Matting

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 either by milling the glaze slightly finer. If this is not sufficient some kaolins listed below are able to increase the glossiness and give a better glaze finish.

Glaze kaolin

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 the glaze opacity.

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 fluid enough to heal itself.

First of all application of 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 will help also in one of  the two following ways:

  1. either to have a higher viscosity: bubbles stay below the glaze surface

  2. or to have a very low viscosity : bubbles travel quickly through the glaze

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

Glaze kaolin

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

Talc

Wollastonite

  • US: Nyad 325M

Dimples

This defect is also sometimes called “orange-peel” effect. 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 appearing on the glaze surface after firing can be due to many causes from raw materials to process. Sometimes they can be due to low quality kaolin. So 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

Crazing and Peeling

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

Crazing happens when the glaze thermal expansion is too high versus body.

Peeling  happens when the glaze thermal expansion is too low versus body.

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

Waviness

Waves on the glaze surface is 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 to improve the surface smoothness

Contact us about Glaze defects after firing

top