Cavity Kilns in Canada – October 2014

By |2019-03-13T10:36:07+00:00April 6th, 2015|0 Comments

In September I did a biochar presentation in Calgary, Canada, which seemed to be a mecca for Permaculture, with participants from several permaculture organizations eager to hear what biochar could do for them. There are several initiatives around the world including from Switzerland, Australia and the US to start a biochar training courses along the lines of Permaculture courses.

The presentation in Calgary was a prelude to a hands-on weekend biochar workshop at beautiful Quantum Leaps Lodge near Golden BC in the Canadian Rockies.

This is a photographic blog of a pyramid kiln built out of a single sheet of steel, and a cylindrical kilns cut from a 2000L propane tank. This is a continuation of work exploring Cavity Kilns, following the development of the Kon-Tiki Deep Cone Kiln at the Ithaka Institute, reported in 5 previous blogs.


Template for pyramid kiln, 4 sides and base out of 2450 x 1225 mm sheet.

  • Choosing a base of 500 mm allows 9 mm to spare on either side of square base

Pyramid kiln, 4 sides and base out of Canadian mild steel sheet

Mark and cut the steel sheet

  • 1 sheet of mild steel, 1/8 inch = 3.2mm. Sheet size = 2450 x 1225 mm
  • Mark centerlines along and across sheet, and mark out the sides of the 4 trapezia
  • Using thin cutoff wheel in grinder cut out the 4 trapezia, leaving intact the residual diamond
  • Cut the square base out of the residual diamond.

Raising and holding the sides ready for tack welding

  • Use the square base or a large square to set the sides square.
  • Take the sides in position.
  • Un-allowed for errors due to thickness of saw cuts may be filled when welding.

Weld the entire seam on the outside and inside for a leak proof kiln

Weld on the bottom

  • Tack the base and run the weld along all edges of entire base

Add stiffening to top of kiln

  • Use 25 mm x 5 mm thick angle iron
  • Cut ends at a compound angle for the 45o miter corners and the slope of the kiln wall.
  • Adjust ends, clamp and re-adjust ends as needed to fit
  • Weld corners and tack angle to sides at ends and center

Weld handles and drain

  • Handles made from 10mm rebar.
  • Turn kiln upside down and mark and cut hole for the drain.
  • Weld on a short nipple with pipe thread to take a 90o galvanized pipe elbow, 1.5 inch or 38 mm is OK
  • Use black pipe or an old pipe that has lost the galvanizing for the piece to be welded.

Making a “cone” kiln out of old 2200 L (500 gal) propane tank

  • Fill tank with water to expel all propane
  • Drain with siphon
  • Cut off 1/3 of propane tank with oxy-acetylene or oxy-propane torch.
  • Legs were kept for use as handles support, but can be optionally cut off.

Install drain and legs

  • Cut a short nipple threaded on one end. Use black pipe or an old pipe that has lost the galvanizing. 1.5 inch or 38 mm is OK.
  • Cut a hole in bottom of kiln, weld nipple around it, install a 90o galvanized elbow, and a sufficiently long horizontal pipe terminating in a plug or valve.
  • Install legs of sufficient length to keep drain clear of ground.


  • Preserves heat in the kiln,
  • Keeps the outside warmer,
  • Preheats combustion air,
  • Stabilizes against wind, and
  • Provides a cool barrier.
  • Shield is made from a roll of 3 ft wide, 12 ft long, thin metal flashing.
  • 8 x 120mm (4in) long threaded rods were welded 50mm from the top of the kiln at 8 equally spaced points.
  • Nuts were placed on each rod.
  • Holes were drilled in the flashing so it would sit 80 mm higher than the kiln rim edge.
  • The flashing was wrapped around the kiln, supported on the rods and held with a second nut on each rod.

Firing The Pyramid Kiln

  • Build an open stack with kindling on top
  • Light kindling
  • When fully flaming and stack collapses add a layer of wood.
  • As the layer reaches pyrolysis temperature, indicated by ash forming on the surfaces, add another layer of wood.
  • Continue till the kiln is full

Loading cylindrical kiln with kindling & setting alight

  • The cylindrical kiln was filled with sticks and brambles, which formed a natural open platform on which to start a kindling fire
  • Cylindrical kiln smokes as it starts on rained-on sticks while pyramid kiln burns cleanly after additional layer is added

Clean pyrolysis and combustion from the cylinder kiln

  • Heat shimmer but no smoke above cylindrical kiln indicated clean pyrolysis of wet sticks, green wood, and stumps up to 1 cu foot (30x30x30 cm) in size, even as additional material is added.
  • Kiln firing continued into evening. Outer surface of wood has ashed, requiring addition of another layer.

Burning paint off barrel to be used for drum kiln.

Quenched biochar in pyramid kiln.

  • The pyramid kiln was quenched by spraying by hose from above.
  • It could have been quenched from below via hose to the drain as shown for the cylindrical kiln


  • Water was slowly introduced to kiln through the drain valve cooling bottom of kiln, while pyrolysis continued on top.
  • Kiln was filled to top with water, which floated up char. Minor pyrogas flaming (yellow flames) and CO combustion (blue flames) continued on top of kiln brimming with water.
  • Top layer was quenched with hose spray

Draining Quench Water

  • Quench water was brown on first drawer from bottom
  • But clear as the higher levels were drained
  • Biochar slumped as quench water drained

Alkaline pH of biochar

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