit slopes upward. The pit is then filled with timber and the top covered with finer and finer layers of overlapping braches, leaves and then finally covered with earth. Openings are left near the down slope and up slope end of the pit. A fire is lit in the lower end of the pit and the slope of the pit causes a draft that carries the fire throughout the timbers. Once the smoke that emerges from the upper opening clears the entire pit is covered with more earth to exclude any air and the whole thing left to cool. The cooling can take a number of days, depending on the size of the pit.
Pit method – woodcut from Pyrotechnica
Pit method - modern Ghana
In the clamp method a large conical mound of timber is piled up around a central log, called the motey peg. The mound is covered with finer branches, leaves and earth, as in the pit method. The motey peg is then removed and the fire started by dropping hot coals into the central shaft, called the motey hole.
Vents around the base of the mound are left open to create a draft between them and the top of the motey hole. Once the smoke of the fire clears the motey hole and vents are covered with earth to exclude air and the whole thing left to cool.
Clamp method in woodcut from Pyrotechnica
Charcoal Burners in the Black Forest c 1900
So, now armed with the knowledge I needed to make charcoal I brought a bag of firewood from the local service station and then looked about the back yard for an appropriate spot to build my pit or clamp.
After a little thinking I decided I didn't have enough dead branches and leaves in my back yard to make an appropriate base to cover with earth. I also figured by landlord would not be so happy with my digging a big hole in the lawn.
I did, however, have the pile of bricks I'd been using as forge. After an hour or two of rearranging the bricks and dropping a steel plate on top I'd come up with a double walled kiln. My plan was to fill the space between the walls, and cover the steel plate, with earth from near the compost heap. That way I could leave the shrubs unscathed and lawn intact.
Then for the most important ingredient, the wood! I bought that as a 20 kg bag of firewood from the local service station. After getting it home I attacked it with an axe to make splits about 10 cm in cross section (I'm wondering if I'd have been better off buying kindling?).
I'd read that it was a good idea to leave some space around each piece of timber to allow the burning gases of the fire to heat the surface of the wood. To this end I stacked the wood in the kiln in alternating perpendicular rows with a gap of 1 cm or so between each piece.
Wood stacked in kiln
Once this was done I placed the metal plate over the kiln and built a makeshift chimney out bricks. Then I filled the walls and covered the kiln with earth. All that remained was to light the fire!
My first impression was of the amount of smoke the kiln produced. After a mere 15 minutes the amount of smoke was prodigious. After 30 minutes a neighbour from three doors up came by to ask if the house was on fire.
I apologised and explained I was making charcoal for blacksmithing. 'Well' she prelied, 'do you know my grandfather was a blacksmith' and 'do you make decorative gates'? She was quite enthusiastic.
She also offered to give me their old gas flue to add to my chimney. I happily accepted this and she was back in 5 minutes with the flue, which I immediately added to the kiln (I figured it might help increase the draft).
The kiln on fire
After about another half hour the neighbour from two doors up, an elderly gent, came by and he was not so enthusiastic.
'What the hell do you think you're doing!' he yelled at me from the street. 'You're filling my back yard with smoke!' 'I've got the washing out!'
Oh! I wandered closer up the drive towards the neighbour (who I will admit I was unfamiliar with), my intention being to apologise and say I'd be finished soon.
As he saw me walking purposely up the drive he gave a little yelp of fright and quickly shuffled back up the street and into his home. Ooops! I didn't think I was quite that scary.
I spent the next 90 minutes of the burn expecting to hear the sounds of a fire truck any second and wondering what the fine would be for unduly bothering them.
I will point that I had checked that it wasn't a total fire ban day. And I did have a hose ready and laid out near by. Maybe I was just paranoid.
Anyway, at the end of that 90 minutes (2 and a half hours after I lit the fire) the thick white smoke cleared to a thin grey smoke and then disappeared to leave just a heat haze over the chimney.
Right, time to shut it down. I grabbed the shovel and covered all the openings, including the chimney, with earth. Every so often I would notice smoke rising from the earth. When this happened I lightly soaked the earth with the garden hose and packed it down till the smoking stopped.
After another three hours had elapsed the kiln had stopped smoking for about an hour and I decided to open it and see what was there. In this respect I was perhaps a little impatient. All description I had read suggested leaving the kiln to cool overnight.
But, being impatient, I decided to open it. After scraping off the dirt I lifted the metal cover and saw the results.
The resulting charcoal
If you compare this picture with the earlier picture 'Wood stacked in kiln' you will notice the pieces of wood to the bottom right of the image have largely kept their overall structure as pieces of wood while the wood to the top left has been reduced to smaller pieces.
This happened because the wood at the top left is nearest the opening of the kiln and, having the greater air supply, has burned hotter. This resulted in quicker evaporation of the water and volatile organic compounds leading to the burning of some of the lignin and cellulose, the solid part of the wood. Since it is the solid wood that makes the bulk of the charcoals mass, burning it leads to a loss of product.
Ideally in the production of charcoal the burning should be stopped when all the water and volatile gasses have been removed but before much of the solid wood has burned.
The wood at the bottom right has been incompletely converted to charcoal. The outer layer is charcoal to a depth of about 2 – 3 cm. Inside the wood is brown but still identifiably unburned wood.
Another requirement for good charcoal production is the evenness of the heat throughout the burn. The more even the heat the more likely for all of the wood to be converted to charcoal at the same time and the less chance of producing wasted or un burnt wood.
My kiln design only had a single air inlet. The clamp method, on the other hand, seems to provide for an unlimited series of vents all around the base of the clamp. This would have, I believe, allowed for more even burning.
The results of the experiment appear in the picture below. About half the volume of wood was completely converted to charcoal (it's in the recycling bin). About a quarter was partially converted (middle pile) and the last quarter was well dried out but had very little of it had been converted to charcoal.
If I were to run the process again, I would add some vents around the bottom of the kiln to help even out the burning.
Dried, charred and charcoaled wood
I won't get the chance to run the same experiment again, though, having since discovered that in NSW it is illegal to burn wood for the purpose of making charcoal in an urban area.
I've also located a source of cheap charcoal, the local charcoal chicken shop in the mall. The guy there is happy to sell me a 25 kg bag for $30.00.
Although $30.00 for 25 kg is too cheap to really consider making my own charcoal from an economic perspective, I'm still keen to have a go with the clamp method using about 100 kg of raw wood.
A possibility for Festival, perhaps, or maybe some time in the middle of winter?
Some links (the last one is a paper on modern production and is very informative about the science):
Copyright 2006 by Gerard Tops. <gerardtops at yahoo.com.au>. Permission is granted for republication in SCA-related publications, provided the author is credited. Addresses change, but a reasonable attempt should be made to ensure that the author is notified of the publication and if possible receives a copy.
If this article is reprinted in a publication, I would appreciate a notice in the publication that you found this article in the Florilegium. I would also appreciate an email to myself, so that I can track which articles are being reprinted. Thanks. -Stefan.