Cracking occurs when a glaze is too tight for a biscuit object. The mud and glaze expand and contract when the kiln heats up and then when it cools down. This is thermal expansion. If the glaze has a higher rate of thermal expansion, it will not contract as much as the mud when the kiln cools. This puts the glaze under stress and causes it to crack.

When Cracking Occurs?

Cracks in the glaze sometimes occur immediately when the kiln is cooling down. If this cracking occurs immediately, there is a crackling sound that you can hear before you open the oven. This sound may sound innocent, but when you hear it, you know there is a problem with the glaze.

Sometimes cracking starts shortly after the oven has been emptied. If this happens, you may have a chance to observe how the cracking pattern is formed. Usually the first crack is a long crack that runs all the way through the ceramic glaze. In a vertical piece, such as a vessel, this crack may run from the bottom to the ceiling. In flat and wide pieces, the crack usually covers the diameter of the base. This is called ‘primary cracking’.

However, cracking may not occur immediately. Cracks in glazes may appear weeks, months or even years later. Glazed ceramics are particularly prone to cracking. This is because glazed ceramics continue to absorb moisture from their surroundings. As this moisture is absorbed, the product expands a small amount. However, this small expansion can increase the tension under the glaze and cause cracks to form.

Conclusion:

Understanding cracks in glazes is important for an artist because it can damage the results of labour and creativity. Understanding the stress caused by cracks and taking precautions can help you create more robust and durable artefacts in Pottery art. Remember that each crack has a story and this story plays an important role in the evolution of Pottery.

Main Cause of Cracks: Thermal Expansion Mismatch

The main cause of cracks is that the glaze is too small compared to the clay layer underneath. When the biscuit is baked, the glaze melts and forms a thin layer on the biscuit. However, in the process the clay also softens and the elements in it melt.

During the firing process, both the clay and the glaze expand. This amount of expansion is called the coefficient of thermal expansion (TGM). Each mud and glaze has its own specific TGM value, which determines how the mud and glaze react to temperature changes.

Need for Harmonised Expansion and Contraction

To prevent cracks, the expansion and contraction rates of glaze and clay must be compatible. If the glaze expands more than the clay when heated, it will contract more than the clay when cooled. In this case, the glaze will stretch over the clay layer and cracks will form.

Consequently, attention to these details, which are important for pottery artists and hobbyists, is a key to preventing glaze cracks. Understanding the compatibility between clay and glaze is an important step in improving the quality of your artefacts and avoiding cracking problems. Remember that every material is different, so it is important to start by knowing the thermal expansion properties of your preferred clay body and glaze.

Let's talk about two suggestions that you may hear from many sources but are not effective. Firstly, it is recommended to slow down the cooling phase of your kiln programme. However, this can often be ineffective in allowing the glaze to conform to the clay body. The second suggestion is to apply the glaze more thinly. However, this too can often lead to cracking where the glaze is unsuitable.

1. New Clay and Secret Discovery

If you have a favourite glaze, the simplest solution is to try another clay. If you are using commercial glazes, you can contact the manufacturer for suggestions on the types of clays that are compatible with that glaze. You can also find other artists sharing their experiences on pottery forums.

2. Trials and Test Benches

By experimenting with various types of mud and testing different glazes, you can determine which combination is closest to the desired result. This is an effective way to learn from your own experience.

3. Check Temperature Ranges

Make sure that your sludge is cooked at the correct temperature range. For example, undesirable results can occur when not cooked at a specific sludge temperature.

4. Adjustment and Balancing

Cracking is usually caused by the glaze contracting more than the clay. Those with chemistry knowledge can solve this problem by adjusting the materials. You can balance the rate of thermal expansion by adding to the clay or adjusting the oxides in the glaze.

As a result, finding the harmony between clay and glaze is a continuous learning process for artists. Discovering your own combinations through trial and error methods will enable you to create your works with the quality and aesthetics you want. 

1) Biscuit Durability

Cracking is a condition that significantly weakens the biscuit and reduces its functionality. It is easy to think that cracking is only a surface problem. After all, the glaze appears to be a thin layer covering the outer surface.

However, when fired, the clay and glaze pass materials with each other. The mixture of these components forms a clay glaze interface.

If the glaze is fired at high temperature, it is important to remember that the glaze does not just remain on the surface. The glaze binds to the clay body and becomes part of the clay surface.

As a result, cracks in the glaze reach up to the clay body and form small cracks on the surface of the clay. These small cracks become points of weakness in the ceramic.

Research shows that the durability of an object with a cracked glaze is compromised. For example, B. Pinto's research states that

Cracks in the glaze ‘create a fracture path that can become a major defect in the object... These defects can easily propagate within the artefact and cause failure under a relatively low external load.
 

2) Bacteria and Contamination

Cracks can harbour bacteria and mould on the glazed surface, which is a hygiene concern.

How to Test for Cracking in the Object

If you want to make sure that the glazed object is not cracked, you need to stress test it.

There are different methods for this, but most involve exposing samples of glazed objects to different temperatures

One approach is to place a glazed test tile in boiling water and leave it for 10 seconds. Gently remove the test tile with tongs and place it in a pot of ice water. Let the tile rest here for 10 seconds. Repeat this process several times to force the glaze.

Then remove the test tile from the water and dry it. You can visually inspect the glaze for cracks.