Welcome to Imerys Performance Minerals

Tableware troubleshooting

TABLEWARE TROUBLESHOOTING

  • Body preparation
  • Casting
  • Drying
  • Glaze application
  • Firing

Body preparation

Residue on sieve

After milling or blunging, measuring the quantity and type of residue found on the sieves is a key step of the ceramic process. If observing residue with a magnetic bar and optical microscope shows the presence of magnetic, colored or unusual particles, there may be a risk of having defects later on in the process.

Casting

Casting spot and / or casting cords

When the slurry is in contact with the mold a segregation of particles may occur creating defects such as casting spots and casting cords after firing. This segregation is caused by the orientation of particles during the filling of the mold.

Adjusting the rheological set up of the slurry is the best way to control and prevent these defects which are usually caused by a low thixotropic slip.

Increasing the density of the suspension or the thixotropy will reduce this problem. The use of a kaolin such as our Grolleg grade or our ball clays improves density and controls thixotropy.

Kaolin

  • UK: Standard Porcelain
     

Ball clay

  • UK: Hywite Superb
  • France: FMT

Drying

Cracks

The root causes of drying cracks are multiple and often stem from the shaping stage, too fast drying, uneven atmosphere (temperature or moisture) or careless storage.

From a raw materials point of view, the use of minerals promotes good particle packing able to absorb or prevent the generation of cracks leading to a significant improvement in 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 stresses. Strength can be increased by the use of finer and stronger kaolins such as Grolleg, Standard Porcelain or Super Standard Porcelain.

 

Powdering during finishing and / or fragile pieces

Finishing and handling operations exert stresses on the pieces causing breakage or defects in the pieces. To facilitate finishing operations and prevent defects from de-molding or firing, it is sometimes necessary to increase the strength of the body with higher plasticity kaolins or clays.

Kaolin

  • UK: Grolleg, Standard Porcelain, Super Standard Porcelain
  • New Zealand: Premium

Ball clay

  • France: FMT

Glaze application

Glaze powdering

After glazing, the surface of the glaze can be powdery or the glaze layer detaches during handling. The adherence of the glaze to the body/substrate is not good enough and can generate defects. The use of a fine kaolin with high binding behavior is the best solution.

UK: Standard Porcelain, Super Standard Porcelain

Firing

Deformation

During the firing of aluminous porcelain pieces, the flux bearing minerals transform into a glass phase and new crystalline species appear. If glass viscosity is high and the crystalline phase is effective, the piece will not deform. However, if the glass phase presents low viscosity and the crystalline phase is poor, then the piece tends to deform. To solve this, the firing cycle and the formula can be adjusted. Certain kaolins, such as our Premium grade, have a mineralogy and particle size distribution which contribute to reducing the risk of deformation.

Kaolin

  • New Zealand: Premium


Black spots

Black spots which appear on the glaze surface after firing can be due to many causes ranging from raw materials to processing. The staining effect of the contaminants can be diluted by more intensive milling or removed by fine sieving or installing magnets at different stages of the process. Cleaning sieves frequently and using magnets reduces the risk of black or colored spots. Some contaminants are not removable at the ceramics plant but can be removed upstream using specific technologies during kaolin processing for example.

Highly processed products

  • France: BIP
  • UK: Grolleg, Standard Porcelain, Super Standard Porcelain
  • New Zealand: Premium

 

Still having issues with your tableware production? Have a question you need to ask us?

Please contact us. We will be only too happy to help.

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Ceramics