After milling or blunging, the measure of the quantity and type of residue found on the sieves is a key step of the ceramic process. The observation of the residue with a magnetic bar and optical microscope will raise the presence of magnetic, colored or unusual particles. This will alert on the risk to have defects later in the process.
When the slurry get in contact with the mold, a segregation of particles can occur creating defect after firing. This segregation can find his origins in the orientation of particles during the filling of the mold.
The rheological set up of the slurry remains the main driver to control and avoid these defects, usually generated by a too fluid or low thixotropic slip.
Increasing the density of the suspension or the thixotropy reduce the occurrence of the defect. The use of a kaolin like Grolleg allows to work at higher density and having a controlled thixotropy.
The root causes of cracks are multiple and often to be found in the shaping steps but also, too fast drying, uneven atmosphere (temperature or moisture), incautious storage are potential causes for the drying cracks.
From a raw materials point of view, the use of minerals allowing a good packing of particles and able to absorb or avoid crack generation lead to significant improvement of the yield.
Cracks can occur when the piece presents thickness differences or at the junction of solid and hollow cast parts. The design of the piece can be modified or the casting time adjusted. The body should be strong enough to absorb the stresses. This can be increased by the use of finer and stronger kaolin like Grolleg, Standard porcelain or Super Standard Porcelain.
Powdering during finishing and / or fragile pieces
The finishing and handling operations apply stresses on the pieces. They can create breakage or defects of the pieces. To facilitate the finishing operation and avoid the creation of defects from de-molding to firing it’s sometimes needed to increase the strength of the body with higher plasticity kaolin or clay.
After glazing , the surface of the glaze can less powdery or the glaze layer is removed during handling. The adherence of the glaze to the body/substrate is not good enough and can generate defects. The use of fine kaolin with high binding behavior will reduce the occurrence of the defect.
During the firing of porcelain, aluminous porcelain pieces, the flux bearing minerals transform in a glass phase. In parallel, new crystalline species are appearing. When the glass viscosity is high and the crystalline phase is important, the piece will not deform. When the glass phase presents a low viscosity and the crystalline phase is poor, then the piece tends to deform. The firing cycle and the formula can be adjusted. Some kaolin like Premium presents the adequate mineralogy and particle size distribution and reduces the risk of deformation.
Black spots appearing on the glaze surface after firing can be due to many causes from raw materials to process. The staining effect of the contaminants can be diluted by a more intensive milling or removed by a fine sieving or installing magnets at different steps of the process. A high frequency cleaning of sieves and magnets reduces the risk of having black or colored spots. Some contaminants are not removable in a ceramic factory but can be removed by specific technologies during the kaolin processing for example.