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Glaze defects after firing

GLAZE DEFECTS AFTER FIRING

Low gloss

Low opacity

Bubbles and pinholes

Dimples

Black spots

Crazing and Peeling

Waviness

Low gloss

For glossy glaze production, the glaze glossiness on the entire tile surface can be too low. This can be due to low glaze fusibility which can be increased by milling the glaze slightly finer. If this is not sufficient some kaolins listed below are able to increase gloss and give a better glaze finish.

Glaze kaolins

  • France: Kaolinor 1C, Kerbrient
  • UK: Kaoshine CNL80

Low opacity

This can occur in semi-opaque or opaque glazes due to low crystallization of zircon precipitates during firing. A high alumina kaolin can improve glaze opacity.

  • Brazil: Kaoshine RC75 and RC70
  • US: Allen G

Low opacity

This can occur in semi-opaque or opaque glazes due to low crystallization of zircon precipitates during firing. A high alumina kaolin can improve glaze opacity.

  • Brazil: Kaoshine RC75 and RC70
  • US: Allen G

Bubbles and pinholes

Bubbles are always present during and after firing in a glaze. However if the bubbles are too large and are not cured during firing, they will appear as a defect on the glaze surface.

Pinholes on the other hand are generated by gases from the body which form larger holes if the glaze is not liquid enough to ‘heal’ itself.

Applying a suitable engobe layer thickness will help to reduce the size of the bubbles which travel through the glaze layer. Then adjusting the molten glaze viscosity can help in one of the two following ways:

  • higher viscosity: bubbles stay below the glaze surface
  • very low viscosity: bubbles travel quickly through the glaze

The above adjustment can be achieved in various ways including using a high alumina kaolin which will increase the viscosity of the glaze.

Glaze kaolin

  • Brazil: Kaoshine RC75
  • US: Allen G

Another way to solve this issue is to replace Mg and Ca carbonates by silicates in the body and glaze to reduce the amount of carbon dioxide emitted during firing.

Talc

  • France: Ceramflux, OOS Ceram  

Wollastonite

  • US: Nyad 325M

Dimples

This defect is also sometimes called ‘orange-peel’ and is caused when bubbles cannot escape from the glaze due to high surface tension and viscosity and remain beneath the glaze surface.  When cooling down the bubbles will shrink and leave a depression on the glaze surface.

Black spots

Black spots on the glaze surface after firing can be due to many causes from raw materials to processing. Sometimes they can be due to low quality kaolin. Switching to a well processed kaolin will directly remove the source of contamination. Some of the purest kaolins for glazes are listed below.

Glaze kaolin

  • France: Kaolinor 1C, Kerbrient
  • Brazil: Kaoshine RC70 and RC75

Crazing and Peeling

These defects are both due to a mismatch of thermal expansion between glaze and body.

  • Crazing occurs when glaze thermal expansion is too high versus body.
  • Peeling occurs when glaze thermal expansion is too low versus body.

The right engobe formulation can help prevent such defects by acting as an interlayer with an intermediate thermal expansion coefficient.

Waviness

Waves on the glaze surface are usually due to high viscosity of the molten glaze which does not have time to extend during firing. The first solution is usually to adjust the kiln by increasing temperature or soaking time, but kaolin also has an influence. A fine kaolin will help improve surface smoothness.

  • France: Kaolinor 1C, Kerbrient
  • UK: Kaoshine CNL80
  • Brazil: Kaoshine RC75

 

Still having issues with glaze defects after firing? Have a question you need to ask us?

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

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