Clay Brick Firing Temperature and Firing Curve

Most clay bricks are fired to a peak of around 1,000 °C, depending on the clay: US tunnel kilns run at up to about 1,090 °C (US EPA), and on our reference lines the peak is typically 950–1,050 °C, up to 1,150 °C. Fired too low, a brick stays porous and weak; fired too high, it bloats, distorts or glazes. The whole firing curve takes 10–40 hours (Brick Industry Association).

What happens on the way up to the peak, and on the way down, decides whether the bricks come out sound. This article walks through the firing curve stage by stage, shows how peak temperature changes the brick, explains brick firing time, and lists the common firing defects with their causes. Industry figures carry their published source; figures marked “on our reference lines” come from the fired clay brick lines YuxingCorp supplies.

Clay Brick Firing Temperature by Stage

The Brick Industry Association (BIA) divides firing into overlapping stages, and other sources add the critical points inside them:

The ranges overlap because a brick heats from the outside in. While the surface is oxidizing, the core may still be releasing water. That is why the firing curve is set by the thickest, densest part of the setting, not by the surface temperature a thermocouple reads.

How Peak Firing Temperature Changes the Brick

Vitrification gives a fired brick its strength and low absorption, and it depends strongly on the peak temperature. A 2024 study of a red brick clay from Hunan, China, fired laboratory specimens at 800–1,050 °C and reported these properties up to 1,000 °C:

Source: Gu and Ling, Science Progress, 2024. The values are for small laboratory specimens, not full-size bricks, but the pattern holds: in this clay, absorption fell most sharply between 900 and 950 °C, and the authors place the onset of viscous-flow sintering around 1,000 °C.

Too much heat is as harmful as too little. In ceramic tests on Tasmanian brick clays fired at 950, 1,000 and 1,050 °C with a two-hour soak, one blend was obviously overfired at 1,050 °C, with roughened surfaces and in some cases deep transverse cracking, and bricks from one of the clays also appeared overfired at that temperature. Color deepened from light red at 950 °C to dark rust red at 1,050 °C (Mineral Resources Tasmania). The BIA gives firing shrinkage of 2.5–4% for typical bricks, rising at higher temperatures, which also produce darker shades.

The useful number for a plant is not one temperature but the firing range: the band between the lowest temperature that meets the product standard and the highest temperature before the clay bloats or distorts. A wide firing range tolerates the temperature differences that exist across any kiln setting; a narrow one needs a gentler curve and tighter control. Test-firing your clay at several temperatures is how the range is found.

Brick Firing Time: How Long the Curve Takes

Firing takes 10–40 hours, and cooling in a tunnel kiln rarely more than 10 hours (BIA TN 9). The US EPA puts drying, firing and cooling together at 20–50 hours for most brick types (AP-42, section 11.3). In a survey of 44 European tunnel kilns for clay blocks, average firing time was 34.1 hours (Zi Brick and Tile Industry International, 2025).

Where a plant sits in that range depends on:

1. Brick size and wall thickness. Heat and gas must reach the core; thick solid bricks need longer than thin-walled hollow blocks.
2. Clay mineralogy. Clays with more organic matter, sulfur or carbonates need a longer oxidation stage.
3. Internal fuel. Fuel mixed into the body burns inside the brick, so its heat release has to be matched to the air supply.
4. Setting density. A tightly packed setting fires more slowly and less evenly. European research recommends 6–8 mm spacing between clay blocks (Zi, 2025).

Faster firing lowers specific energy use (Zi, 2025), but only if the clay and the setting allow it. Shortening the cycle is a commissioning task, done step by step, not a setting chosen on paper.

The Firing Curve in a Tunnel Kiln

In a tunnel kiln the firing curve is laid out along the length of the kiln. Each car passes the same temperature profile at the same speed, so every brick follows the same curve. On our reference lines a tunnel kiln is at least 90 m long: about 25 m of preheating, 40 m of firing and 25 m of cooling. The curve is adjusted with three controls:

• Push rate, which sets how long each car spends in each zone.
• Fuel distribution, through the fuel feeding points or burners along the firing zone.
• Air and draught, which set how fast hot gas moves through the setting and how much oxygen reaches the bricks.

On our lines, most of the heat usually comes from internal fuel mixed into the body, such as coal gangue, coal or fly ash, topped up with external fuel. Internal fuel ignites in the preheating zone, so the oxidation stage starts inside the brick earlier than the external fuel alone would suggest. Matching air supply to that heat release is one of the main tasks of firing adjustment during commissioning, which is part of our kiln supply scope. The tunnel kiln for bricks article explains kiln zones and sizing.

In Bull’s trench and Hoffman kilns the fire moves through stationary bricks, and the curve each brick sees depends on how the fire is fed and how the bricks are placed. That is why brick quality in those kilns varies more with position. Our comparison of Hoffman, tunnel and Bull’s trench kilns covers the differences.

Common Firing Defects in Clay Bricks and Their Causes

Sources for the table: BIA TN 9; US EPA AP-42; Ceramic Industry (2003); Digitalfire; Mineral Resources Tasmania.

A few of these need more explanation:

• Black core and bloating are linked. Ceramic Industry describes black coring as unburned carbon from organic matter in iron-bearing clay, made worse by a fast temperature rise in the early part of firing and by densely packed loads that restrict airflow. Gas left in the core when the surface vitrifies can then swell the brick, which is why the remedy for bloating includes completing oxidation before vitrification starts.
• Quartz inversion matters most on cooling. Free quartz changes volume suddenly at about 573 °C. If one part of the brick passes through that point before another, the stress can crack it; Digitalfire notes that the risk rises with large quartz grains and thick sections.
• Lime blowing shows up later. In the Tasmanian tests, blends made with a sand containing 10.9% CaO showed bloating at 1,050 °C in one case and, in another, blowing two to three weeks after firing. Ceramic Industry notes that the size of the lime particles, more than their presence, decides whether pops appear. Fine grinding and screening are the usual fixes.
• Efflorescence starts in the raw material. One Tasmanian clay with 0.41% total sulfur gave moderate to heavy sulfate efflorescence. Some plants add barium carbonate before extrusion to keep sulfates from rising to the brick surface (AP-42).

Most firing defects trace back to two things: a raw material whose behavior was not tested, and a curve set for one part of the setting instead of all of it. The clay brick manufacturing process article shows where raw material testing and drying sit before the kiln.

FAQ

What temperature are clay bricks fired at?

Vitrification, which gives the brick its strength, runs from about 871 to 1,316 °C (BIA Technical Note 9), and US tunnel kilns reach a maximum of about 1,090 °C (US EPA AP-42). On our reference lines the peak is typically 950–1,050 °C, up to 1,150 °C. The right peak depends on your clay’s firing range.

How long does it take to fire a clay brick?

Firing takes 10–40 hours, with cooling in a tunnel kiln rarely more than 10 hours (BIA TN 9). Drying, firing and cooling together take 20–50 hours for most brick types (AP-42). In 44 European tunnel kilns, average firing time was 34.1 hours (Zi, 2025). Thick bricks and clays rich in organic matter need longer.

What is the difference between brick firing and brick burning temperature?

They mean the same thing. “Burning” is an older term that is still common in many brick-making regions. Both refer to the peak temperature the brick reaches in the kiln, typically around 1,000 °C, and to the curve that leads up to and down from it.

What happens if bricks are underfired?

Underfired bricks have not vitrified enough. They stay porous, absorb more water and are weaker. In one laboratory study, water absorption fell from 18% at 800 °C to 6% at 1,000 °C, and compressive strength roughly doubled (Gu and Ling, 2024). Pale color and a dull sound are the usual signs.

Why do bricks crack in the kiln?

Early cracks come from heating too fast while water is still leaving the brick. Cracks formed on cooling usually come from cooling fast or unevenly through quartz inversion at about 573 °C. Check residual moisture after the dryer, the preheating rate and the cooling-zone air before blaming the clay.

Does firing temperature change brick color?

Yes. Higher firing temperatures give darker shades and more shrinkage (BIA TN 9). In the Tasmanian tests, bricks went from light red at 950 °C to medium rust red at 1,000 °C and dark rust red at 1,050 °C. Iron content and kiln atmosphere also affect color; flashing in a reducing atmosphere is used to change surface color on purpose.

Planning a Fired Clay Brick Plant?

The firing curve is set during commissioning of the tunnel kiln on our fired clay brick production line, from kiln drying-out and ignition through firing adjustment. Send raw material samples first: the test firing tells you the firing range your kiln has to hold.

Sources

• Brick Industry Association, Technical Notes on Brick Construction 9: Manufacturing of Brick (December 2006): https://gobrick.com/media/file/9-manufacturing-of-brick.pdf
• US EPA, AP-42 Compilation of Air Emissions Factors, Section 11.3: Brick and Structural Clay Product Manufacturing: https://www.epa.gov/sites/default/files/2020-10/documents/c11s03.pdf
• X. Gu and Y. Ling, Characterization and properties of Chinese red clay for use as ceramic and construction materials, Science Progress 107(1), 2024: https://pmc.ncbi.nlm.nih.gov/articles/PMC10908243
• Mineral Resources Tasmania, Technical Report 9, pp. 181–184, Ceramic investigations: brick manufacturing tests by de-aired extrusion: https://mrt.stgro-2780.saas.squiz.cloud/mrtdoc/dominfo/download/TR9_181_184/TR9_181_184.pdf
• Zi Brick and Tile Industry International, Energy efficiency – the order of the day (part 3 of 3), ZI 01/2025: https://www.zi-online.info/en/artikel/energy-efficiency-the-order-of-the-day-part-3-of-3-4205190.html
• Ceramic Industry, Identifying and Correcting Clay Body Defects (2003): https://www.ceramicindustry.com/articles/ppp-identifying-and-correcting-clay-body-defects/
• Digitalfire Reference Library, Quartz inversion: https://digitalfire.com/glossary/quartz+inversion