Clay Brick Manufacturing Process: Step by Step

Clay bricks are made in eight steps. The raw material is tested, dug and stockpiled, then crushed, ground and mixed with water. A vacuum extruder pushes the mix through a die as a continuous column, which is wire-cut into bricks. The green bricks are set on kiln cars, dried, fired at around 1,000 °C, cooled, unloaded and packed.

The same eight steps apply whether a plant makes 240 × 115 × 53 mm solid bricks or 390 × 190 × 190 mm hollow blocks. What changes from plant to plant is the raw material, the forming route, how the bricks are set and dried, and the kiln. This guide follows a modern extruded-brick plant with a tunnel kiln, the configuration most new plants choose, and gives the figures that control each step. Industry figures come from the Brick Industry Association (BIA) and the US EPA; figures marked “on our reference lines” come from the fired clay brick lines YuxingCorp supplies.

The Clay Brick Manufacturing Process at a Glance

The BIA groups the same work into six phases: mining and storage, preparation, forming, drying, firing and cooling, and de-hacking and storage (BIA Technical Note 9). Splitting testing, cutting and setting into their own steps makes it easier to see where the equipment and the decisions sit.

Steps 1–3: Testing, Storing and Preparing the Raw Material

Testing and stockpiling

Clay bricks are made from three families of natural clay: surface clays, shales (clays compressed until they are almost slate) and fire clays mined at depth (BIA TN 9). Modern plants also use coal gangue, fly ash, river silt and mine tailings. On our reference lines coal gangue with shale is the most common body: gangue typically makes up 30–70% of the mix and fly ash can reach 20–40%.

The raw material test comes first because it decides almost everything that follows:

1. Plasticity tells you whether the body will extrude cleanly, and how fine it must be ground.
2. Drying behavior tells you how fast the green bricks can be dried, and whether they are strong enough to be stacked high while still wet.
3. Firing range tells you the peak temperature and how tightly the kiln must hold it.
4. Calorific value tells you how much of the firing heat can come from fuel inside the body.
5. Sulfur and fluorine tell you what the kiln will emit. Raw materials typically contain about 0.01–0.06% fluorine, much of which leaves the kiln as hydrogen fluoride, and sulfur in the clay is the main source of sulfur dioxide (US EPA AP-42, section 11.3).

Clay and shale are mined in open pits and stockpiled, and ground material is held in silos or piles that are typically enclosed (AP-42). A stockpile is more than storage: it lets the plant blend material from different parts of the pit, so that the extruder and the kiln see a consistent body from week to week.

Preparing the clay body

Preparation turns lumpy, variable pit material into a uniform body. A primary crusher breaks the lumps; grinding mills (dry pan, roller mill or hammer mill) and screens then produce the fine fraction (AP-42). Hard materials such as shale and coal gangue need more crushing and grinding than plastic surface clay, and bricks with thin webs, such as perforated bricks and hollow blocks, need a finer body than solid bricks.

Water is added and the body is mixed until its moisture is even. This is also where internal fuel enters the process. On our reference lines most of the firing heat usually comes from fuel mixed into the body: coal, coal gangue, fly ash or slag. Bodies rich in coal gangue can be fired on internal fuel alone. Some plants add barium carbonate at this stage to stop soluble sulfates from rising to the brick surface and leaving white deposits (AP-42).

Step 4: Forming the Bricks by Vacuum Extrusion

Most clay bricks are formed by the stiff-mud process: the body, at 10–15% water, is de-aired in a vacuum chamber at 375–725 mm of mercury, then forced through a die as a continuous column (BIA TN 9). About 90% of US bricks are made this way. De-airing removes the air pockets that would otherwise weaken the brick and cause laminations.

On our reference lines, two-stage vacuum extruders form the column at an extrusion pressure of up to 4.0 MPa with vacuum to −0.092 MPa. The upper stage mixes the body and feeds it into the vacuum chamber; the lower stage compresses and extrudes it. Hollow and perforated bricks must be vacuum-extruded. Changing the die switches the line from one brick type to another.

Two other forming routes exist (BIA TN 9):

• Soft mud, with 20–30% water, pressed into molds lubricated with sand or water. It suits handmade-look facing bricks and plastic clays that do not extrude well.
• Dry press, with up to 10% water, pressed in steel molds at 3.4–10.3 MPa. It is now rare; AP-42 notes there may be no US plants still using it.

For a plant making standard, perforated and hollow bricks in volume, extrusion is the practical choice. Our article on the vacuum brick extruder covers how the machine is specified.

Step 5: Cutting and Setting: One-Setting or Two-Setting

The column leaves the die and is cut by wire: first into strips by an automatic strip cutter, then into individual bricks by a blank cutter (AP-42 describes wire cutting after extrusion). The green bricks are then set onto kiln cars, either by an automatic setting machine under PLC control or by a six-axis robot with custom grippers.

How the bricks are set decides how the dryer and kiln are arranged. On our reference lines there are two routes:

The choice depends on how strong the green bricks are when wet, which is one more reason the raw material test comes first.

Step 6: Drying the Green Bricks

Wet bricks leave the cutter with 7–30% moisture, depending on the forming method (BIA TN 9). They are dried at about 38–204 °C, usually for 24–48 hours, and shrink 2–4% as the water leaves (BIA TN 9). Most dryers take their heat from the cooling zone of the kiln, at about 204 °C, with gas or other fuel as a supplement (AP-42).

Drying must be slow at first. If the surface dries and shrinks before the core, the brick cracks, and those cracks open further in the kiln. Dryer air temperature, humidity and air speed are set during commissioning to suit the clay, and they are the first things to check when cracked bricks appear.

Steps 7 and 8: Firing, Cooling and Packing

Firing clay bricks in a tunnel kiln

Firing turns dried clay into ceramic. The BIA describes five stages, which overlap: final drying of free water up to about 204 °C, dehydration from about 149 to 982 °C, oxidation from 538 to 982 °C, vitrification from 871 to 1,316 °C, and, for some products, flashing in a reducing atmosphere near the end to change the surface color (BIA TN 9). Firing takes 10–40 hours. On our reference lines the peak firing temperature is typically 950–1,050 °C, up to 1,150 °C, depending on the body. Our article on clay brick firing temperature explains each stage and the defects that come from getting it wrong.

Tunnel kilns are the most common kiln for this work (BIA TN 9). Kiln cars loaded with bricks move through a long tunnel with a preheating zone, a firing zone and a cooling zone; US tunnel kilns are about 104–152 m long (AP-42). On our reference lines a tunnel kiln is at least 90 m long, with a firing zone of about 40 m and about 25 m each for preheating and cooling. External fuel tops up the heat from the internal fuel: coal, natural gas, coal gas or biomass pellets. The tunnel kiln for bricks article covers zones, sizing and fuel in detail.

Cooling, unloading and packing

Bricks cool inside the kiln as they move toward the exit. In tunnel kilns cooling rarely takes more than 10 hours; in periodic kilns it takes 5–24 hours (BIA TN 9). Cooling cannot be rushed: bricks that cool too fast crack. The bricks also shrink a further 2.5–4% during firing, and higher firing temperatures give more shrinkage and darker colors (BIA TN 9), so die dimensions are set to allow for both drying and firing shrinkage.

At the end of the line, fired bricks are unloaded from the kiln cars and packed automatically on our reference lines. Each production batch is then checked against the standard the market uses. In the United States, building bricks of clay or shale are covered by ASTM C62-23, which sorts them into three grades by resistance to freezing when wet (ASTM C62).

What the Process Means for Plant Size

The kiln sets the capacity of a clay brick plant, and every other step is sized to keep it full. On our reference lines, plants are commonly built for 60–100 million bricks a year in standard brick equivalent, about 250,000–300,000 bricks a day. Typical products are standard bricks of 240 × 115 × 53 mm, perforated bricks of 240 × 115 × 90 mm and 270 × 130 × 118 mm, hollow blocks of 390 × 190 × 190 mm, and 240 × 115 × 75 mm bricks for export markets.

Three points follow for anyone planning a plant:

1. Preparation is sized for the worst raw material, not the average. Hard shale or gangue in the pit sets the crusher and mill size.
2. The extruder and setting equipment must match the kiln’s daily output, with margin for die changes and cleaning.
3. The dryer and the kiln are one system. The setting route fixes the dryer type, and the dryer usually depends on kiln waste heat.

If you are weighing fired clay bricks against other wall materials, our comparison of AAC blocks and red bricks sets out how the products and the plants differ.

FAQ

How are clay bricks made, step by step?

The raw material is tested, mined and stockpiled, then crushed, ground and mixed with water. A vacuum extruder forms a column that is wire-cut into bricks. The green bricks are set onto kiln cars, dried at about 38–204 °C, fired in a kiln, cooled, unloaded and packed (BIA Technical Note 9). Most modern plants fire in a tunnel kiln.

What temperature are clay bricks fired at?

Vitrification, the stage that gives a brick its strength, takes place between about 871 and 1,316 °C (BIA TN 9), and US tunnel kilns reach a maximum of about 1,090 °C in the firing zone (US EPA AP-42). On our reference lines the peak is typically 950–1,050 °C, up to 1,150 °C, depending on the clay.

How long does it take to make a clay brick?

Drying usually takes 24–48 hours and firing 10–40 hours, with 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). Raw material stockpiling and preparation come before that.

Can fly ash or coal gangue replace clay in fired bricks?

Partly, and sometimes almost entirely. On our reference lines coal gangue typically makes up 30–70% of the body, usually blended with shale, and fly ash can reach 20–40%. Gangue also carries fuel, so gangue-rich bodies can be fired on internal fuel alone. A raw material test confirms the right blend.

Why are clay bricks extruded under vacuum?

De-airing removes air trapped in the clay, which gives denser, stronger bricks and prevents laminations. The BIA puts the vacuum at 375–725 mm of mercury for stiff-mud extrusion. Hollow and perforated bricks, with their thin webs, must be vacuum-extruded; on our reference lines vacuum reaches −0.092 MPa.

What is the difference between burnt clay bricks and unfired bricks?

Burnt clay bricks are fired in a kiln, which turns clay into ceramic. Unfired bricks are bonded by cement or by autoclave curing instead. Concrete blocks come from a brick making machinery line, and sand-lime and fly ash bricks from an autoclaved brick production line; neither needs a kiln.

Planning a Fired Clay Brick Plant?

The steps above are configured for 60–100 million bricks a year on our fired clay brick production line, including tunnel kiln design, refractory masonry and commissioning. Start by sending raw material samples: the test decides the forming, setting and fuel route for your plant.

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
• ASTM International, ASTM C62-23 Standard Specification for Building Brick (Solid Masonry Units Made from Clay or Shale): https://store.astm.org/c0062-23.html