What Does Shrinkage in Brick Making Mean?

Shrinkage is a critical factor in brick production, occurring at different stages of the process. Understanding how and why shrinkage happens helps manufacturers minimize defects like cracking and warping in the final product.

During the drying process, as water evaporates from the raw brick (called the green body or blank), the particles move closer together, causing the overall volume to shrink. This is known as drying shrinkage.

To measure this, manufacturers use a value called drying line shrinkage, which is the percentage reduction in length of the blank compared to its original length.

If the drying shrinkage is too high and drying occurs too quickly, the brick may develop serious surface cracks and become unusable.

In general, drying line shrinkage should be kept below 6%. If it exceeds this, the raw material should be adjusted—often by mixing in less plastic or more granular materials to reduce shrinkage.

After drying, bricks are fired in a kiln. During this stage, additional shrinkage occurs due to:

Physical and chemical changes in the material

Burn-off of organic or volatile components

This is known as firing shrinkage, and it results in a finished brick that is lighter and slightly smaller than the dried blank.

Firing shrinkage is also calculated as a percentage—specifically, the reduction in length from the dried blank to the final fired brick.

The drying sensitivity coefficient measures how likely a clay body is to crack during drying. It’s influenced by how unevenly moisture evaporates:

The outer layers of the brick dry and shrink faster.

The inner core remains wetter and resists shrinking.

This mismatch in shrinkage creates internal stress.

If the stress exceeds the elasticity limit of the clay (typically 1–2%), surface cracking or “swelling” can occur.

A coefficient less than 1 means the material is easy to dry with minimal risk of cracking.

A coefficient greater than 2 indicates a high risk of cracking, requiring adjustments to the raw mix—such as adding sand or other non-plastic materials—to reduce sensitivity.

In general, the more plastic a clay is (i.e., the more moldable it is), the higher its drying shrinkage and sensitivity. This means highly plastic clays must be managed carefully during drying to avoid defects.

Controlling shrinkage—both during drying and firing—is essential for producing high-quality bricks. Monitoring the drying line shrinkage, firing shrinkage, and drying sensitivity coefficient ensures the bricks remain strong, stable, and crack-free. Adjusting the raw material mix is often the key to solving shrinkage-related issues in brick manufacturing.