Ceramic raw materials influence drying shrinkage by changing body plasticity, water demand, particle packing and moisture-removal pathways.
Fine, highly plastic materials such as ball clay and bentonite improve forming behavior and green strength, but they normally require more water and can increase shrinkage and drying time. Lower-plasticity materials such as kaolin, quartz and calcined grog can reduce shrinkage, but excessive additions may reduce workability and dry strength. The best formulation is therefore not the one with the lowest shrinkage; it is the one that balances forming, demolding, drying and dimensional stability.
Definition: What Is Drying Shrinkage?
Drying shrinkage is the dimensional contraction that occurs when water leaves a wet ceramic body and the particles move closer together.
It is commonly calculated as:
Drying shrinkage = (wet length − dry length) ÷ wet length × 100%.
Digitalfire’s drying-shrinkage reference reports that a typical plastic pottery clay may shrink by approximately 6%, while highly plastic bodies may reach about 7.5%. Casting bodies can be as low as approximately 2% because their higher solid loading reduces the amount of water that must be removed. Actual values should always be compared using standardized sample preparation and moisture content.
How Different Raw Materials Affect Drying Shrinkage
1. Ball clay increases plasticity and shrinkage sensitivity
Ball clay contains fine particles with high surface area. It improves suspension stability, particle bonding, workability and dry strength. However, its higher water demand can slow drying and increase dimensional contraction.
When too much ball clay is used, differences between thick and thin sections can create moisture gradients, internal stress and cracking.
2. Bentonite has a strong effect at low additions
Bentonite contains extremely fine, highly active particles with strong water absorption and swelling behavior.
Digitalfire’s bentonite reference notes that additions of about 2% can significantly improve plasticity and dry strength in some bodies. Higher additions, however, can slow moisture movement, increase drying shrinkage and make the body more sensitive to cracking.
Its use should therefore be controlled by testing rather than by a fixed recipe percentage.
3. Kaolin supports dimensional control
Kaolin generally has coarser particles and lower plasticity than ball clay or bentonite. It normally requires less forming water and produces lower drying shrinkage.
Kaolin is also important for whiteness and refractory structure. However, a body containing too much low-plasticity kaolin may lack green strength or demolding resistance. Ball clay, plasticizers or improved particle packing may be needed to restore processing performance.
4. Quartz and grog create a non-shrinking framework
Quartz, calcined clay and grog do not undergo the same plastic drying contraction as raw clay minerals. They dilute the plastic fraction and create a more dimensionally stable particle framework.
Digitalfire’s grog reference explains that grog can reduce drying shrinkage and help stop small cracks from propagating. However, excessive non-plastic material lowers workability and dry strength. Poorly selected quartz size may also create stress during later firing.
5. Solid loading and particle packing determine water removal
The study Influence of Solid Loading on Drying-Shrinkage Behaviour found that total shrinkage in slip-cast bodies decreased as suspension solid loading increased.
At higher solid loading, less water must be removed from each unit volume, and particles begin in a more closely packed structure. Yet high solid loading is useful only when viscosity, deflocculation and mold-filling behavior remain controllable.
Raw-Material Control Checklist
Mineralogy: Identify kaolinite, illite, smectite and other clay minerals.
Plastic fraction: Avoid excessive ball clay or bentonite.
Particle distribution: Balance fine-particle bonding with coarse-particle stability.
Solid loading: Reduce free water without sacrificing slurry flow.
Batch testing: Prepare shrinkage bars at standardized moisture content.
Combined validation: Compare shrinkage, dry strength, demolding and cracking together.
Raw materials influence drying shrinkage by controlling how much water a ceramic body requires, how particles pack and how moisture escapes. Highly plastic clays improve forming but often increase shrinkage. Non-plastic materials reduce contraction but can weaken processing performance. A stable ceramic formulation should not target minimum shrinkage alone; it should create a repeatable shrinkage range suited to the product geometry and forming process.
FAQ
Q1: Does increasing ball clay always increase drying shrinkage?
A: It generally increases shrinkage sensitivity, but the result also depends on clay type, particle size, solid loading, deflocculation and the overall body formulation.
Q2: Can quartz addition eliminate drying cracks?
A: Quartz can reduce the proportion of shrinking clay, but it cannot correct uneven drying, wall-thickness variation or excessive airflow.
Q3: How can a factory detect raw-material batch changes?
A: Prepare standard test bars using the same moisture content, forming pressure and drying conditions, then compare drying shrinkage, weight loss and dry strength over time.