Kiln Types

Part of Brick Making

Overview of kiln designs from simple updraft to downdraft.

Why This Matters

The kiln is the most important piece of infrastructure in brick production. Its design determines how hot you can fire, how evenly heat distributes, how much fuel you burn, and how many bricks you can produce per firing. Choosing the right kiln for your situation — your available materials, labor force, fuel supply, and production needs — is a critical early decision in any rebuilding effort.

A community that picks the wrong kiln design wastes enormous amounts of fuel and labor. A simple pit kiln uses three to five times more fuel per brick than a well-designed downdraft kiln. Over thousands of bricks, that fuel difference translates into weeks of woodcutting labor saved, or forests that remain standing to provide building timber instead of being burned for heat.

Starting simple and upgrading as skills and resources grow is the right strategy. Your first kiln should be something you can build in a day. Your tenth should be a permanent structure that serves for years. This article covers the progression from simplest to most sophisticated.

Clamp Kiln (Simplest)

A clamp is not a permanent structure — it is a carefully stacked pile of green bricks with fuel channels built into the base. The bricks themselves form the kiln walls, and the entire structure is fired as a unit.

Construction

1. Choose a flat, well-drained site sheltered from wind
2. Lay parallel rows of fired bricks or stones to form fire channels — tunnels about 300mm wide and 300mm tall, spaced 400-500mm apart
3. Fill the channels loosely with combustible material: brushwood bundles, dried dung cakes, or coal
4. Stack green bricks above the channels using an open setting pattern with finger-width gaps between bricks
5. Build the stack 1-2 meters high, tapering slightly inward for stability
6. Coat the exterior with a 50-75mm layer of mud mixed with straw, leaving ventilation holes at the base and a few at the top
7. Light the fire channels from one end and let the fire advance slowly through

Advantages

• No permanent infrastructure required — build anywhere
• Can fire hundreds to thousands of bricks at once
• Materials are just the bricks themselves plus fuel and mud
• Good first kiln for a new community

Disadvantages

• Very uneven firing — outer bricks overfire, center may underfire
• High fuel consumption (3-5x more than permanent kilns)
• 20-40% of bricks may be rejects
• Cannot reuse — each firing requires rebuilding
• Difficult to control temperature

Capacity

Typically 1,000 to 10,000 bricks depending on size. A small clamp of 1,000 bricks can be built by 2-3 people in a day.

Pit Kiln

A step up from the clamp — a permanent or semi-permanent hole in the ground that retains heat better.

Construction

1. Dig a pit 1-2 meters deep, 2-3 meters in diameter, in well-drained soil (avoid areas with high water tables)
2. Line the pit walls with previously fired bricks or stones if available — this improves insulation and longevity
3. Dig a narrow trench from outside the pit to the bottom to serve as a fire channel and air supply
4. Stack green bricks in the pit using the open setting pattern
5. Cover the top with broken pottery, fired brick fragments, or a mud-and-straw dome with a central smoke hole

Advantages

• The earth insulates the firing, reducing fuel consumption by 30-50% compared to a clamp
• Below-grade construction shelters from wind
• Can be reused many times if lined
• Easier to control temperature by covering and uncovering the top

Disadvantages

• Drainage can be a problem — a flooded pit ruins everything
• Loading and unloading requires climbing in and out
• Limited capacity (typically 200-500 bricks)
• Bottom bricks may overheat while top bricks underfire

Updraft Kiln (Intermediate)

The first true permanent kiln design. Heat enters from a firebox at the base, rises through the ware, and exits through an opening at the top.

Construction

1. Build a circular or rectangular chamber from fired bricks, stone, or rammed earth, with walls 200-300mm thick
2. The floor is a perforated platform (grate) — either arched brickwork with gaps or a grid of bars — supported above a firebox cavity
3. The firebox is a vaulted chamber below and to one side of the main chamber, with a stoking hole accessible from outside
4. The top of the kiln is either open (covered with broken pottery during firing) or has a permanent dome with removable plugs
5. A loading door in the side wall, bricked up with mud mortar for each firing

Design Details

• Chamber diameter: 1-3 meters for small operations, up to 5 meters for large production
• Wall thickness: Minimum 200mm of brick or 300mm of earth
• Firebox size: Roughly 1/4 to 1/3 the volume of the ware chamber
• Floor perforations: 20-30% open area for heat passage
• Top opening: Adjustable — wide open during early stages, gradually closed as temperature rises

Advantages

• Reusable for hundreds of firings
• Much more even heat than clamps or pits
• Better fuel efficiency — thick walls retain heat
• Temperature can be controlled by adjusting fuel feed and top damper
• Can reach 1000-1100°C with hardwood, higher with charcoal

Disadvantages

• Requires fired bricks or stone to build (chicken-and-egg problem — fire your first bricks in a clamp)
• Top of kiln is always cooler than bottom — 50-100°C gradient
• Relatively high fuel consumption compared to downdraft designs
• Heat escaping from the top is wasted energy

Capacity

Typically 500-2,000 bricks depending on chamber size. A 2-meter diameter kiln holds roughly 800-1,200 standard bricks.

Downdraft Kiln (Advanced)

The most fuel-efficient and even-firing traditional kiln design. Heat is forced up and over an internal wall (the bag wall), then drawn down through the ware and out through flues in the floor.

How It Works

1. The firebox is at one side or multiple sides of the kiln
2. Flames and hot gases rise from the firebox and hit the kiln roof (or an arched crown)
3. Unable to escape upward, the heat is deflected downward through the stack of bricks
4. Cooled gases exit through channels in the floor connected to an external chimney
5. The chimney’s draft pulls air through the entire system

The critical insight is that heat is forced to pass through the entire stack from top to bottom, giving much more even firing than an updraft kiln where heat simply rises past the bricks.

Construction

1. Foundation: Lay a solid brick platform with built-in flue channels running to the chimney
2. Walls: Build circular or rectangular walls 300-450mm thick from firebrick or dense fired brick. Height typically 2-3 meters
3. Bag wall: An internal wall, roughly 2/3 the height of the main chamber, positioned between the firebox and the ware space. This forces heat upward before it can reach the bricks
4. Crown: Build an arched or domed roof. This reflects heat downward. The crown must be structurally self-supporting — use a temporary wooden form during construction, then burn it out
5. Firebox: One or more fireboxes along one side (or around the circumference for round kilns). Each has a stoking port and ash pit
6. Chimney: A tall brick chimney (3-6 meters) connected to the floor flues. The taller the chimney, the stronger the draft, and the hotter you can fire
7. Damper: A sliding brick or metal plate at the chimney base to control draft strength
8. Loading door: A section of wall that is bricked up for each firing and broken out for unloading

Design Proportions

Element	Guideline
Chamber height	2-3x the diameter for efficient circulation
Bag wall height	60-70% of chamber height
Bag wall gap (to ceiling)	300-500mm — enough for gases to pass over
Floor flue area	10-15% of floor area
Chimney height	1.5-2x kiln height minimum
Chimney internal diameter	300-500mm for small kilns
Firebox depth	600-900mm

Advantages

• Most even firing of any traditional design — temperature variation under 30°C across the entire load
• 40-60% less fuel than updraft kilns for the same capacity
• Can reach 1200°C+ with hardwood and good draft
• Excellent temperature control via damper adjustment
• Lasts decades with proper maintenance

Disadvantages

• Most complex to build — requires significant masonry skill
• Needs firebrick for the hottest zones (firebox, bag wall, crown)
• Chimney construction requires scaffolding
• Requires understanding of draft dynamics to operate well
• Not worth the investment until you have ongoing, large-scale brick production

Capacity

Typically 1,000-5,000 bricks per firing. Large industrial downdraft kilns have held 20,000+ bricks.

Comparison Table

Feature	Clamp	Pit	Updraft	Downdraft
Permanence	Temporary	Semi-permanent	Permanent	Permanent
Build time	1 day	1-2 days	1-2 weeks	3-6 weeks
Build materials	Mud only	Earth, some brick	Fired brick/stone	Firebrick essential
Fuel efficiency	Poor	Fair	Good	Excellent
Temperature evenness	Poor (100°C+ variation)	Fair (80°C+)	Good (50-80°C)	Excellent (under 30°C)
Max temperature	800-950°C	850-1000°C	1000-1100°C	1200°C+
Reject rate	20-40%	15-30%	10-20%	5-10%
Typical capacity	1,000-10,000	200-500	500-2,000	1,000-5,000
Skill required	Low	Low-Medium	Medium	High

The Progression Strategy

For a rebuilding community, the recommended approach:

1. Week 1-2: Build a clamp. Fire your first batch of bricks. Accept 30% rejects. Use the good bricks for your next kiln.
2. Month 1: Build a simple updraft kiln from your first-generation bricks. This becomes your primary production kiln.
3. Month 3-6: After producing thousands of bricks and developing your skills, build a downdraft kiln for high-quality, fuel-efficient production.
4. Keep the updraft kiln for overflow production and for pre-firing experimental clay batches.

Firebrick Bootstrap

You need firebrick to build the hottest parts of a downdraft kiln, but firebrick requires high temperatures to make. The solution: use your updraft kiln with charcoal fuel and bellows-assisted draft to fire a small batch of high-alumina clay bricks to firebrick temperatures. Use these to line your downdraft kiln’s firebox and bag wall.

Maintenance and Repair

All permanent kilns require maintenance:

• After every firing: Inspect for cracks in walls, crown, and flues. Fill cracks with fireclay mortar before the next firing.
• Every 5-10 firings: Replace any spalled or deteriorated bricks in the firebox and bag wall. These areas experience the most thermal stress.
• Annually: Inspect the chimney for cracks and blockages. Clean ash from floor flues. Check that the damper moves freely.
• Grate/floor: Replace damaged floor supports. A collapsed grate can destroy an entire kiln load.

Apply fireclay wash (thin slurry of fireclay and water) to interior surfaces annually. This seals micro-cracks and extends the kiln’s life.