Many of the defects found during sorting after firing are generated at previous stages of the process: casting, drying and glazing.
After first firing, the shape of the crack denotes its 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.
If the origin of the crack is heating or pre-heating, the potential causes are too high moisture due to incomplete drying of the piece. This requires the pre-heating curve to be adjusted.
For an issue with cooling, if the firing curve cannot be adjusted, the best way to solve the problem is to decrease the coefficient of thermal. This can be done by decreasing the total amount of quartz could either by reducing the quartz itself or by using higher alumina content clays such as:
- UK Devon: Hycast VC, Hycast Rapide
A suitable glaze/body thermal expansion coefficient match is also crucial to avoid thermal shock.
In FFC production, the use of a lower dilatation chamotte could be recommended.
- France: Clayrac Epurée
- UK: Molochite®
Creeping (Deformation or Pyroplasticity)
Creeping is a crucial parameter which needs to be kept under control in both vitreous-china and fine-fireclay production.
Variations in creeping can have an impact on the geometry of the product itself.
To reduce the creeping in vitreous china:
Reduce total amount of feldspar, and prefer K feldspar to Na feldspar. Alternatively, reduce firing temperature, making sure water absorption levels remain correct.
- Use more refractory plastic materials (clay and kaolin)
- Increase green density of the cast piece – better rheology
To reduce creeping in fine fireclay:
- Increase the amount of chamotte in the slip
- Or use a coarser chamotte
- Avoid the introduction of fluxes
- France: Clayrac SMD, SHD, Clayrac Epurée
- UK: Molochite®
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