Updraft Kiln

Part of Kiln Design

Building and operating a simple updraft kiln for pottery.

Why This Matters

The updraft kiln is the first permanent kiln most communities build, and for good reason. It bridges the enormous gap between pit firing — unpredictable, wasteful, and limited to low temperatures — and the sophisticated downdraft designs that require refractory bricks and chimney engineering. An updraft kiln built from locally made bricks can reliably reach 1,000-1,100°C, consistently enough to produce functional stoneware, simple glazes, and the refractory materials needed for your next kiln.

The key innovation is separation. In a pit kiln, pots sit directly in the fuel — exposed to flame contact, ash deposits, carbon trapping, and wild temperature swings. The updraft kiln places the fire below and the pots above, separated by a perforated floor. Hot gases rise through the floor and bathe the ware evenly (or at least more evenly than a pit fire). This single change improves success rates from 50-70% to 80-95%, eliminates most surface defects, and makes glazing possible.

For a rebuilding community, the updraft kiln is the workhorse that produces everything from cooking vessels and water storage to roof tiles, drain pipes, and kiln furniture for larger kiln construction. It is the technology that makes a settlement self-sufficient in ceramics.

Design Principles

Understanding why the updraft kiln works helps you build a better one and troubleshoot problems.

Airflow

Combustion air enters through the firebox opening at the base. The burning fuel heats this air to high temperature. Hot gases, being lighter than cool air, rise naturally — this is the “updraft.” They pass through holes or gaps in the floor separating the firebox from the firing chamber, flow upward through and around the ware, and exit through an opening at the top.

The driving force is the temperature difference between the hot gases inside and the cooler air outside. Hotter fires create stronger draft, pulling more air in and maintaining combustion. This is a self-reinforcing cycle — up to a point. If the top opening is too large, heat escapes too fast; too small, and the fire is starved of draft.

Heat Distribution

The fundamental limitation of the updraft design: heat enters at the bottom and exits at the top, creating a vertical temperature gradient. The bottom of the firing chamber (nearest the fire) is always the hottest; the top is the coolest. In a poorly designed updraft kiln, this gradient can exceed 200°C from bottom to top.

Strategies to reduce the gradient:

• Stack the load loosely at the bottom (less resistance to gas flow) and tightly at the top (forces gases to slow down and transfer more heat)
• Use a bag wall — a partial internal wall that deflects rising gases sideways before they go up, distributing heat more evenly
• Multiple firebox openings around the perimeter instead of a single opening

Construction: Step by Step

Foundation

1. Select a level site with good drainage. The kiln will be heavy — soft ground may settle unevenly.
2. Lay a foundation pad of packed gravel or flat stones, 15-20 cm thick, extending 30 cm beyond the kiln walls in all directions.
3. The foundation must support the full weight of the kiln structure plus a load of ware (potentially 200-400 kg combined).

Firebox (Lower Chamber)

The firebox is where fuel burns. It occupies the lower 30-40 cm of the structure.

1. Floor: Lay a single course of bricks flat on the foundation to create the firebox floor. This protects the foundation from direct heat.

2. Walls: Build circular or rectangular walls 15-20 cm thick (one brick width for standard bricks, or two courses of smaller bricks). Interior diameter: 60-100 cm for a practical kiln. Height: 30-40 cm.

3. Stoking port: Leave an opening in one wall approximately 20 cm wide and 25 cm tall for feeding fuel. This opening should face away from prevailing wind (wind blowing into the firebox causes uncontrollable flare-ups).

4. Optional ash pit: Dig a shallow pit (10-15 cm) below the firebox floor level beneath a grate of clay bars. This allows ash to fall away from the combustion zone, maintaining airflow. Without an ash pit, you must rake ashes out through the stoking port during long firings.

The Floor (Separator)

The floor separating the firebox from the firing chamber is a critical component. It must:

• Support the weight of the ware above
• Allow hot gases to pass through
• Withstand direct flame contact from below

Construction options:

Option A — Clay bars with gaps. Make thick clay bars (5-8 cm diameter, length matching the kiln interior) and lay them across the firebox with 3-5 cm gaps between them. Support the ends on a ledge built into the kiln wall (a protruding course of bricks). This is the simplest approach and allows easy replacement of broken bars.

Option B — Perforated clay slab. Form a thick slab (8-10 cm) of refractory clay mixed with heavy grog, and punch holes (4-6 cm diameter) through it in a grid pattern before drying. Fire it separately if possible. This is stronger but harder to replace if it cracks.

Option C — Brick shelf with gaps. Lay bricks across the kiln with small gaps between them. Less ideal because standard bricks may sag at high temperature, but workable for earthenware-temperature kilns.

Method	Strength	Gas Flow	Replaceability	Best For
Clay bars	Moderate	Excellent	Easy (individual bars)	General use
Perforated slab	High	Good	Difficult	Heavy loads
Brick shelf	Low-Moderate	Moderate	Easy	Low-temp firing

Firing Chamber (Upper Chamber)

1. Walls: Continue the walls upward from the firebox for another 40-60 cm. Use the same brick construction. The firing chamber should be slightly narrower than the firebox — tapering inward by 5-10 cm creates a natural shelf for the floor elements and helps direct heat inward.

2. Height: The total interior height of the firing chamber depends on your production needs. 40 cm accommodates a single layer of medium pots. 60 cm allows two layers — the lower layer of larger pieces, the upper of smaller ones.

3. Spy hole: Leave a small opening (3-5 cm) in the wall at the midpoint of the firing chamber. This allows you to peer inside during firing to assess temperature by color and to insert test pieces.

Top Cover

The top of the kiln can be:

Open top with shard cover. Simply leave the top open and cover it during firing with a layer of pot shards and broken kiln furniture. This provides basic heat retention while allowing gases to exit. Adjust the coverage to control airflow — more shards = slower, hotter; fewer = more draft, potentially cooler.

Permanent dome with vent. Build a dome over the firing chamber with a central vent hole (15-20 cm diameter). A flat stone or brick serves as a movable damper over this hole. This is superior for temperature control but requires dome-building skills (corbelling is easier than a true arch for beginners).

Flat roof with chimney. Lay flat slabs or bricks across the top, supported by the walls. Leave an opening for a short chimney (3-5 courses of bricks, 15-20 cm interior). The chimney increases draft, which is beneficial for reaching higher temperatures.

Operating the Updraft Kiln

Loading

1. Place kiln shelves (flat refractory slabs or stacked bricks) inside the firing chamber to create levels. Support shelves on small clay props (cylinders or cubes) at least 5 cm tall to allow gas circulation between levels.

2. Load the bottom level with your largest, thickest-walled pieces — they can tolerate the higher temperature at the bottom. Place them with gaps between for gas flow.

3. Load upper levels with smaller, thinner pieces. Never stack pots directly on top of each other — use shelves or props between layers.

4. Leave the top 10-15 cm empty to allow gases to collect and exit evenly.

Seal Contact Points

Wherever a pot might touch a shelf or another pot, dust the contact point with fine sand or alumina powder (ground from white stone or kaolin). This prevents pieces from fusing together during firing — a common and heartbreaking loss.

Firing Schedule

A typical updraft kiln firing takes 8-14 hours for earthenware, 12-18 hours for stoneware:

Water smoking (0-3 hours, ambient to 300°C). Light a small fire and feed it sparingly. The goal is to drive remaining moisture from the ware very slowly. Visible steam from the top vent is normal. If you hear a sharp crack or pop, you are heating too fast — reduce fuel immediately.

Steady climb (3-6 hours, 300-600°C). Increase fuel gradually. Feed smaller pieces more frequently rather than adding large logs infrequently. The goal is a steady, even temperature rise — approximately 100°C per hour is safe for most clay bodies.

Full fire (6-10 hours, 600-1,000°C). Feed the firebox as fast as it will consume fuel. Use dry, split hardwood for maximum heat. At this stage, combustion should be fierce — flames should be visible coming through the floor holes into the firing chamber.

Soak (10-12 hours, at peak temperature). Hold peak temperature for 1-2 hours by maintaining a consistent fuel feed rate. This ensures the entire load reaches maturity, not just the pieces nearest the fire.

Shutdown and cooling (hours 12-24+). Stop feeding fuel. Seal the stoking port with bricks and clay. Close the top damper (or cover with more shards). Follow the cooling schedule for your specific clay body — slow is always safer.

Phase	Time	Target Temp	Fuel Rate	Key Risk
Water smoking	0-3 hrs	20-300°C	Very light	Steam explosions
Climb	3-6 hrs	300-600°C	Moderate	Quartz inversion (573°C)
Full fire	6-10 hrs	600-1,000°C	Maximum	Under-firing if too conservative
Soak	10-12 hrs	At peak	Steady	Running out of fuel
Cooling	12-24+ hrs	Peak → ambient	None	Opening too early (dunting)

Judging Temperature Without Instruments

Since you likely have no pyrometer, use these visual indicators by looking through the spy hole in a dimmed environment (shade your eyes and avoid direct flame glare):

Color Seen Inside Kiln	Approximate Temperature
No visible glow	Below 500°C
Faintest dark red (barely visible)	500-550°C
Dark cherry red	600-700°C
Cherry red	700-800°C
Bright cherry red	800-900°C
Orange	900-1,000°C
Yellow-orange	1,000-1,100°C
Yellow	1,100-1,200°C

Draw trials: Place small test rings of your clay body on the top shelf, accessible through the spy hole. Pull one out with a long stick every hour during the full-fire phase. Compare the pulled ring to a reference set of previously fired rings at known temperatures. When the draw trial matches your target maturity, begin the soak phase.

Common Problems and Solutions

Problem: Temperature plateau — cannot get above 800°C. Causes: Insufficient draft, wet fuel, ash accumulation blocking airflow. Solution: Rake ashes from the firebox. Open the top vent wider to increase draft. Switch to drier, denser fuel. If draft remains poor, extend the chimney height (stack loose bricks to add temporary height).

Problem: Pots on the bottom shelf are over-fired while top shelf is under-fired. Cause: Excessive temperature gradient (normal in updraft kilns but manageable). Solution: Add a bag wall — a partial wall (half the height of the firing chamber) between the firebox floor holes and the ware. This deflects rising heat sideways, distributing it more evenly. Also load heat-resistant pieces (thick storage jars, kiln furniture) on the bottom as sacrificial items.

Problem: Pots crack during firing. Cause: Rising temperature too fast (especially through 573°C quartz inversion), or residual moisture. Solution: Extend the water-smoking phase to 4 hours minimum. Ensure all ware has been air-dried for at least 2 weeks before loading. Add grog to your clay body (15-20% by volume).

Problem: Glaze crawling or pinholing. Cause: Organic material in the clay body or glaze burning out after the glaze has sealed the surface, trapping gas bubbles. Solution: Fire more slowly through the 800-1,000°C range to allow complete burnout before the glaze melts and seals. Ensure bisque firing (first firing without glaze) reaches at least 900°C to eliminate organics before glaze firing.

Fuel Reserves

Calculate your fuel needs before starting a firing. Running out of fuel mid-firing — with the kiln at 700°C and no more wood — wastes everything: the fuel already burned, the pots (which will be under-fired), and your time. A standard updraft kiln firing requires 150-250 kg of dry split wood. Have at least 20% more than your estimate on hand. Stack it close enough to the kiln for easy access but far enough (2+ meters) that radiant heat does not ignite your fuel supply.