• Low flow-ability

    1. Depending on the application device (bell, double disc, airless spray gun), rheology of the glaze must be adjusted. In some cases the glaze does not flow enough and application on the tiles will not be correct or the pumps will not work properly.

    2. This is due to high viscosity and/or to a high yield value. One solution is to adjust the water and/or additives content. But to keep the same slip density, switching to a more fluid kaolin or clay can be the solution.

  • Glaze sedimentation
    1. The glaze is composed mostly of fritt and kaolin. But during preparation and storage the glaze can sediment more or less quickly depending on the kaolin used. Also the settled cake at the bottom of the tank will be more or less difficult to stir back to suspension. By using the appropriate kaolin the above issues can be solved.
  • Residue on sieve
    1. During engobe preparation and depending on the milling stage and mesh size used on the sieve, residue can be found on the sieves. If the quantity of residue is too high, a low residue ball clay will help to reduce it.

  • Drying time

    1. The drying time of glaze and engobe must be adjusted to obtain a mat surface just before the following application.

    2. If the drying of engobe or glaze is too quick before the next application, the glaze will be absorbed too fast leading possibly to holes in the layer and a gradient of particles size and composition across the layer itself.

    3. If the glaze dries too slowly, at the decoration step it can lead to a loss of definition of the inks due to expansion of the ink dots on a wet surface.

  • Powdering or low adhesion
    1. When the glaze layer is not cohesive enough, after drying the layer can either transform into powder on the surface which will stick to the printing rollers, or create defects. To reduce powdering,  the following kaolins can be used to bring more cohesion to the glaze layer.
  • Ridges
    1. During application by bell , the glaze curtain is trembling or wavy, which leads to ridges on the tile surface. This is usually due to low viscosity of the glaze. A controlled glaze viscosity around 40-50 s (Ford cup 4 mm ) is usually a good compromise depending on the slip density.

  • Matting
    1. 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.
  • Low opacity
    1. 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

    1. 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.

    2. 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.

    3. 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.

    4. 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 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

  • Dimples
    1. 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
    1. 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.
  • Crazing and Peeling

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

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

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

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

  • Waviness
    1. 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