Numerous defects in the sorting after firing are generated by the previous steps of the process: casting, drying, glazing.

Firing cracks

After the first firing, the shape of the crack could tell about the origin: if the crack is sharp, the origin is the cooling. If the crack is smooth, then the origin is the pre-heating or heating.




For an issue in heating or pre-heating, the potential issues are a too high moisture due to incomplete drying of the piece. Otherwise, the pre-heating curve has to be adjusted.

For an issue in the cooling, if the firing curve could not be adjusted, we can consider that the best way to solve the issue is to decrease the thermal expansion coefficient. The total quartz amount could be decreased either by a reduction of the quartz itself, or by using higher alumina content clays:

A proper glaze/body thermal expansion coefficient matching is also mandatory to avoid thermal shock.

In FFC production, the use of a lower dilatation chamotte could be recommended.

Creeping (Deformation or Pyroplasticity)

Either in vitreous-china or in fine-fireclay, creeping is a key parameter to keep under control.


Indeed, a variation in the creeping could have an impact on the geometry of the product itself.

To reduce the creeping of vitreous china:

Reduce total amount of feldspar, and prefer K feldspar to Na feldspar, or reduce firing temperature, but always ensure the water absorption is still correct

  • Use more refractory plastic materials (clay and kaolin)
  • Increase green density of the cast piece – better rheology

To reduce the creeping of fine fireclay:

  • Increase the amount of chamotte in the slip
  • Or use a coarser chamotte
  • Avoid the introduction of fluxes


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