Updraught kiln
The first machine that holds a temperature — pottery, brick and tile, and the training ground for every furnace above it.
Home: Ceramics and glass, Level 1 · Foothold, level 2 of 4 in its track. Serves 3 steps.
Every build on this site has hazards that injure or kill — quicklime, carbon monoxide, molten metal, collapsing ground. Read the safety notice before starting, and read this build's own failure modes below.
What it is and why this shape
An open bonfire firing reaches perhaps 700 °C, unevenly, and breaks a third of what you put in it. A kiln does three things a bonfire cannot: it contains the heat, it directs the flame past the ware, and it lets you control the rate at which temperature rises and falls.
That third one is what actually matters. Pottery does not break because a kiln is too cool — it breaks because the temperature changed too fast. A kiln is really a machine for going slowly.
The updraught form is the simplest that works: fire below, ware above, flame drawn up through the load and out of the top. It is less fuel-efficient than a downdraught or a cross-draught kiln and considerably easier to build, which is the right trade for a first one. Learn to hold and read a temperature here, then apply exactly the same skills to lime, brick and the bloomery.
Vertical section
Fig 1
Earth / subsoilMasonry / brickGravel / aggregateFire / hot zoneVoid / air
Perforated floor — hole pattern
Fig 2
Masonry / brickGravel / aggregateVoid / air
Keyed parts
| 1 | Damper / temporary top cover | Broken shards and clay slabs. Controls draught and, critically, slows the cooling. |
| 2 | Ware chamber | Ware stacked with gaps everywhere so flame reaches every surface. Never packed solid. |
| 3 | Perforated floor (bag floor) | Carries the load and spreads the flame. Holes total ~15% of the floor area. |
| 4 | Firebox | Combustion happens here, not in the ware chamber. Depth matters more than width. |
| 5 | Firemouth, 2 off opposite | Two, fired alternately, so heat enters evenly instead of favouring one side. |
| 6 | Grate bars or a raised hearth | Lets ash fall clear and air come up under the fuel. |
| 7 | Ash pit and primary air | Adjustable — this is your main temperature control. |
| 8 | Wall, 230–300 mm | Brick, or clay with heavy temper. Thicker at the firebox where erosion is worst. |
| 9 | Buttress / binding hoop | A kiln pushes outwards when hot. Ties or buttresses stop it opening at the joints. |
Specifications
| Internal bore | 1.10 m | Round is stronger and fires more evenly than square |
| Ware chamber height | 1.10 m | ≈ 0.9 m³ usable after allowing for stacking gaps |
| Firebox depth | 0.50 m | Below the perforated floor. Deep enough to hold a real fire |
| Wall thickness | 250 mm | 300 mm around the fireboxes where erosion is worst |
| Perforated floor open area | ≈ 15% | Graded: larger holes at the centre, smaller at the perimeter |
| Firemouths | 2, opposite | Fired alternately. One firemouth gives a permanently hot side |
| Working temperature | 900–1,050 °C | Earthenware and brick. Stoneware needs a different kiln |
| Firing cycle | 18–30 hours | Including a long slow start and a much longer controlled cool |
| Fuel per firing | 0.7–1.2 m³ wood | Dry, split small. Updraught kilns are not fuel-efficient |
| Cooling | ≥ 24 h, sealed | Longer than the firing. Rushing this is the main cause of losses |
Bill of materials
| Material | Quantity | Notes and substitutes |
|---|---|---|
| Fired brick or clay blocks | ≈ 900 | Build the first kiln from sun-dried clay blocks; it will fire its own replacements. |
| Clay mortar | ≈ 0.4 m³ | Clay and sand. Never lime mortar — it fails at these temperatures. |
| Floor slabs | ≈ 1.2 m² | Thick fired slabs, drilled or moulded with the hole pattern before firing. |
| Grate bars | 6–8 | Iron if available. Substitute: a raised clay hearth with side air channels. |
| Binding hoop or buttresses | 2 hoops or 4 buttresses | A kiln expands and pushes outward when hot. Something must resist that. |
| Shards and props | as needed | For stacking ware clear of the floor and of each other. |
| Damper slabs | 4–6 | To close the top progressively during cooling. |
Tools: Trowel · Plumb line and string · Long poker and rake · Shovel · Something to judge colour by — see the procedure
Construction
Site and foundation
Dry, firm ground, clear of anything flammable, with the firemouths facing across the prevailing wind rather than into it. Dig out and ram a level foundation. A kiln that settles unevenly cracks through the wall on the first or second firing.
✓ Check: Foundation level within ±10 mm and firm enough not to print under a heel.
Build the firebox and firemouths
Raise the firebox walls with the two firemouths opposite each other and the ash pit below. Corbel the firemouth arches or turn them over a former. Use the thickest walls here — this is where the kiln erodes.
✓ Check: Both firemouths the same size and at the same height, or one will always draw harder.
Set the perforated floor
Bed the slabs on the firebox walls with the graded hole pattern from Fig 2: bigger holes at the centre, smaller towards the perimeter where flame naturally wants to rush up. Leave a small expansion gap at the wall and fill it with sand, not mortar, so the floor can move.
✓ Check: Floor bears evenly on all supports with no rock, and no hole sits directly under where a pot will stand.
Raise the ware chamber
Continue the wall up, keeping it plumb and the bore even. Build in the binding hoops or buttresses as you go. Leave the top open — a temporary damper of slabs gives you better control than a fixed chimney on a kiln this size.
✓ Check: Plumb within 15 mm over full height; bore even within ±30 mm.
Dry the kiln, then fire it empty
Air-dry for three weeks. Then fire it slowly and empty, over a full day, to drive out the last water and to let the structure crack where it is going to crack — which it will. Repair those cracks with clay mortar before you risk a load in it.
✓ Check: No steam from the wall at red heat, and any first-firing cracks found and patched.
Stack the ware for flame, not for volume
Stack so hot gas can reach every surface: pieces on shards, not on the floor; gaps between everything; heavier and coarser ware low where it is hottest, finer ware higher. The temptation to fit in one more pot is how you get an unevenly fired load and lose several.
✓ Check: You can see daylight through the load from below before you close it up.
Water-smoke, slowly
The first 4–6 hours are a very small fire — barely more than a smoulder — driving out physical water. Rush this and pots explode, and one exploding pot takes its neighbours with it. You are done when a cold blade held over the top stops fogging.
✓ Check: No condensation on a cold metal blade held in the exhaust.
Raise the heat, alternating firemouths
Now build the fire steadily over 6–10 hours, stoking the two firemouths alternately and controlling the rate at the ash pit rather than by piling on fuel. Aim for a steady climb, not a fast one. Judge temperature by colour in dim light: dull red ~700 °C, cherry ~800, orange ~900, bright orange-yellow ~1,000.
✓ Check: The whole chamber glows the same colour top to bottom. A dark zone means a stacking or floor-hole problem.
Soak, then seal
Hold at temperature for 1–2 hours so the heat penetrates the thickness of the ware rather than just its surface. Then stop stoking and seal everything — firemouths, ash pit and top — with clay and shards.
✓ Check: No draught anywhere. Any leak becomes a cold spot and a crack.
Cool slower than you fired
Leave it sealed for at least 24 hours, longer for a big load. More ware is lost to fast cooling than to anything else, because cooling failures happen silently after the exciting part is over. Do not open to peek. Unpack only when you can hold a piece in a bare hand.
✓ Check: Ware comes out cool to the touch, ringing when tapped rather than sounding dull.
Tolerances that matter
| Dimension | Target | Why it matters |
|---|---|---|
| Floor open area | ≈ 15%, graded | Too little and the kiln will not draw; too much and flame rushes through without giving up heat. |
| Wall plumb | ±15 mm over height | A leaning wall pushes itself over as it expands at temperature. |
| Water-smoking period | ≥ 4 h at low heat | Physical water flashing to steam is the main cause of pots exploding. |
| Temperature climb | ≤ 100 °C per hour | Faster than this stresses the ware and the kiln structure alike. |
| Cooling time | ≥ firing time | Thermal shock on cooling causes more losses than anything during the firing. |
| Floor expansion gap | 10–15 mm, sand-filled | Mortared solid, the floor lifts or cracks the wall as it expands. |
How this fails
Ware that was not bone dry. Trapped water becomes steam and the piece bursts, taking its neighbours with it. Dry ware for weeks, then warm it near the kiln before loading.
Rushing the cooling. The most common loss and the most avoidable. Everything survived the firing and then cracks in the night because someone opened the top to look.
Lime mortar in the structure. It decomposes at kiln temperature and the joint fails. Clay mortar only.
Firing one firemouth. Gives a permanently hot side and a permanently cold side. Alternate them, consistently.
Packing the load solid. Flame cannot reach the middle, so the outside is overfired and the middle is raw.
No hoops or buttresses. The kiln expands hot and opens at the joints, and each firing widens the crack until a wall goes.
Limestone or shell in the clay body. It converts to quicklime in the firing, then slakes on damp air afterwards and blows pits out of the finished ware days later.
How you know it worked
VerificationA full load has come out with under 5% losses, ware rings rather than thuds when tapped, and a fired test piece does not soften or slump when returned to the same temperature.
Scaling and variants
Smaller: a pit firing or a 0.5 m barrel kiln reaches 700–800 °C, needs almost no construction, and is the right way to learn to read heat and to test a clay body before committing weeks of building.
Larger: scaling an updraught kiln up wastes progressively more fuel, because the heat leaves out of the top by design. Beyond about 1.5 m bore, build a downdraught kiln instead: the flame goes up the side, is turned at a domed roof, drawn down through the ware and exits at the floor into a chimney. Far more even, considerably more efficient, and it will reach stoneware temperatures — but it needs a real chimney and a dome, so build the updraught version first.
Same structure, other jobs: enlarge the firebox and this kiln burns lime; load it with green bricks and it becomes a brick kiln. The skill you are actually acquiring here is holding and reading a temperature, and that transfers to every furnace above this tier.