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Book:	Woodlands
Chapter:	11 Firewood and charcoal
Section:	Charcoal
Metadata:	Details	Buy this book

Charcoal is formed when wood or other organic materials are heated out of contact with air. Charcoal has a high carbon content, and burns at a higher temperature than wood. Charcoal making is an ancient process, used in Europe for at least 5,000 years, and has been of huge importance in the development of civilisation. Charcoal was the smelting fuel of the bronze and iron ages. The liquor produced during charcoal making was used by the Ancient Egyptians in the embalming process, and it's likely that glass making, which requires the high temperatures possible with charcoal, was also developed during this period.

Charcoal is still used in industrial processes, for barbecue fuel and for other uses (see below). The popularity of barbecues in Britain and elsewhere has meant a great increase in demand for charcoal. From being virtually a dying craft, there are now many portable kilns in operation in the UK. This not only helps keep woodland management viable, but also reduces imports of foreign charcoal, which is not always produced from properly managed forest resources. Charcoal from British hardwood is lighter in weight than imported charcoal from tropical hardwood, and is easier to light and faster to reach cooking temperature.

Charcoal making is hard and dirty work, and the profits for the small-scale operator are low. However, it is a useful process for adding value to the management of derelict coppices or other neglected woodland, as it uses waste wood which would otherwise have no value. In-cycle coppices (see coppicing) produce very little waste wood, and are managed for higher value products.

Charcoal burning operations can be categorised as follows:

Large scale

Large commercial operators may use four or more kilns on a static site, with machinery for loading and unloading. Collection of tarry residues, recycling of gases, use of retort kilns and other processes are designed to increase efficiency and profitability. Marketing is likely to be on a national scale, for various uses.

Commercial viability is dependent on efficient processes, good marketing and the relative cost of imported charcoal. Various organisations including the Forestry Commission and the Forestry Contracting Association (see appendix B) have studied the commercial viability of charcoal production in Britain in recent years.

Retort kilns are more efficient than ring kilns, as they use a separate heat source to bring the wood to a sufficiently high temperature for carbonization to take place. At the time of writing, one kiln manufacturer in Britain is producing a small retort kiln for charcoal production (see appendix B). Continuous carbonizing retorts, which are very efficient but have a high capital cost, are in use in other countries (Hollingdale, Krishnan and Robinson, 1999).

Medium scale

A small operator or voluntary group with one or two portable ring kilns should be able to produce a few tons of charcoal a year, for which they could find a local market for barbecue fuel, with additional other uses possible. This scale of charcoal production is only likely to be commercially viable if it's tied in with other woodland management enterprises, or if it has a significant voluntary input.

Small scale

Small amounts of wood can be converted into charcoal in a kiln made from an old oil drum, for home use or resale locally for barbecues.

The details that follow relate to medium scale operations. Use of old oil drums is described below.

Ring kilns

The traditional earth kilns are now used only for demonstration purposes, and most charcoal burners use portable metal ring kilns.

Design

Ring kilns comprise a bottomless steel cylinder, with a removable lid, chimneys and air inlet/outlet channels. The air inlets and chimneys can be adjusted to steer the hot gases around the kiln to result in even and thorough carbonization. A 6' kiln, for example, will normally have six inlet/outlet points, with three chimneys that can be moved from one point to another. Various sizes of kiln are available from kiln makers, or can be made by light engineering firms. The optimum size will depend on the scale of the operation, the number of operators, the requirement for portability and other factors. The sizes listed below give a guide to suitable dimensions.

The larger diameter kilns are made in two stacking rings, to give sufficient height while still making the kiln portable and easy to load/unload. Some stacking kilns have a tapering top ring, but this is more difficult to move around on site as it cannot be rolled along the ground. The diameter/height ratio is important, as for example an 8' (2.4m) diameter kiln with a height of less than about 6' (1.8m) will not burn efficiently. Using only one ring from a double kiln will not produce an efficient yield. Note that at least two people are needed to lift the upper ring and lid into position.

Tall, narrow kilns, for example 5' (1.5m) diameter by 2'6" (0.8m) height also work efficiently. An old oil drum makes a useful small kiln of similar proportion.

As they are subject to heat stress and physical damage, ring kilns must be made of a good quality steel, to give an 8-10 year working life. The lower ring should be made of 3mm steel, and the upper ring and lid of 2mm steel. Construction must be simple and robust, to avoid problems with tar accumulation. Design details for a 7'6" (2.3m) diameter double ring kiln are given in Hollingdale, Krishnan and Robinson (1999).

Details of loading, igniting and operating will vary with the type of kiln and the operator, but the general principles for operating a double ring kiln are described below.

The air dried wood is packed tightly into the kiln and then ignited, and burning is controlled by restricting the air supply. Too much air results in the wood burning to ash, which not only wastes the wood and the preparation time, but is also likely to damage the kiln. Too little air results in only partial conversion to charcoal.

Output

The dryness of the wood is important to the efficiency of the operation. If wood is dried to 20% moisture content or less, one tonne of charcoal can be produced from four tonnes of wood, giving a charcoal yield of 25%. Under normal woodland conditions in Britain, wood air-dried for a year or more is seldom this dry, and one tonne of charcoal from six tonnes of wood, or a charcoal yield of 16%, is the more normal conversion rate. In drier climates, charcoal yields of 25-30% are easier to achieve.

The wood or 'charge' not only loses weight during conversion to charcoal, but also loses volume. Typically it will shrink by 25-30%, so for efficient use, the kiln must be packed tightly and to the top, finishing with a shallow cone to leave the minimum of space under the lid. Charcoal yield is also affected by the dimensions of the ring kiln and other design details, and by the skill of the operator in controlling the burn.

The capacity and yield figures are for guidance only. The capacity given is total capacity; the amount of wood a kiln will hold will depend on the uniformity of the wood and how closely it is packed. Charcoal yield will depend on the factors given above.

In British woodland conditions, the rule of thumb is that about half a tonne (500kg) of charcoal will be produced from 3 tonnes of wood in a double ring kiln (7'6"-8' diameter), equivalent to a 16% yield. Note that 2.5m³ of air dried wood weighs about 1 tonne.

Operating programme

In Britain, most charcoal burning in ring kilns is carried out during the summer months. This is because most operators will be doing felling, coppicing and other woodland management work in the winter, and also because conversion to charcoal is quicker and more efficient in dry weather. Long hours of daylight, dry ground conditions and the chance of warm, dry weather are important practical considerations.

For efficient commercial operation, at least two kilns are needed, running in overlapping cycles. While one kiln is burning, another can be cooling and so on. Hollingdale, Krishnan and Robinson (1999) give a detailed operating programme for two double ring kilns over a 5 day week, each used twice in the 5 days and producing in total 2-3 tonnes of charcoal a week. This relates to tropical climates, and is not directly applicable to British conditions.

For voluntary groups with one kiln, it may be necessary to load and burn over a day and a night, and then return a few days later to empty the kiln. At least one person must stay on site until the kiln is cool, and the security of the site should also be considered. A kiln full of charcoal is an obvious target for arsonists.

By increasing the draught a quicker burn can be achieved, but less charcoal will be produced than by a slower burn. This may however be the best way for voluntary groups to operate, where supervision over 30 hours or more is difficult. Smaller kilns have faster turnaround times than larger kilns. A 6ft (1.8m) single ring kiln can be filled, burned and shut down in 12 hours for example. It's important to get the kiln well lit at the start of the burn, or burning time will be extended. Dry wood, dry ground and dry weather also speed up the burning time.

Some operators only burn at night, to reduce the nuisance caused by smoke and other emissions.

Materials

Species of wood

Most charcoal making uses thinnings or poor quality waste that is too small or awkwardly shaped for other uses, so a mixture of species is normally available. Species vary in the quality and quantity of charcoal produced, as indicated below. It's best to fill the kiln with one species at a time, as this will make the burning process easier to manage and give more consistent results. The charcoal can be mixed before bagging as required.

Ash, sycamore, beech, hornbeam and elm make good quality charcoal for barbecue use. Sycamore is quick and easy to convert to charcoal, because of the thin bark and ease of burning, even when 'green' or in wet weather conditions.
Oak gives the best return by weight, but it's generally a waste to use it for charcoal, when higher value may be gained from other uses. Oak is slow to convert to charcoal, because of the thick bark.
Birch and hazel are less productive by weight, but make reasonable charcoal. Birch is rather friable and tends to break up in transit.
Alder charcoal breaks down very fine and is used for gunpowder.
Chestnut charcoal tends to break up, but it burns very hot so is valuable in forging metal, and may find a market with blacksmiths.
Charcoal made from willow tends to spit in use and doesn't burn as long as other hardwoods, but has the advantage of lighting easily. Mix it 50/50 with other species before bagging for barbecue use.
Softwood charcoal burns well, but it breaks up easily in transit and is very dusty.

Size, seasoning and stacking

Lengths of 600-900mm (2'6"-3'), with a minimum top diameter of 50mm (2") and maximum butt diameter of 150m (6") are best, in straight lengths without forks. For efficient conversion rates, the kiln must be packed tightly with the minimum of air space. Because of the extra work involved, it's not usually worth splitting larger diameter wood for charcoal making. A more profitable use for waste wood of this size is to split it and sell it for firewood.

The wood should be cut in winter when the sap is down, and air dried in a stack for at least a year before it is converted to charcoal. Wood cut in the growing season, when the sap is up, will take longer to dry than wood cut in the winter. Chestnut should be dried for two years. Dried wood is much easier to light and cleaner to burn and produces more charcoal per weight of wood.

Where operators are using bought-in supplies of wood, the price paid must reflect whether or not the wood is dried. Trying to keep up production by burning insufficiently dried wood will lead to production problems.

Correct stacking is important for efficient working. Species should be kept separate. Stacking should include sorting by thickness of material, as thicker wood is required for the centre and top of the kiln, and thinnest around the edges. A layer of sheeting under the top layer of the stack, but not enclosing the sides, will help speed drying.

Sand

Sand is required for sealing around the lid and base of the kiln. A cubic metre of sand, sufficient for about four burns, costs £20-30, so try to source from within the wood if possible. Alternatively, dry stone-free soil can be used.

Siting the kiln

Always check with your local planning authority before starting any charcoal burning operation as planning permission may be needed. Small, temporary operations are likely to be exempt from regulations relating to the change in use of land, but the planning authority should always be consulted.

It is cheaper to transport charcoal than to transport the wood required to make it. This is because the raw material weighs about 5 times the finished product. The wood must be stacked and air dried for at least a year before burning, and for commercial efficiency, the wood should only be handled and stacked once before it is burnt. There are two options:

Stack the felled wood at the nearest suitable site for burning, and move the kiln to this site when the wood is dry and ready for conversion.
Keep the kiln on a semi-permanent site, and transport the raw material to this site for stacking, drying and conversion.

The choice will depend on the size of the operation, the availability of suitable kiln sites, the type of transport available and other factors. In practice, a semi-permanent site to which the raw material is moved is normally best. Suitable sites for burning (see below) may be in short supply, and the extra cost and time of transporting the raw material may be outweighed by the benefits of working from one site. In addition to the kiln, other equipment and facilities are needed which are easier to set up efficiently on a semi-permanent site. The efficiency of the system is crucial for commercial operators, but less important for voluntary groups.

Within the wood, the kiln should be sited with care. Note the following:

The kiln must be sited on flat, dry ground. Water must not be able to run into the kiln, so a ridge or raised area with sufficient level ground is the best choice. Broadleaved woodlands tend to be in wet areas or on sloping ground which were not suitable for cultivation, so finding the perfect site may not be easy! Don't site the kiln on peaty ground, as there is a high risk of setting fire to the peat resulting in an 'earth burn', which will be difficult to put out. Light sandy soils give better results than heavy clays or other wet ground, because a light, well-drained soil more readily absorbs the moisture given off during carbonisation.
The kiln must be sited near a track or road for easy transport. In coppice woodlands, a location which is fairly central to the coupes to be cut over the following few years will reduce haulage distance for the raw material. Sufficient space to stack wood nearby for air-drying is needed, to minimise handling.
Charcoal burning produces a significant amount of smoke, so consider the effect on neighbours, roads, airports and other land users. Burning at night may be the best option to lessen adverse effects on neighbours. A site sheltered from the prevailing winds will be best in windy areas. However, bear in mind that charcoal burning usually takes place during the summer months, and trying to burn during an anticyclone, when there is little movement of air to disperse the smoke, can also cause problems.
A nearby water supply is useful, as water is required for safety precautions, as well as for the domestic needs of the charcoal burners.
Plan for several burns on a site, even for portable kilns. The first burn on new ground is not efficient, because of the moisture in the ground. Burn some brash on it first to dry it out a bit. With use the site will get baked and burn efficiency will increase.

Tools and other equipment

Tools

Pick or mattock for levelling the ground to site the kiln
Saws, billhooks for preparing the wood
Shovels for shovelling sand and charcoal
Wheelbarrow for transporting materials
Buckets, stirrup pumps, fire beaters for fire precautions

Grading/bagging devices

It's worth making up a device that grades the charcoal, and makes it easier to bag it up. Shovelling the loose material into bags is slow and inefficient, and it is difficult to keep the outside of the bags clean. Clean bags are essential for marketing. See below for further details.

Transport

Transport of raw materials and product within the wood and to the point of sale is likely to be crucial to the success of all but the smallest of operations. Dumper trucks, old tractors and trailers and other old and easy-to-maintain machinery tend to be used within the wood by small operators, especially as they are unattractive to thieves and vandals. A trailer, pick-up truck or other vehicle will normally be needed for transport to point of sale. Vehicles must be properly maintained and used only by competent operators.

Storage and accommodation

A lockable caravan or shed is useful for storing tools, sacks and other equipment, and as shelter for the operators.

Safety precautions

General

The ground for at least 2 metres around the kiln must be clear of leaves, sticks, empty charcoal bags and other flammable material. Most charcoal burning takes place in summer, when there is a risk of fire spreading through the woodland.
Keep a similar area clear of any hazards so that operators can move quickly and safely away from a flare or other danger. Don't leave tools where they may be tripped over by an operator moving out of the smoke.
Inform the fire brigade in advance of the site and time of the burn.
A supply of water and firebeaters should be immediately to hand. If a piped supply is not available, ensure at least 20 litres is to hand in a jerrican or similar container.
There should always be at least two people on the site, in case of accident or emergency.
The kiln must be closely supervised throughout the burn, and during the cooling down period (see below).
Where the public have access, the site must be cordoned off and clearly signed.
Empty and full bags must be stored in a secure place where they do not constitute a fire risk.
Insurance through the Forestry Contracting Association is available for coppice workers and charcoal burners. Insurance for voluntary groups is available through BTCV.

Personal protective equipment

Appropriate personal protective equipment should be worn, as listed below.

Close-fitting, flame-resistant clothing should be worn.
Leather gloves with gauntlets should be worn for loading and unloading, to give protection to the hands and wrists.
Wear safety boots to protect the feet from dropped timber or other hazards.
Face masks should be worn by operators working near the kiln during the burning process, to avoid inhalation of fumes.
Face masks should be worn during emptying and bagging up, because of the dust produced.

Operating precautions

Take great care when handling kiln sections, lids, chimneys and other parts of the kiln to avoid trapped fingers.
Check to see if animals have created burrows under the kiln, as this can allow excess air into the kiln making the burn difficult to control.
Wood gas is produced during the burning process, which burns with a very hot and almost invisible flame. The pressure in the kiln will drive gas out of vents, chimneys and any gaps left in the kiln joints.
Opening the lid or breaking any seals during the burn
is dangerous as the entry of air can result in an explosion and the expulsion of burning gas. Even when cold, opening the kiln should be done with great care. Insufficiently burnt charcoal, which still has a high volatile content, will re-ignite when air is admitted.

Further safety points are included below, in the description of the burning process.

Operating procedure

This describes the operating procedure for an 8' diameter double ring kiln, with eight outlet/inlet channels and four chimneys.

Loading

Select long, straight logs about 150mm (6") diameter to use as 'stringers', which are placed as shown in the base of the kiln. Between every other pair of stringers, place kindling of paper, leaves and dry twigs, to provide four lighting points for a kiln with eight inlets. Across the stringers lay partly charred pieces of wood ('brown ends' or 'brands') from the previous burn, to form ducts which will draw the air into the centre of the kiln. The centre can also be filled with kindling and brown ends.

Load the kiln with layers of wood, packing the pieces as tightly as possible with the larger pieces towards the centre. For double kilns, lift the upper ring into position, and fill that to the top.

Finish with a conical shape to match the shape of the lid, so maximising the load. Fit the lid in position, and open the steam release valves.

Alternative methods of loading and lighting can be used according to the design of the kiln and personal preference. Some prefer to start with the kindling and brown ends in the bottom centre of the kiln, build ducts as described above, and then push a lighted rag, soaked in paraffin, through a duct into the centre of the load.

On smaller kilns, you can place a chimney temporarily in the centre of the kiln on top of some kindling, and then build up the load of wood around it. You then remove the chimney to leave a vent down which some lighted charcoal is dropped to start the fire.

Lighting

With inlets at the base and the vents in the lid open, light the kindling. On kilns with more than one lighting point, it's best to start at a point away from the wind, to make sure that this part of the load is well alight before you light the windward side, which will burn more quickly.

Allow the kiln to heat up for about 30-60 minutes until the kiln is very hot. The traditional test for temperature is when spit bounces off the surface of the kiln! During this time, fill the joint between the two rings, and depending on the design of the kiln, between the lid and the ring. During this period, large amounts of steam will emit from the steam release holes, caused by the moisture being driven out of the wood. Place the chimneys in position on alternate inlet/outlet channels.

Reducing the draught

As each part of the kiln reaches the required temperature, cover the base of the kiln with sand or soil, but leave the ends of the eight air inlet/outlet channels open.
Using sand or soil, cover the ends of the four air inlet/outlet channels which are supporting the chimneys.
Close the steam release holes.

Air should now enter the kiln only through the four air inlet channels at the base, from where it rises up through the kiln, with combustion gases drawn down and out through the chimneys. This is known as reverse draught.

Control of charring

After 15-30 minutes, thick white smoke should rise from each chimney. If smoke production slows from any chimney, temporarily remove the sand from the air inlet below to increase the air flow. Seal it off again as soon as sufficient smoke is being produced.

During charring tar is produced, which is deposited in the outlet channels and chimneys. If smoke emission from any of the chimneys reduces noticeably, remove the chimney using heat-proof gloves or thick sacking to protect the hands, and clean out the tar with a long stick. Also clean the outlet channel, and push a long stick into the centre of the kiln to make sure the gas flow is not restricted.

After about eight hours, move the chimneys onto the adjacent air channels. This converts inlets to outlets and vice versa, and encourages an even burn. Take great care as all the metalwork will be extremely hot.

Cooling

Charring is complete when the colour of the smoke from all the chimneys turn bluish and slightly transparent. This should occur 16-24 hours after lighting, for an 8' diameter double ring kiln. A test for moisture can be made by removing one of the chimneys and holding a spade over the outlet. If no moisture condenses on the spade, then charring is complete.

Remove all chimneys and seal outlets with sand. Add more sand to the seal around the rim and steam release ports to ensure that the whole kiln is absolutely airtight. Leave the kiln to cool for 16-24 hours before opening.

Unloading

During unloading, buckets of water, sand or soil should be ready in case of any fires breaking out in the charcoal.

Wait until the outside of the kiln is cold before you open it. If you open it too soon, while part of the charcoal is still alight, you are likely to lose the whole load as it re-ignites, as well as damaging the kiln. If a part is still seen to be alight, cover and re-seal the kiln immediately and leave it for a further cooling period.

The volume of wood will have greatly reduced during charring, allowing the upper ring to be removed from double ring kilns. Lever up the side of the lower ring, to remove the inlet/outlet channels. Then tip up the bottom ring to leave the pile of charcoal easily accessible for bagging. The alternative, of getting down inside the kiln, is dirty and unpleasant work. Depending on the system used, roll the ring to the alternative burning site, where it can be loaded by another team while the first load is bagged up.

Grading and marketing

Grading is required to ensure the charcoal is suitable for its end use. Barbecue charcoal must be sieved to remove the dust or fines, which can also be marketed. Effective marketing of barbecue charcoal and other charcoal products is essential for the success of any charcoal-making project.

Grading for barbecue charcoal

The British Standard and European Norm for barbecue charcoal (EN 1860-2) is currently under discussion. The following points give a guide to standards.

Charcoal must be uniformly black. EN 1860 states that 85% of the charcoal should pass through a 120mm mesh, and rest on a 20mm mesh. The generally accepted size range for barbecue charcoal across Europe is from 80mm to 20mm.

Chemical tests can also be carried out to check fixed carbon content, volatile matter, ash content and moisture content. Most charcoal makers rely on experience to judge the quality of their product. A simple test of burning the product will check that it ignites easily and burns clear, leaving only ash. Any smoke will indicate excessive volatile matter.

Well made hardwood charcoal has a high fixed carbon content, and a calorific value of about 7200 kcal/kg.

Bagging up

Charcoal can absorb moisture from the atmosphere up to about 5% by weight, but this does not affect quality, and it can be safely packaged. However, if rain gets onto the unpackaged charcoal, it will absorb more moisture which will then seep out when it is packaged. Two-ply paper bags are recommended for packaging. Plastic bags are not recommended as they will cause condensation of the moisture in the charcoal, resulting in unsightly moisture on the inside of the bag.

Most charcoal is sold by weight when packed, with '3kg when packed' being typical. Selling by volume is not recommended as the material settles in transit.

For sales appeal, you must keep the bags clean while bagging up. Have water and soap available so that you can wash your hands frequently to avoid getting the bags dirty. To reduce the dust hazard, wear a face mask and set up the sieve/chute to the leeward side of the pile of charcoal, so the dust is blown away from you.

For efficient grading and bagging use a device which incorporates a sieve for removing dust and a chute for filling the sacks. A sieve chute of the type shown (Hollingdale, A.C., Krishnan, R and Robinson A. P., 1999) is fairly simple to construct, and should allow at least 60 bags an hour to be filled by two people. For stability, the lower end of the chute can be supported on the edge of the kiln ring, with the dust then falling back into the ring for collection as required.

The device shown below was designed by the charcoal burning team at Rutland Water Nature Reserve, and allows speedy and clean filling of sacks. A similar device could be made up by a blacksmith or light engineering firm.

The bags are sealed by stapling or by using a stitching machine which stitches a tape to close the top of the bag.

Marketing barbecue charcoal

Timing is important where the main product is barbecue charcoal, as sales are strongly related to season, and affected by weather. Nearly all sales are in the summer, with peaks during school holidays, bank holidays and in fine weather. The main outlets for small operators are petrol stations, caravan and camping parks and garden centres. Most outlets have limited storage space and will not want to hold large stocks. It's therefore important for the producer to be able to maintain regular deliveries throughout the summer, with enough capacity to take advantage of any heat waves. This in turn must be closely linked to the production capacity, bearing in mind the period of up to a year required for drying the wood.

The size of bag or packaging type can vary, and marketing should be directed at particular uses and outlets. Campers and holidaymakers will normally prefer small bags or 'throwaway' barbecues in foil trays. Various forms of packaging are used, including those with bags which are designed to be lit, to avoid the user having to handle the charcoal. Restaurants, cafes, pubs, hotels, activity centres and other regular users will use large sacks, economically packaged.

For sales appeal and to compete with imported charcoal, the importance of local production, reduced transport, links with beneficial woodland management and other values need to be highlighted. Bags may be available through local coppice groups or kiln suppliers. Through personal and community links, local publicity and consistently good production and delivery, a strong local market can be developed.

VAT at the current rate (17.5% in 2001) is chargeable on all fuel charcoal by the charcoal maker to all wholesalers, retail outlets or to other charcoal makers. Charcoal for fuel sold directly by the charcoal maker to the final consumer is charged at the current fuel rate (5% in 2001).

Fines

Charcoal fines are the dust produced from charcoal burning. Fines are used as a soil conditioner and an additive to compost, to raise the pH and keep the compost 'sweet'. A mixture of 10% charcoal to 90% composted wood chips makes a good mulch or compost.

Commercial operators can convert fines to charcoal brickettes, but this can only be done in specialist kilns. Fines are also used in charcoal filters. Small operators would probably need to combine together in order to produce a marketable quantity for these uses.

Fines or dust, particularly of sycamore, may find a lucrative market with herbal apothecaries.

Artists' charcoal

Straight thin stems of spindle, willow, oak, grape vine and other species can be converted to artists' charcoal by placing the stems in a biscuit tin, and then firing it during a normal burn. Spindle makes the best artists' charcoal, but willow is normally used because it is readily available in the required stem form. Provided a market can be found, this can be one of the most profitable charcoal products.

Jewellers

Some jewellers use blocks of charcoal as a heat source for metalworking.

Small-scale charcoal burning

Small amounts of charcoal can easily be made in a DIY kiln made from a 45 gallon or similar size steel drum, in good condition without holes. Drums should be obtainable from farmers or small industrial units, but check for any contents which may be toxic.

Drum kilns are useful for small operations, or for use in woodlands with poor access, or those that lack suitable sites for larger kilns.

Small diameter branches of fast-burning species will give the best results. Large pieces or slow-burning hardwoods will not reach a sufficiently high temperature and will only be partly carbonised. Willow, sycamore, birch or softwoods are suitable.

Wood should be 50-100mm (2-4") diameter, and 300-360mm (12-14") long, air dried for at least six months. Fairly even-sized pieces of the same species will give the best results.

Loading and burning time should be about 3-4 hours, with about 12 hours' cooling required.

Although the volume of wood converted in a drum is much smaller than in a ring kiln, the dangers are similar. Note carefully the safety precautions above (see above). Take great care to avoid injury from sudden flames or 'flare outs' which may occur as the volatiles are driven out during the burning or 'white smoke' phase. Only one person at a time should be required to control the process, with assistants and onlookers keeping at least 2m clear of the drum.

Making a drum kiln

Remove the screw cap from the top. Wearing safety goggles, use a cold chisel to make three more holes, 50mm (2") in diameter. These will form the vents.
Turn the drum over and cut out the base, about 15mm (¹/₂") from the rim. Keep the base to form a lid. Knock up the cut edge to form a lip around the rim.
Place the drum, open end up, on three bricks. This permits air flow to the vents.
Place paper, kindling and brown ends into the base of the drum and light.
Once the flames from the kindling have died down and the brown ends are alight, gently drop the prepared wood onto the fire, taking care to avoid any flames. Using the larger pieces first, fill randomly to the top with wood.
When the wood is burning well, use a long-handled shovel to pile sand or soil around the base to reduce air entry, leaving a gap of about 100mm (4"). Place the lid on top, propping it on one side to leave a small gap for smoke to escape.
Thick, white, damp smoke will be emitted, from the moisture driven out of the wood. When this slows, knock the side of the drum to settle the contents. A further emission of white smoke will result.
Depending on the dryness of the wood and other factors, after between 2-4 hours the dense white smoke should change to thin blue smoke. This change indicates that the wood has converted to charcoal and that the charcoal is starting to burn. Block the air gap at the base with sand and close the lid to stop the burn. Add turves, soil or sand to block any gaps around the rim.
Leave to cool for about 12 hours. Carefully tip the drum over onto a plastic sheet or similar, and shovel the charcoal into bags or other containers.

Note that other methods can be used to convert charcoal in a drum, which involve either upending the drum or rolling it over during the burning process. There are extra hazards involved in doing this, and these methods are not recommended.