Selecting suitable clay for the ceramic formers is very crucial because around 50% of the formulation of ceramic hand formers consists of clay. Below are the properties that we expect clay to contribute to the slip/body. This is suitable for both ball clay and China clay.
High Fluidity: (Low Deflocculant demand)
Typically a clay with low deflocculant demand is considered high-fluid clay. High-fluidity clay helps the slip to maintain stable rheological properties for a longer time. This property is very important for ceramic former slip in avoiding flow mark defects. As the thickness required for the ceramic former body is low, the possibility of flow marks is higher. These imperfections/defects can be avoided by using high-fluid clay.
More accurate fluidity of the clay slurry can be tested using a Brookfield viscometer.
Cast Rate:
A low cast rate of clay is better for proper densification and former casting, as it allows time for the manual casting process. A low cast rate also indicates that the clay has high dry strength.
Dry and fired Strength of clay:
Dry MOR (strength) of the hand former fully depends on the clay in the formulation. Therefore, clay should have a minimum cast dry MOR of 100 kgf/cm², which can help the former body achieve enough strength for handling and transportation from the casting to the firing stage.
Fired MOR & Thermal Shock Resistance:
High fired strength and thermal shock are important final properties of the ceramic hand formers. Therefore, the selected clay should also support these properties. High alumina content in the clay helps support these properties. The target of 35% Al₂O₃ in clay is satisfactory to support the above properties.
Low organic and iron content:
Low organic impurities and low iron content help the body have fewer defects. So checking at the loss of ignition at 1200 degrees Celsius and the Fe₂O₃ content in the chemical analysis is vital to understanding the impurities.
Residue or free silica:
A low residue percentage may indirectly indicate low free silica content in the clay, which is important to have good thermal shock resistance in the body and a low chance of dunts to the body after firing.
Key takeaways:
Following are the recommended points for selecting clay for the hand former formulation:
- DD% of less than 0.2% is a favorable starting point.
- A cast rate of 2.0 mm – 3.0 mm for 30 minutes is the optimum.
- Cast Dry MOR should be a minimum of 80 kgf/cm² for the clay.
- Fe₂O₃ content in the clay should be around 1.0%, and LOI should be around 12%.
- Clay residue at the 200# sieve should be less than 2%.
Did this article help you understand clay selection better for ceramic formers production? Please let us know in the comments. Also, check out the Ceramic formers production process & Defects in Ceramic formers.
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