Temperature Stages

Part of Pottery and Ceramics

Understanding the key temperature thresholds in pottery firing — what happens to clay at each stage and why controlling the heat curve determines success or failure.

Why This Matters

Pottery firing is not just “make it hot.” Clay undergoes a series of irreversible chemical and physical transformations at specific temperatures. Rushing past a critical stage shatters pots. Stopping too early produces vessels that dissolve in water. Going too high melts your work into a puddle on the kiln floor.

Without a thermometer or pyrometer — which you almost certainly will not have in a rebuilding scenario — you must learn to read temperature by observing color, sound, and the behavior of test pieces. Potters did this successfully for 10,000 years before any measuring instrument existed. The key is understanding what happens at each stage so you know what to look for.

Getting the firing schedule right means the difference between a 90% success rate and a 90% loss rate. When every pot represents hours of gathering clay, processing it, forming it, drying it, and gathering fuel, you cannot afford to lose entire kiln loads to preventable firing errors.

The Complete Firing Curve

Stage 1: Water Smoking (20-200C)

What happens: Free water (the water you added to make the clay workable) evaporates from the clay body.

Duration: Slow — 2-4 hours minimum. This is where most kiln disasters occur.

Critical rule: If any moisture remains when the temperature rises above 100C, the water flashes to steam and expands 1,600 times in volume. The result is explosive shattering — pots literally blow apart, and flying shards can damage neighboring pieces.

How to manage:

• Start with a very small fire or open the kiln door/vents fully
• Pots should feel warm, not hot, for the first hour
• Hold the temperature below 100C for at least 1 hour
• Gradually increase over the next 1-2 hours
• You can test readiness by holding a cool piece of metal or glass above the kiln opening — if it fogs with condensation, water is still escaping

The Number One Rule

Never rush the water smoking stage. Bone-dry pots still contain 1-3% residual moisture trapped in the clay structure. Even if a pot has air-dried for weeks, it still needs slow water smoking. Thick-walled pots and slab-built pieces are especially vulnerable.

Stage 2: Organic Burnout (200-500C)

What happens: Organic matter trapped in the clay (plant roots, carbon, humus) burns away. The clay may darken temporarily as carbon particles form, then lighten as they burn off.

Duration: 1-2 hours at a moderate rate of temperature increase.

Visible signs:

• Smoke may emerge from the kiln, especially with clay dug from organic-rich soil
• Pots may temporarily turn gray or black before lightening

Key concern: If the kiln atmosphere is too reducing (oxygen-starved) during this stage, carbon gets trapped in the clay body. This is called a carbon core — a dark gray or black layer visible when you break a pot. Carbon cores weaken the ceramic and can cause glaze blistering in later firings.

Management: Keep the kiln well-ventilated (dampers open, feed holes uncovered) during this stage. You want plenty of airflow to burn out all the carbon.

Stage 3: Ceramic Change / Quartz Inversion (500-600C)

What happens: This is the most critical transformation. At 573C (the quartz inversion point), the silica crystals in clay undergo a sudden structural rearrangement, expanding by about 2% in volume. This expansion happens quickly and unevenly through the pot wall.

Why it matters: If the temperature changes too rapidly through this zone — either heating up or cooling down — the outer surface of the pot expands or contracts before the interior does. The resulting stress cracks the pot. This is the second most common cause of kiln losses after steam explosions.

Duration: Pass through 500-600C at a slow, steady rate. Allow at least 1 hour for this 100-degree range.

Visible signs: No dramatic visual change. Pots begin to glow a very faint red in total darkness.

Quartz Inversion on Cooling

The quartz inversion is equally dangerous on the way down. At 573C during cooling, the silica contracts suddenly. This is why you must cool the kiln slowly — do not open the kiln until well below 300C. Many potters who survive the firing lose pots by opening the kiln too early to look.

Stage 4: Dehydroxylation (450-700C)

What happens: Chemically bound water (water molecules that are part of the clay crystal structure, not free water) is driven off. This is an irreversible change — once this water is gone, the clay can never be reconstituted by adding water again. The material is now ceramic, not clay.

Visible signs: Pots begin to glow dull red (visible in a darkened room above 500C).

Significance: This is the minimum threshold for “fired pottery.” Material that has passed through dehydroxylation is permanent — it will not dissolve in water. However, it is still porous and relatively weak.

Stage 5: Sintering and Vitrification (800-1,300C)

What happens: Clay particles begin to fuse together. Glass phases form between the particles, filling pores and densifying the body. The higher the temperature, the more vitrification occurs.

Temperature	Degree of Vitrification	Typical Ware
800-900C	Minimal — porous, weak	Basic earthenware, flowerpots
900-1,050C	Partial — functional earthenware	Cooking pots, bricks, most historic pottery
1,050-1,200C	Substantial — some translucency	Stoneware
1,200-1,300C	Near-complete — dense, waterproof	High-fired stoneware, porcelain clays
Above 1,300C	Over-vitrification — clay bloats, warps, melts	Usually a disaster unless using refractory clay

Visible signs by color (observed through a peephole or gap in the kiln):

Color	Approximate Temperature
No visible glow	Below 500C
Faint red (dark room only)	500-600C
Dark red	600-700C
Cherry red	700-800C
Bright cherry red	800-900C
Orange	900-1,000C
Yellow-orange	1,000-1,100C
Yellow	1,100-1,200C
Light yellow / white-yellow	1,200-1,300C
White	Above 1,300C

Practice Reading Color

Before your first real firing, practice observing heat colors by heating metal in a forge or fire pit. A piece of iron passes through the same color sequence. Memorize what cherry red looks like versus orange versus yellow. This skill replaces a pyrometer.

Stage 6: Glaze Melting (If Glazing)

If the kiln contains glazed ware, the glaze materials begin to melt and flow at their designed temperature (typically 900-1,250C depending on the glaze recipe). The kiln must reach this temperature and hold it (“soaking”) for 15-30 minutes to allow the glaze to fully mature, smooth out, and release trapped gas bubbles.

Kiln Atmosphere

The chemical environment inside the kiln profoundly affects the finished pottery.

Oxidation Firing

• Conditions: Plenty of air entering the kiln. Flames burn cleanly. No smoke.
• Effect: Iron in the clay produces red, orange, and brown colors. Copper in glazes produces green.
• How to achieve: Keep dampers and vents open. Do not pack fuel so tightly that it smothers.

Reduction Firing

• Conditions: Restricted airflow. Incomplete combustion. Smoky atmosphere.
• Effect: Iron in the clay produces gray, blue-gray, or black. Copper in glazes produces deep red (oxblood). Carbon may embed in the clay surface.
• How to achieve: Close dampers partially. Add green wood or damp fuel to generate smoke. Restrict air intake.
• Timing: Reduction is typically done during the 900-1,200C range. Reducing during lower temperatures risks trapping carbon.

The Cooling Curve

Cooling is not simply the absence of firing. It has its own critical stages.

1. Initial cooling (peak to 1,000C): Can proceed at a natural rate. Close the kiln completely and let it cool undisturbed.
2. Quartz inversion zone (600-500C): Slow down. Do not open the kiln. Do not introduce cold air. The silica contracts at 573C and rapid cooling cracks pots.
3. Below 300C: Kiln can be cracked open slightly to speed cooling.
4. Unloading: Wait until pots are cool enough to handle comfortably (below 50C). Removing hot pots and setting them on cold surfaces thermal-shocks them.

Total cooling time: For a small kiln, 12-24 hours. For a large kiln, 24-48 hours. Patience during cooling is just as important as control during heating.

Firing Schedule Summary

Phase	Temperature Range	Rate	Duration	Key Risk
Water smoking	20-200C	Very slow	2-4 hours	Steam explosion
Organic burnout	200-500C	Moderate	1-2 hours	Carbon core
Quartz inversion (up)	500-600C	Slow	1+ hour	Thermal cracking
Dehydroxylation	450-700C	Moderate	Overlaps with above	None if controlled
Sintering	800-1,100C+	Can be faster	2-4 hours	Under/over-firing
Peak / soak	Target temp	Hold steady	15-60 minutes	Glaze maturity
Cooling to 600C	Peak to 600C	Natural	4-12 hours	None
Quartz inversion (down)	600-500C	Slow	1+ hour	Thermal cracking
Final cooling	500C to room temp	Natural to moderate	8-24 hours	Thermal shock

Pyrometric Cones (DIY Temperature Indicators)

Without a thermometer, you can make simple temperature indicators from clay and flux mixtures.

1. Make small cones (3-4 cm tall, pyramidal) from different clay/flux blends:

   • Pure clay — bends at approximately 1,250-1,300C
   • 75% clay + 25% wood ash — bends at approximately 1,100C
   • 50% clay + 50% wood ash — bends at approximately 950C
   • 25% clay + 75% wood ash — bends at approximately 850C

2. Place these cones inside the kiln where you can see them through a peephole.

3. When a cone bends and touches the shelf, you have reached its temperature.

4. Calibrate your cones by experimenting and recording results. Over time, you build a reliable set of temperature indicators tuned to your specific materials.

Mastering the firing curve transforms pottery from a gamble into a reliable manufacturing process. Every kiln load teaches you something about your specific clay, fuel, and kiln behavior.