Why Do Green Clay Bricks Crack During Drying? Causes, Process Control and Practical Solutions

Cracking during drying is a common quality problem in clay brick manufacturing. Even when the extrusion and cutting processes appear normal, freshly formed green bricks may develop surface cracks, edge damage, internal fractures, or deformation before entering the firing kiln.

These defects can reduce the number of saleable bricks and create additional handling, rework, and production costs.

In most cases, drying cracks are not caused by a single factor. They may result from unsuitable raw material preparation, excessive forming moisture, uneven moisture distribution, inappropriate stacking, or an improperly controlled drying schedule.

For brick manufacturers, the key is to identify where moisture and stress are becoming uneven throughout the production process.

This guide explains the main causes of green brick cracking and provides practical methods for evaluating and improving the clay brick drying process.

1. Why Do Green Clay Bricks Crack During Drying?

Freshly extruded clay bricks contain water needed to provide plasticity during forming.

During drying, this water gradually moves from the interior toward the surface and evaporates into the surrounding air. As moisture is removed, the clay body shrinks.

If different parts of the brick lose moisture at significantly different rates, uneven shrinkage can develop. The resulting internal stress may exceed the strength of the green body and cause cracks.

Common types of drying defects

Surface cracking: Fine cracks appear on the surface, sometimes shortly after drying begins.

Edge cracking: Corners and edges develop cracks due to uneven moisture removal, handling damage, or localized stress.

Internal cracking: Cracks develop inside the brick body and may not be visible until the brick is broken or fired.

Deformation: The green brick loses its intended shape because the material cannot maintain its structure during moisture removal.

Layer separation: Weak bonding or uneven material distribution can cause separation within the extruded body.

Identifying the defect type is the first step toward finding its actual cause.

2. Raw Material Preparation: The First Point to Check

The drying process begins before the brick enters the dryer.

Raw material composition, particle size distribution, plasticity, and mixing uniformity all affect forming behavior and subsequent shrinkage.

Particle size distribution

An unsuitable particle size distribution may influence the packing structure and moisture movement within the green brick.

If the feed contains excessive coarse particles, the material may require additional crushing, screening, or blending.

If the mixture contains too many fine particles, its water demand and shrinkage behavior may also change.

The appropriate particle grading should be established through raw material testing and production trials.

Material mixing and aging

Uneven mixing can create different moisture levels in different parts of the feed.

Aging or conditioning allows moisture to distribute more evenly through the prepared material, depending on the raw material and process arrangement.

Before extrusion, check whether the material is sufficiently mixed and whether moisture conditions remain consistent between production batches.

What should manufacturers monitor?

•  -Raw material source and batch variation

•  -Particle size distribution

•  -Plasticity and workability

•  -Natural moisture content

•  -Added water during mixing

•  -Aging or conditioning time

•  -Material temperature, where relevant

Stable raw material preparation provides a more consistent starting point for extrusion and drying.

3. Control Forming Moisture Before Extrusion

Excessive forming moisture is one possible reason for drying problems.

When too much water is added, the green brick may contain more moisture than the drying system can remove under the intended operating schedule.

This can increase drying time and may increase the risk of deformation or cracking if moisture removal is uneven.

However, reducing water content without considering material plasticity can also create problems.

If the mixture is too dry, extrusion resistance may increase, feeding may become unstable, and the green body may have insufficient strength or poor surface quality.

How to establish suitable forming moisture

Rather than using one fixed moisture percentage for all clay materials, manufacturers should establish an appropriate operating range through tests.

A practical procedure includes:

1.   1. Measure the moisture content of the prepared feed.

2.   2. Check the plasticity and extrusion behavior.

3.   3. Inspect the extruded clay column for tearing, surface defects, and dimensional instability.

4.   4. Test green brick strength and shape retention after cutting.

5.   5. Record drying shrinkage and visible cracking.

6.   6. Adjust water addition and mixing conditions based on the results.

Forming moisture should be controlled together with extrusion pressure, vacuum conditions, and raw material preparation.

4. Drying Speed and Temperature Must Be Properly Coordinated

A tunnel dryer is designed to remove moisture from green bricks under controlled conditions before firing.

The drying process involves heat transfer, moisture movement, air circulation, and exhaust.

If the drying environment changes too quickly, the surface may lose moisture faster than the interior can release it. This creates moisture gradients and differential shrinkage.

Why an unsuitable drying schedule causes cracks

Excessive initial drying intensity: Rapid moisture removal from the surface can increase internal stress.

Uneven airflow: Different areas of the drying chamber may experience different drying conditions.

Insufficient moisture exhaust: Moisture released from the bricks must be removed from the drying environment. Inadequate exhaust can reduce drying effectiveness.

Unbalanced temperature and humidity: Temperature alone does not determine drying performance. Relative humidity and air movement also affect moisture transfer.

What should a drying curve include?

A practical drying schedule should define:

•  -Initial drying temperature

•  -Relative humidity

•  -Air velocity and circulation

•  -Moisture removal rate

•  -Exhaust conditions

•  -Intermediate drying stages

•  -Final residual moisture before firing

The schedule should be adjusted according to raw material sensitivity, green brick dimensions, stacking density, and seasonal conditions.

There is no universal temperature or drying time suitable for every clay brick factory.

5. Stacking Patterns Also Affect Drying Quality

The arrangement of green bricks can influence airflow and moisture removal.

If bricks are stacked too closely, airflow channels may become restricted. This can create differences in drying conditions between the inside and outside of the stack.

If stacking is unstable, the lower layers may deform under the weight of the bricks above them.

Important stacking considerations

Airflow channels: Maintain suitable passages for drying air to reach the required surfaces.

Stacking stability: Ensure that the arrangement can support the load without excessive deformation.

Brick spacing: Follow the designed spacing requirements for the specific brick type and dryer configuration.

Loading consistency: Avoid large differences in stack geometry between drying batches.

Handling accuracy: Prevent impact and excessive gripping force during loading and transfer.

Automatic brick stacking systems can help improve positioning accuracy and repeatability when correctly configured for the brick dimensions and drying process.

6. How Can a Tunnel Dryer Help Reduce Drying Defects?

A tunnel dryer provides a controlled environment for removing moisture before the firing stage.

Compared with uncontrolled natural drying, an appropriately designed mechanical drying system allows manufacturers to manage airflow, temperature, humidity, and material movement more systematically.

Key equipment and process features

Air circulation system: Helps distribute drying air throughout the chamber.

Temperature control: Allows operators to adjust drying conditions according to the process schedule.

Humidity and exhaust management: Supports moisture removal from the drying environment.

Material transfer system: Coordinates brick movement through the drying zones.

Process monitoring: Helps identify variations between different drying sections.

A tunnel dryer cannot automatically eliminate every drying defect. Its effectiveness depends on the coordination between raw materials, forming moisture, stacking, airflow, and operating parameters.

Yingfeng Machinery provides brick dryer and tunnel kiln systems designed for integration with clay brick production lines. The appropriate configuration should be determined by the required production capacity, brick dimensions, moisture conditions, and site layout.

7. A Practical Troubleshooting Checklist

When cracks appear during drying, investigate the process in sequence rather than immediately changing the dryer temperature.

These are diagnostic possibilities, not definitive diagnoses. The actual cause should be confirmed through inspection and production records.

8. How to Build a More Stable Brick Drying Process

A systematic approach is more effective than adjusting individual parameters without understanding the complete process.

Step 1: Establish raw material records

Record material sources, particle size, plasticity, and moisture content.

Step 2: Standardize forming conditions

Monitor water addition, mixing, extrusion, vacuum conditions, and green brick dimensions.

Step 3: Check stacking consistency

Use a repeatable stacking pattern that supports the bricks and provides suitable airflow passages.

Step 4: Monitor drying conditions

Record temperature, humidity, airflow, exhaust conditions, and residual moisture at different stages.

Step 5: Evaluate fired product quality

Compare drying records with final product defects, including cracking, deformation, and dimensional variation.

Step 6: Adjust one group of variables at a time

Controlled adjustments make it easier to identify which process changes are responsible for improvements.

9.HONGYUN Machinery's Brick Drying and Kiln Solutions

HONGYUN provides brick drying systems, tunnel kilns, and related equipment for clay brick production lines.

A drying solution should be selected according to the characteristics of the raw material, required green brick output, brick dimensions, stacking arrangement, local climate, and firing system.

For projects experiencing drying cracks, an engineering assessment should examine the entire route from raw material preparation to extrusion, stacking, drying, and firing.

FAQ

 1.Why do clay bricks crack during drying?

Drying cracks may result from uneven moisture removal, excessive forming moisture, unsuitable raw material preparation, poor stacking, or an inappropriate drying schedule. The actual cause should be identified by examining the complete production process.

 2. Does faster drying always improve production efficiency?

No. Excessively rapid moisture removal can increase shrinkage differences and cracking risks. Drying speed should be balanced with raw material sensitivity, brick dimensions, airflow, and the required final moisture condition.

 3. How does a tunnel dryer help prevent brick cracking?

A properly designed tunnel dryer allows temperature, humidity, airflow, and material movement to be managed more systematically. This can help reduce uneven drying when the system is correctly matched to the bricks and production process.

 4. Can automatic brick stacking reduce drying defects?

It can help improve stacking consistency, positioning accuracy, and repeatability. However, the stacking pattern must also provide suitable airflow and support for the green bricks.

 5. What moisture content should green bricks have before firing?

There is no universal value for every brick material and kiln system. The required residual moisture should be established according to raw material properties, brick dimensions, dryer design, and firing requirements.

 6. Should I replace my dryer if the bricks keep cracking?

Not necessarily. First check raw material preparation, forming moisture, extrusion quality, stacking patterns, airflow, and the actual drying curve. Equipment modification or replacement should follow a technical diagnosis.

Conclusion

Reducing drying cracks in clay bricks requires coordinated control of raw material preparation, forming moisture, extrusion, stacking, airflow, and drying conditions.

A tunnel dryer is an important part of this process, but its performance depends on how well it is integrated with the complete brick production line.

For manufacturers seeking more consistent green brick quality, the most reliable starting point is to identify the actual source of moisture imbalance and shrinkage stress before changing equipment or operating parameters.