Many factors play a role in the formation of pinholes. Factors such as the type of clay you use, the way you shape, the glaze you choose, the glaze application and both biscuit and glaze firing programmes can cause this situation.

This article looks at each stage of the pottery production process. By proceeding in a systematic way, we hope to help you identify what might be causing pinholes in your glaze. Fortunately, there are solutions that you can try at each stage, so you can eliminate the problem.

What is a Pore?

Pores in glazes are small holes that form on the surface of the liquid glaze and remain after cooling. These holes can be of various sizes and are usually as small as a pinhole. In addition to creating an aesthetically unpleasant appearance in glazes, pores can also cause functional problems such as liquid permeability.

Causes of Pore Formation:

Tips to Prevent Pore Formation:

Conclusion:

Although pore formation in glazes can be a frustrating problem, it is not impossible to overcome. Careful selection of clay and glaze, optimising biscuit and glaze firing programmes and applying glaze application techniques correctly will ensure a poreless and flawless glaze.

Extra Tips:

Remember: The art of pottery is a process that requires patience and experience. Do not be demotivated if you encounter pore problems. As you continue to experiment and learn, you will have the ability to create perfect artefacts.

Thick and Large Parts

Larger and thicker pieces may not receive enough heat during biscuit baking. This may cause the carbons and gases in the clay to remain incompletely combusted. During glaze firing, these gases may form pores in the glaze as they escape.

Rough Surfaces Can Also Be a Problem

A surface that remains rough after glazing can prevent the glaze from adhering properly. Air pockets and small cavities can lead to bubbles and pores in the glaze layer.

Solutions

Bonus Tips:

By following these tips, you too can make pore-free, smooth and eye-catching glazed pottery!

Remember: Practice makes perfect. Keep experimenting and discover the most ideal techniques!

Stacking your products on top of each other makes it more difficult for moisture and gases to escape and prevents the pots below from cooking sufficiently. Therefore, avoid overfilling your kiln and make sure there is sufficient airflow between your pots.

Ensure Air Flow:

Sufficient airflow prevents carbonaceous material from burning through the clay and pinhole formation. 

Bake Bisque at a Higher Temperature:

For pottery fired at a low temperature, try firing biscuits at a temperature 2 cones higher than you plan to glaze. This prevents the pottery from forming gas during the glaze phase.

For flood mud, note that darker coloured muds contain more organic matter and minerals and may require a higher biscuit baking temperature.

Cook Slowly and Add Waiting Times:

Slow cooking and adding one or two waiting times to your programme allows the gases to better escape from the clay. 

By following these simple tips, you can optimise your biscuit baking process and achieve smooth, flawless glazes.

Bonus Tip:

Don't be afraid to experiment with your biscuit baking programme and find out what works best for your clay and glazes. Every clay and glaze is different, so you need to experiment to get the best results.

Glaze Absorption:

Dust and Dirt:

Glaze Thickness:

Additional Tips:

Glaze Materials

Some glaze materials may cause more pitting than others. An example of this is the mineral rutile. Rutile is added to glazes to give them interesting and varied colours.

A glaze containing rutile can contain beautiful looking crystal patterns, streaks and flecks. However, it can also increase the likelihood of pitting. Other glaze materials that can increase pitting are zinc, strontium carbonate and some glaze binders.

Solution:

If you are making your own glaze, you can see if there is a difference by changing the ingredients. However, if you are buying ready-mixed glazes, you may want to consider using an alternative option.

Glaze Viscosity and Surface Tension

Some glazes are more viscous than others. A viscous glaze is a glaze that is not very flowing or fluid, even when melted during the firing programme.

Because a viscous glaze does not flow or move much, the glaze will have a more difficult time sealing pits that may form during firing. High alumina content or the presence of materials such as zirconia can make the glaze more solid.

Another issue is glaze surface tension. Some glazes have a higher surface tension in the molten state. This can make it difficult for bubbles to break through the glaze surface. Bubbles may not be able to penetrate the glaze or they may break the surface, but there is not enough time for the pits left behind to fuse in the development glaze.

Solution:

Viscosity and surface tension do not depend on each other. Thick glazes do not necessarily have a high surface tension and vice versa.

However, it is possible to reduce viscosity and surface tension by adjusting the glaze in several ways:

Reduce the amount of alumina silicate in the glaze. Clay is a form of alumina silicate. It is added to the glaze to harden it to prevent the biscuit from slipping. But the glaze with higher clay content will be more solid and viscous.

By adding more fluids to the glaze, the glaze will prolong the time the glaze remains molten during glaze firing. This gives it more time to repair pits formed on the surface by the expulsion of gases.

Glaze Consistency

If your glaze is too thick or viscous, you may not get a good application on the biscuit. A glaze that is too thick is less likely to form a well adhered, even layer on the biscuit.

If the glaze is too thick in application, the glaze is more likely to flake off the biscuit before or during baking. Also, as mentioned above, an excessively thick layer of glaze is more likely to encounter pitting problems.
 

Solutions:

Add a small amount of deflocculant, such as glaze sodium silicate. This will thin the glaze without diluting it with water and make it less viscous. A deflocculated glaze will adhere more successfully to the biscuit and will be less likely to form pits.

I hope this text will help you. Please note that this text is not an exact translation and is for information purposes only. It is always recommended to consult experts for technical issues.