Firing Temperatures

Part of Brick Making

Temperature ranges for different brick types and how to judge heat without instruments.

Why This Matters

In a rebuilding scenario, the difference between a brick that lasts generations and one that crumbles in the first rain comes down to firing temperature. Too low and the clay never sinters — the particles never fuse into a solid mass. Too high and bricks warp, crack, or melt into a glassy slag that is useless for construction. Getting the temperature right is the single most critical step in brick production.

Modern kilns use pyrometers and electronic controllers. You will have neither. For thousands of years, potters and brickmakers judged temperature by the color of the fire, the behavior of test pieces, and the feel of radiant heat on exposed skin. These methods are surprisingly accurate once you calibrate your eye, and they remain the gold standard when instruments are unavailable.

Understanding firing temperatures also lets you produce specialized bricks — soft bricks for interior walls that insulate well, hard engineering bricks for foundations and chimneys, and firebricks that can line your kilns and forges for years of service. Each requires a different temperature range, and knowing those ranges expands what you can build.

Temperature Ranges by Brick Type

Different clays and different end uses demand different peak temperatures. Here is a practical reference:

Brick Type	Temperature Range	Color of Fire	Key Properties
Sun-dried / adobe	No firing	N/A	Water-soluble, insulating, temporary
Low-fired earthenware	600-800°C	Dull red to cherry red	Porous, soft, adequate for sheltered walls
Standard building brick	900-1050°C	Bright cherry to orange	Strong, weather-resistant, most common
Hard engineering brick	1050-1200°C	Orange to yellow-orange	Very dense, low water absorption
Firebrick (refractory)	1200-1400°C	Yellow to white-yellow	Withstands repeated heating cycles
Stoneware-grade	1200-1300°C	Yellow-white	Near-vitrified, almost waterproof

Clay Variation

These ranges assume common sedimentary clays. Your local clay may contain fluxes (iron, feldspar, lime) that lower the sintering point by 50-100°C, or refractory minerals (kaolin, alumina) that raise it. Always fire test pieces first.

Judging Temperature by Fire Color

This is the oldest and most reliable field method. In a darkened kiln shed, observe the glow of the bricks themselves (not the flames) through the spy hole:

1. First visible glow (~475°C) — A faint, barely perceptible dark red appears on brick surfaces. This is the “black heat” stage. Water is being driven off and organic matter is burning away. Do not rush past this stage or bricks will crack from steam pressure.

2. Dull red (500-650°C) — The interior glows an even, dark red. All chemical water has been expelled. The bricks are becoming ceramic but are still very weak.

3. Cherry red (700-800°C) — A clear, bright red like ripe cherries. Carbon burnout is complete. Low-fired bricks can be finished here if they will be used only in sheltered locations.

4. Bright cherry to dark orange (850-950°C) — The glow intensifies and shifts toward orange. Sintering begins in earnest — clay particles are fusing at their contact points. This is the minimum for weather-resistant bricks.

5. Orange (1000-1100°C) — A clear, uniform orange throughout the kiln. Standard building bricks reach full strength here. The brick surface may show a slight gloss.

6. Yellow-orange (1100-1200°C) — Approaching engineering-brick territory. Bricks shrink noticeably and become very dense. Risk of warping increases.

7. Yellow to white (1200°C+) — Firebrick and stoneware range. Only high-alumina or kaolin-rich clays survive without deforming. Common clays will slump or bloat.

Practice Your Eye

Before your first large firing, practice color-reading with a small test kiln. Fire it slowly, pull test pieces at intervals, and note the color when you removed each one. After cooling, test each piece by scratching with a nail, dropping from waist height, and soaking in water overnight. This builds your personal calibration.

The Spit Test and Other Field Methods

Beyond color reading, experienced brickmakers use several physical tests:

The spit test. Spit on a cooled test brick. If the saliva is absorbed instantly with a hissing sound, the brick is underfired and too porous. If it beads up and rolls off, the brick may be overfired or near vitrification. A well-fired brick absorbs the moisture slowly over 2-3 seconds.

The ring test. Tap two cooled bricks together. An underfired brick produces a dull thud. A properly fired brick rings with a clear, metallic note. An overfired or cracked brick produces a dead, flat sound.

The scratch test. Draw a steel nail across the brick surface. If the nail leaves a deep groove easily, the brick is underfired. A well-fired brick resists scratching and may even dull the nail. An overfired brick may show a glassy surface that the nail skates across.

The break test. Snap a brick in half. Examine the cross-section. Underfired bricks show a dark core surrounded by a lighter fired shell. A fully fired brick has a uniform color throughout. If the core is darker, you need a longer soak time at peak temperature.

The water absorption test. Weigh a dry brick, soak it in water for 24 hours, and weigh it again. Calculate the percentage increase. Good building bricks absorb 5-15% of their weight. Under 5% suggests overfiring (brittle). Over 20% means underfiring (will not weather well).

Controlling the Firing Curve

Temperature alone is not enough — the rate of temperature change matters enormously.

Phase 1: Water Smoking (Ambient to 600°C)

This is the most dangerous phase. Raise temperature no faster than 50-80°C per hour. Free water trapped in the clay turns to steam, and if it cannot escape fast enough, bricks explode. In a wood-fired kiln, use small, low fires and keep dampers partially open. This phase should take 8-12 hours minimum. You will see steam rising from the kiln stack.

Phase 2: Carbon Burnout (600-900°C)

Organic matter in the clay burns away. Raise temperature at 80-100°C per hour. Keep oxygen flowing — a smoky, reducing atmosphere at this stage traps carbon in the brick, causing black cores that weaken the structure. If you see heavy black smoke, open dampers wider. This phase takes 3-6 hours.

Phase 3: Sintering and Peak (900°C to target)

Push toward your target temperature at 50-100°C per hour. The exact peak depends on your brick type (see table above). Once you reach peak, maintain it for 4-8 hours. This “soak” ensures the heat penetrates to the center of the densest stacks. Bricks near the fire may reach temperature quickly, but the center of the kiln lags behind.

Phase 4: Cooling (Target to ambient)

This is the second most dangerous phase. Cool no faster than 50°C per hour down to 573°C — at this temperature, quartz in the clay undergoes a crystal structure change (the “quartz inversion”) that causes sudden contraction. If cooling is too fast, bricks crack. Below 573°C, you can allow natural cooling, which typically takes 2-4 days for a large kiln. Do not open the kiln until bricks are cool enough to handle with bare hands.

Quartz Inversion

At 573°C, quartz changes from beta to alpha form and contracts about 2% in volume. This happens both on heating and cooling. Slow your rate through this range in both directions. Cracks from quartz inversion are distinctive — they are clean, sharp breaks rather than the ragged fractures of steam explosions.

Fuel and Temperature Relationships

Different fuels produce different maximum temperatures. Knowing your fuel’s ceiling helps you plan what types of bricks you can produce:

Fuel	Practical Max Temperature	Notes
Dried grass, straw	600-700°C	Too cool for quality bricks, useful for adobe hardening
Softwood (pine, spruce)	900-1000°C	Standard building bricks possible with good airflow
Hardwood (oak, hickory)	1000-1150°C	Engineering bricks achievable with forced draft
Charcoal	1100-1300°C	Excellent for firebricks, burns cleaner
Coal (bituminous)	1200-1400°C	Highest temperatures, but introduces sulfur
Bellows-assisted charcoal	1300-1500°C	Can approach stoneware and refractory temperatures

The key variable is airflow. A well-designed downdraft kiln with natural chimney draft can push hardwood 100-150°C above what an open fire achieves. Adding bellows or a blower can push temperatures another 100-200°C.

Making and Using Pyrometric Cones

If you can source or make clays of known composition, you can create simple pyrometric cones — small pyramids of clay formulated to bend at specific temperatures. Place them visible through the spy hole. When the cone bends and touches its base, you have reached that temperature.

To make rough cones without laboratory supplies:

1. Take your standard brick clay and form a cone about 5 cm tall with a triangular cross-section
2. Fire a test batch at different known temperatures (calibrated by brick color and results)
3. Note which batch bent at which color — these become your reference cones
4. For higher temperatures, mix in sand (raises bending point) or wood ash (lowers it)

Three cones are traditionally used: one rated 25°C below target (the “guide cone”), one at target (the “firing cone”), and one 25°C above (the “guard cone”). When the guide bends, prepare to finish. When the firing cone bends, begin cooling. If the guard cone bends, you have overfired.

Common Firing Problems and Solutions

Problem	Cause	Solution
Bricks explode during firing	Too-fast water smoking	Slow down Phase 1, ensure bricks are bone-dry before loading
Black cores in broken bricks	Carbon trapped by reducing atmosphere	More air during 600-900°C phase, slower firing
Warped or slumped bricks	Overfired or clay too fusible	Lower peak temperature or add sand to clay body
Surface cracks after cooling	Too-fast cooling through quartz inversion	Seal kiln tighter, cool more gradually
Uneven hardness in same load	Poor kiln loading or hot spots	Improve stacking pattern, add bag walls to distribute heat
Bricks too soft and porous	Underfired	Higher peak temperature or longer soak time
Glassy surface, brittle	Overfired, surface vitrified	Reduce peak temperature by one color grade