Fired Bricks
Part of Brick Making
Properties, grades, and production principles of kiln-fired bricks.
Why This Matters
Fired bricks are among the most durable building materials ever created. Roman bricks are still structurally sound after 2,000 years. Sun-dried bricks dissolve in rain within seasons. The difference is fire — sustained heat above 800 C permanently transforms clay minerals into a hard, weather-resistant ceramic material.
For a rebuilding community, fired bricks are the gateway to permanent architecture. They resist water, support heavy loads, withstand fire, and require no maintenance. A building made from well-fired bricks outlasts every generation that uses it. Chimneys, wells, ovens, kilns, foundations, and water channels all demand fired brick because no alternative material offers the same combination of strength, water resistance, and fire resistance.
Understanding what happens inside a brick during firing — and what properties result — helps you produce consistent quality, troubleshoot failures, and choose the right grade of brick for each construction application.
What Happens During Firing
The Chemical Transformation
Firing transforms clay through a series of irreversible chemical changes:
| Temperature Range | Process | What Happens |
|---|---|---|
| 20-100 C | Pore water evaporation | Free water leaves. No permanent change. |
| 100-200 C | Adsorbed water loss | Water film around particles evaporates. |
| 200-350 C | Organic burnout | Plant matter, roots, carbon burn away. |
| 350-500 C | Crystal water loss | Water chemically bonded in clay minerals is driven off. Clay can no longer be reconstituted with water. |
| 500-700 C | Quartz inversion | Silica in the clay undergoes a crystal structure change at 573 C, causing a slight expansion. Must heat through this range slowly. |
| 700-900 C | Sintering begins | Clay particles start fusing together at contact points. The brick hardens significantly. |
| 900-1,100 C | Vitrification | Glass-forming flux minerals melt and flow between particles, binding them into a dense, strong body. Porosity decreases. |
| Above 1,100 C | Over-firing risk | Excessive melting causes the brick to bloat, warp, or fuse to neighboring bricks. |
The Critical Point: 573 C
The quartz inversion at 573 C causes a sudden 2% volume change. If the kiln temperature rises too quickly through this range, the expansion cracks bricks. Heat slowly through 500-600 C and cool slowly back through the same range.
Irreversibility
Once clay has been heated above approximately 500 C, it can never become plastic clay again. Adding water to a ground-up fired brick produces grog (crusite aggregate), not clay. This irreversible transformation is what makes fired bricks permanent — they do not dissolve, soften, or revert when exposed to rain.
Grades of Fired Brick
Not all fired bricks are equal. Temperature, duration, and position in the kiln determine the final grade.
First-Class (Well-Fired)
- Fired to full vitrification temperature (900-1,050 C for most clays)
- Uniform color throughout — no dark core
- Rings clearly when two bricks are tapped together
- Sharp, clean edges and corners
- Smooth fracture surface when broken
- Water absorption below 15% (water beads briefly before absorbing)
- Compressive strength: 10-20 MPa (can support multi-story buildings)
Use for: Load-bearing walls, foundations, chimneys, water channels, wells, any exposed or structural application.
Second-Class (Moderately Fired)
- Fired to sintering but incomplete vitrification (750-900 C)
- Slightly softer than first-class
- May have minor color variation
- Sounds less resonant when tapped
- Water absorption 15-25%
- Compressive strength: 5-10 MPa
Use for: Interior walls, partitions, upper stories, paving in covered areas, backing brick behind a first-class face.
Third-Class (Under-Fired)
- Fired below full sintering temperature (below 750 C)
- Soft — can be scratched with a steel nail
- Often lighter or more mottled in color
- Dull sound when tapped
- Water absorption above 25%
- Compressive strength: 2-5 MPa
Use for: Temporary structures, garden walls, fill material, crush for grog. Not suitable for structural or water-contact applications.
Over-Fired (Clinker)
- Fired above vitrification range (above 1,100 C)
- Partially melted, dark-colored, often misshapen
- Extremely hard — difficult to cut or break
- Very low water absorption
- Irregular shape limits use in standard walls
Use for: Drainage channels (excellent water resistance), hardcore fill, paving where appearance does not matter, crush for aggregate.
Properties of Well-Fired Bricks
Compressive Strength
A well-fired brick can support enormous loads. Standard fired bricks withstand 10-20 MPa of compressive force — meaning a single brick (roughly 65 cm2 top surface) can support 65,000-130,000 Newtons (approximately 6.5-13 tonnes) before crushing.
This is why brick buildings can be many stories tall. The limiting factor is usually the mortar, not the brick.
Water Resistance
Vitrification creates a glass-like matrix within the brick that resists water penetration. A well-fired brick in a rainstorm absorbs water slowly from the surface, but the core remains dry. This is why fired bricks last centuries in wet climates where mud bricks dissolve within years.
Testing water resistance: Drop water onto a brick surface.
| Observation | Water Absorption Class |
|---|---|
| Water beads and sits for 5+ seconds | Low absorption — excellent |
| Water absorbs within 1-3 seconds | Moderate — acceptable for most uses |
| Water absorbs instantly, dark stain spreads | High absorption — under-fired or porous |
Fire Resistance
Fired bricks have already survived temperatures of 800-1,050 C. They do not burn, and they resist structural failure in building fires where temperatures rarely exceed 700-800 C. This makes them essential for chimneys, hearths, ovens, and kiln construction.
Thermal Mass
Bricks absorb heat during the day and release it at night, moderating interior temperatures. A brick wall 30 cm thick provides 8-12 hours of thermal lag — heat that enters the outer face in the afternoon reaches the inner face in the evening, warming the interior after sunset. This passive temperature regulation reduces heating and cooling needs.
Achieving Consistent Quality
Kiln Loading
Brick quality depends heavily on position within the kiln:
- Center bricks: Receive the most uniform, highest heat. Produce first-class bricks.
- Top bricks: Heat rises, so these fire well but may over-fire near the crown.
- Bottom bricks: Cooler zone. Under-firing is common without adequate fire channels.
- Edge bricks: Lose heat through kiln walls. Under-fire unless walls are well-insulated.
To maximize first-class output, load the kiln so the most important bricks are in the center, and accept that edge and bottom bricks will be lower grade.
Fuel and Firing Time
| Fuel | Peak Temperature | Firing Duration | Notes |
|---|---|---|---|
| Hardwood | 900-1,050 C | 24-48 hours at peak | Sustained, controllable heat |
| Softwood | 800-950 C | 12-24 hours at peak | Burns fast, needs constant feeding |
| Charcoal | 1,000-1,200 C | 24-36 hours at peak | Hottest, risk of over-firing |
| Dried dung | 700-900 C | 36-60 hours at peak | Low temperature, needs extended soak |
| Coal | 1,000-1,300 C | 24-48 hours at peak | Very hot, excellent if available |
Temperature Indicators (Without Thermometers)
| Visual Indicator | Approximate Temperature |
|---|---|
| No visible glow | Below 500 C |
| Faint dark red glow (visible only at night) | 500-600 C |
| Dark cherry red | 700-800 C |
| Cherry red | 800-900 C |
| Bright cherry red | 900-1,000 C |
| Orange | 1,000-1,100 C |
| Yellow-orange | 1,100-1,200 C |
Target bright cherry red to orange for most brick clays. Hold this temperature for at least 6-12 hours (soaking) to ensure heat penetrates the core of each brick.
Quality Control After Firing
Sort every brick after unpacking the kiln. Do not mix grades — using a third-class brick in a structural wall invites failure.
The Five-Point Check
- Tap test: Strike two bricks together. First-class bricks ring. Lower grades thud.
- Visual inspection: Uniform color, no cracks, sharp edges.
- Scratch test: A steel nail should not easily scratch a first-class brick.
- Drop test: Drop from 1 meter onto hard ground. First-class bricks survive intact. Under-fired bricks break.
- Water test: Immerse a brick in water for 24 hours. Weigh before and after. Weight gain above 20% indicates under-firing.
Reject Cracked Bricks
A cracked brick has already failed structurally. Even if the crack is small, it concentrates stress and will propagate under load or freeze-thaw cycles. Crush cracked bricks for grog — they are not safe for building.
Using Under-Fired Bricks
Do not discard lower-grade bricks. They have legitimate uses:
- Grog: Crush and use as temper in the next batch of bricks. This is the highest-value use.
- Fill and leveling: Use as rubble fill behind retaining walls or under floors.
- Interior partitions: Non-load-bearing interior walls do not need first-class bricks.
- Garden walls: Short, non-structural walls in sheltered locations.
- Drainage: Crushed under-fired brick makes serviceable drainage aggregate.
- Re-fire: Under-fired bricks can be stacked in the next kiln load for a second firing. The outer layers may have already converted, reducing shrinkage cracking risk.