Home >  Resources >  Charcoal Guide Contents

• Basic Principles
• Instructions for a Small Metal Kiln
• Common Misconceptions
• Effect of Kiln Size on Operation
• Scientific Background
• Bibliography

Basic Principles

• Wood in addition to any moisture content is mainly composed of cellulose, hemicellulose, and lignin which are compounds of carbon, hydrogen, and oxygen.
• Charcoal is mainly carbon. The process of making charcoal is that of converting wood into carbon, i.e. carbonization.
• Carbonization is the process of heating the wood to remove the moisture and to break down by pyrolysis the wood compounds into carbon plus a range of volatile compounds containing the hydrogen, oxygen, and some of the carbon.
• Although the process is sometimes referred to colloquially as "charcoal burning", pyrolysis is quite distinct from complete combustion in which oxygen from the air combines with the wood compounds to eventually produce just carbon dioxide and water.
• The process of carbonization inherently involves the release of substantial amounts of volatile compounds which must be vented from the kiln.
• Removal of moisture takes place typically at temperatures of 110-150°C.
• Pyrolysis occurs at temperatures in the region of 260-380°C. Better purer charcoal is obtained with temperatures at the upper end of this range.
• The process of heating and removing moisture from the wood requires heat energy to be supplied, while the chemical reactions of pyrolysis are slightly exothermic, i.e. they release heat, but this is much less than that released by combustion.
• The process of carbonization can take place in the absence of air provided there is a source of heat, e.g. in a retort with external heating. In a kiln, however, heat is provided by burning directly within the kiln a proportion of the wood charge to be carbonized, and so a limited air supply is required.

Positioning and setting up the kiln

1. Make sure the ground where the kiln is to be placed is roughly level.
2. Position the 4 metal air inlet channels and 4 air inlet/outlet channels (with fittings for the chimneys) alternately in a spoke pattern. The inner ends of the 4 inlet only channels should be fairly close to the centre of the base of the kiln. The 4 inlet/outlet channels can be skewed somewhat so the inner ends are further away from the centre of the kiln while the chimneys when fitted will still be right next to the kiln walls — this is to facilitate the "reverse draught" mode (see bullet 32 below).
3. Roll the kiln body into position on top the air inlet and outlet channels, ensuring that the gaps between the air channels are open all the way the round.

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Creating the lighting channels

4. Place pairs of pieces of wood slightly thicker than the inlet and outlet channels between the channels to form stringers for lighting channels.
5. Pack the lighting channels with dry newspaper, kindling and firelighters. The newspaper and kindling can usefully be soaked in a moderately flammable liquid like cooking oil, but not a highly volatile flammable liquid such as white spirit, let alone explosively flammable petrol.
6. Place wood across the stringers to form the roof of the lighting channels — incompletely charred wood from the previous burn is ideal for this purpose.
7. Any spaces not yet used at the bottom of the kiln can be filled with more kindling or small pieces of wood aligned radially to the kiln.

As a simplification for the small kiln the wooden stringers can be dispensed with and all the space between the metal air channels filled with kindling etc. Pieces of wood to cover the kindling layer are placed across the metal air channels.

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Preparing the wood charge

8. Use well seasoned wood if available.
9. Wood that has been air dried during the best time for drying, i.e. spring and early summer, is better than freshly cut wood.
10. Cut the wood short enough to fit in the kiln.
11. If stacking a layer of the charge vertically then cut the wood for that to a consistent length.
12. Cut larger diameter pieces shorter, especially if using freshly cut wood, so they can be placed in the hotter middle part of the kiln.
13. It is best to split the largest pieces (above 150mm diameter).
14. Don’t mix very large and very small diameter pieces in the same charge.

Filling the kiln

15. Start filling the kiln with pieces of wood laid horizontally on top of the air inlet and outlet and lighting channels.
16. In line with the filling of traditional earth covered kilns the bulk of the kiln can optionally be filled with wood stacked vertically. This means that the material collapses more slowly during the burn, so maintaining lower resistance to the flow of gases through the kiln for longer.
17. Stack the wood vertically round a central pyramid arrangement outwards. This should make the stack collapse inwards away from the cooler wall of the kiln.
18. Place the larger diameter pieces of wood nearer the centre and towards the bottom of the kiln where it always gets hotter than the near the walls or the top.
19. Finish filling to the top of the kiln by stacking horizontally.
20. The kiln can be filled slightly above the top of the walls (up to 100mm) because the charge will sink before the lid needs to go on.

The usual advice is to pack the kiln as neatly as possible to maximise the amount of wood in the charge. However, it may be much quicker and less effort to not worry and just throw the wood in higgledy-piggledy. In this case wood can be stacked well above the top of the walls (up to300mm), because it will sink more as the burn gets underway.

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Lighting and initial moisture removal

21. Set light to the lighting channels; starting with the channels on leeward side, and leaving a few minutes before lighting the channels on the windward side.
22. The quickest way to get the kiln to heat up is to leave the lid off until the carbonizing temperature is reached. This usually takes 30-60 minutes.
    There are a number of indicators of this temperature:
    • the wall of the kiln becomes much too hot to touch,
    • the rusty colour of the kiln wall darkens,
    • water will sizzle if splashed onto the kiln,
    • the kiln is too hot to stand close to comfortably, or
    • the copious steam billowing out of the kiln begins to be tinged with darker yellowish smoke.
23. It should be possible to light the wood at the top of the kiln by throwing on hot embers from the base — aim for near the kiln wall where the air isn’t swamped by steam. Alternatively, the wood at the top may light spontaneously. Leave to burn for several minutes to ensure the upper part of the kiln gets really hot.
24. Once the kiln is hot enough lift the lid into position — if the top of the charge is alight, manoeuvre the lid using logs inserted into the handles. If the charge has sunk below the top of the kiln wall, sit the lid down on the lip just inside the rim of the kiln wall to put the flames out. If the top of the charge is still above the kiln wall, sit the lid down on the charge — if the top of the charge is already alight it may still flame around the edges for a while.
25. Insert a pair of logs across the top of the kiln wall, so as to leave a gap between the lid and the kiln wall all the way round. If the top of the wood charge is still above the kiln wall insert the logs to raise the lid above the charge — the charge will soon sink to leave the gap in the same way.

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Controlling the charring of the wood

The process of charring the wood, i.e. carbonizing it, is one of controlling the air supply at the bottom of the kiln and the exhaust flow at the top, so as to maintain the temperature for pyrolysis. Precise control of the various air inlets and vents is required to even out temperature differences and hence charring rates in different parts of the kiln.

26. As the moisture evaporates and the wood dries out and the pyrolysis reactions progress, the carbonization process becomes progressively more exothermic, tending to increase the kiln temperature. To compensate for this the air inlets and the exhaust vents need to be progressively restricted.
27. Restrict the air supply progressively by:
• first blocking up the spaces between the air inlet channels bit-by-bit with sand or soil, and
• blocking the 4 air inlet/outlets, leaving just the 4 air inlets open.
28. Restrict the exhaust progressively by:
    • first removing the spacing logs and lowering the lid on to the kiln wall,
    • then sealing the perimeter of the lid with sand/soil, and
    • then partially covering the 4 steam vents on the lid.
29. Sand (e.g. conventional building sand) is much more effective at sealing than soil — organic matter in top soil may burn, while clay will shrink as it dries out.
30. To limit the difference in temperature between the top and bottom of the kiln the progressive steps to restrict the air supply should be applied slightly ahead of those to restrict the exhaust.
31. To avoid the side of the kiln facing the prevailing wind getting hotter and carbonization of the wood on this side finishing before the rest of the wood the inlets and vents should be restricted on the windward side first.
32. The most restricted state of the kiln which still allows pyrolysis is the so-called "reverse draught" mode. This is achieved by fitting the 4 chimneys to the inlet/outlet channels and closing the steam vents on the lid by covering and sealing with sand/soil. In this mode gases circulate upwards in the hotter middle part of the kiln and then sink down near the walls before being vented through the outlet channels and chimneys. The chimneys will draw once they have warmed up by being adjacent to the hot kiln wall — this usually takes 5-10 minutes. For a small kiln the gas flow in the reverse draught mode is insufficient to maintain adequate carbonization temperature except towards the very end of the pyrolysis phase.
33. In the "reverse draught" mode some chimneys may not draw properly. Sometimes a chimney position can be made to draw by swapping the chimney with one which is already hot from gases venting through it.
34. The process of charring the wood takes 5-7 hours in the small kiln depending on the degree of air supply and exhaust restriction applied.

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Shutting down the kiln

As the drying out and pyrolysis phase progresses the exhaust changes from the white of mostly steam to a thicker, darker, yellowish smoke, and finally to a clearer, thin, bluish smoke. The blue smoke is from the charcoal itself starting to burn.

35. When the thin blue smoke is issuing from all 4 steam vents on the lid or from all 4 chimneys if reverse draught has been applied, the pyrolysis phase is complete and the kiln can be shut down and allowed to cool.
36. Thoroughly seal up all air inlets and any remaining gaps at the base of the kiln with sand/soil.
37. Thoroughly seal up the lid perimeter with sand/soil.
38. Cover all 4 steam vents in the lid with blocks and seal with sand/soil.
39. Remove the chimneys and cover or bung up the chimney fittings in the air inlet/outlet channels.
40. The kiln can take between 6 and 12 hours to cool before the walls can be touched and the lid removed safely. It may take 24 hours before the kiln is completely cold.

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Emptying the kiln

41. Lift the lid off.
42. Remove the kiln body by lifting up one side and then rolling it away to leave the finished carbonized charge behind.
43. If parts of the charge are still hot douse with water.
44. Sort out incompletely charred wood from charcoal — the latter is lightweight, black all the way through, and will break up easily; while the incompletely charred wood (usually from the top and sides of the kiln) is heavier, looks like brown wood, and can’t be broken when struck.
45. Sieve the rest of the charcoal to separate reasonable sized pieces from the very fine charcoal and ash concentrated at the bottom of the kiln.