We followed Toby Hoad, he of The Greenwood Workshop, for a few days to see how charcoal is made the old-fashioned way. He sources timber from woodlands that he manages here in Purbeck – these days, with the help of a couple of fine French cob-style horses, which we also covered away back in the day. You’ll find that report snuffling somewhere around on this website, too. He runs courses on traditional woodturning and furniture-making. There’s always something going on – follow our link to his website.
Full article from edition 6 with extra images by Michael A Deane. Words by Geoffrey Stubbs
Charcoal has an astonishing range of uses – a primary force behind mankind’s progress to becoming our planet’s most destructive species. Among the boasts – it’s a palliative, a constituent of gunpowder and played a critical role in the development of metallurgy. It was still energising industry into the late seventeen-hundreds, enabling The Blightish Empire well before coal presented its sooty face, and as recently as the 1940’s there existed a horseless carriage powered by charcoal. The char-car, perhaps?
Almost entirely carbon, it is a more efficient heat generator than fossil fuel and less polluting. It was in use on an industrial scale across most of Europe and northern America for several centuries; eventually precipitating the wholesale deforestation of both regions. Coppiced woods once supplied most raw materials with minimal impact on surrounding forestry but the greed for energy in emergent industrialised economies reeked devastation. They say that in Finland there’s not a tree more than three hundred years old due to the demand for charcoal, and it’s a fair bet that much of Purbeck ended up that way too. That’s how good it is… or was.
Once coal and oil replaced charcoal in the engines of industry, this source of ultimately renewable energy was left in the cold. The shift to fossil fuel and a decline in woodland industries meant coppices were left to overburden, subsequently they ceased to support the diversity of flora and fauna which they sustained when managed. We will be observing the transformation of Toby’s woods with interest as less canopy permits more sunlight to hit the ground – we’re already we’re seeing a resurgence of plant species denoting once semi-ancient woodland.
The Charring Process
Charcoal is made by charring wood, which is not quite burning it, very, very slowly, removing most of the organic matter and leaving a carbonised shell. This is done by restricting oxygen to the combustion process, and that’s pretty much the gist of it. Over the next couple of pages we’re going to make this simple process seem far more complex.
Toby’s raw material is derived from the thinning of two over-stood planted woodlands; one surrounding his workshop on Rempstone Estate, another on nearby Forestry Commission property. The ‘burn’ mix mostly comprises, in equal parts, ash, beech and silver birch, proportionately less sycamore and, in large part, overgrown hazel from an old coppice which Toby is gradually reinstating for his woodland furniture enterprise.
After felling and cutting the logs into more-or-less four-foot lengths, Toby stacks the timber close to the track and leaves it to season for at least three months. Some of this stock will go to winter fuel but, as Toby explains, ‘One of the advantages of charcoal is that I can make use of the smaller branches, down to about yea-width [gesticulates a gnarled fist], which were going to waste.’ The bigger logs will be cut again later to a more manageable size and split prior to pyrolysis.
Heat is required, not flame. Hydrogen, oxygen and tars escaping as gases are driven from the tissue of the wood fibre, creating pores through it as they go.
It’s more a chemical reaction using heat as a catalyst than actual burning, though initially that heat is generated by fire. Escaping gases drain the vascular structure, i.e. the pipework, leaving a honeycomb-like residue of porous carbon which has vastly more internal surface area than a solid log, a major factor behind its superior ability to heat.
Under average conditions, 100 parts of wood yield about 60 parts by volume, or 25 parts by weight, of charcoal; small-scale production on the spot often yields only about 50%; large-scale was efficient to about 90% even by the seventeenth century. The operation is so delicate that it was generally left to colliers (professional charcoal burners).
Charcoal is obtained by heating wood until complete pyrolysis (carbonization) occurs, leaving only carbon and inorganic ash. In many parts of the world, charcoal is still produced semi-industrially, by burning a pile of wood that has been mostly covered with mud or bricks. The heat generated by burning part of the wood and the volatile byproducts pyrolyzes the rest of the pile. The limited supply of oxygen prevents the charcoal from burning. A more modern alternative is to heat the wood in an airtight metal vessel, which is much less polluting and allows the volatile products to be condensed.
The original vascular structure of the wood and the pores created by escaping gases combine to produce a light and porous material. By starting with a dense wood-like material, such as nutshells or peach stones, one obtains a form of charcoal with particularly fine pores (and hence a much larger pore surface area), called activated carbon, which is used as an adsorbent for a wide range of chemical substances
Charring is a chemical process of incomplete combustion of certain solids when subjected to high heat. The resulting residue matter is called Char. By the action of heat, charring removes hydrogen and oxygen from the solid, so that the remaining char is composed primarily of carbon. Polymers like thermoset or most solid organic compounds like wood or biological tissue, exhibit charring behaviour.
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible. The word is coined from the Greek-derived elements pyr “fire” and lysis “separating”.
Pyrolysis is a case of thermolysis, and is most commonly used for organic materials, being, therefore, one of the processes involved in charring. The pyrolysis of wood, which starts at 200–300 °C (390–570 °F), occurs for example in fires where solid fuels are burning or when vegetation comes into contact with lava in volcanic eruptions. In general, pyrolysis of organic substances produces gas and liquid products and leaves a solid residue richer in carbon content, char. Extreme pyrolysis, which leaves mostly carbon as the residue, is called carbonization.
The process is used heavily in the chemical industry, for example, to produce charcoal, activated carbon, methanol, and other chemicals from wood, to convert ethylene dichloride into vinyl chloride to make PVC, to produce coke from coal, to convert biomass into syngas and biochar, to turn waste into safely disposable substances, and for transforming medium-weight hydrocarbons from oil into lighter ones like gasoline. These specialized uses of pyrolysis may be called various names, such as dry distillation, destructive distillation, or cracking.
Pyrolysis also plays an important role in several cooking procedures, such as baking, frying, grilling, and caramelizing. In addition, it is a tool of chemical analysis, for example, in mass spectrometry and in carbon-14 dating. Indeed, many important chemical substances, such as phosphorus and sulfuric acid, were first obtained by this process. Pyrolysis has been assumed to take place during catagenesis, the conversion of buried organic matter to fossil fuels. It is also the basis of pyrography. In their embalming process, the ancient Egyptians used a mixture of substances, including methanol, which they obtained from the pyrolysis of wood.
Pyrolysis differs from other high-temperature processes like combustion and hydrolysis in that it does not involve reactions with oxygen, water, or any other reagents. In practice, it is not possible to achieve a completely oxygen-free atmosphere. Because some oxygen is present in any pyrolysis system, a small amount of oxidation occurs.
Pyrolysis has been used since ancient times for turning wood into charcoal on an industrial scale. Besides wood, the process can also use sawdust and other wood waste products.