Which parts of a kiln can actually break?

We are often asked whether it is normal for fired glazes to crack and what can be done about it.

The Problem

It is important to distinguish between the type of ceramic. If we consider densely fired ceramics such as stoneware or porcelain, this is usually only a cosmetic problem, as a dense body prevents, for example, a drinking liquid from running through the vessel. The situation is different with porous earthenware. If cracks appear in the fired glaze sooner or later, the drinking liquid will always run through the body, and the residue will be visible on the saucer or table!

Adjusting Thermal Expansion of Clay and Glaze

The fact that a glaze generally cracks after being removed from the kiln is due to the different coefficients of thermal expansion (TEC) of the clay and glaze. During the firing process, the viscous glaze is firmly bonded to the body at the final temperature. Upon cooling, the glaze becomes hard and brittle. If the glaze has a higher TEC than the body, this will lead to cracking. Sometimes the opposite is also the case, where the glaze has a lower TEC than the body. This can lead to flaking of the glaze. However, such cases are less common in practice, as a glaze can withstand compressive stress up to ten times higher than tensile stress!

Cracking can be counteracted by reducing the thermal expansion of the glaze. To reduce thermal expansion, our low TEC frit 1038 can be used for glossy glazes or 1039 for matte glazes. Depending on the difference in thermal expansion between the body and glaze (coarse crack network = small difference, fine crack network = large difference), additions of between 5% and 50% are required. Please note that increasing the addition changes the chemical composition of the base glaze. Therefore, you should always test the firing result beforehand! As an alternative to reducing the thermal expansion of the glaze, increasing the thermal expansion of the body is sometimes used. This can be achieved, for example, by kneading quartz powder into the clay. This process usually involves additional effort and is therefore less frequently used (if you don't make your own clay).

It's all about the clay

Many customers use a universal clay for low-temperature firing, which has a wide firing range of 1000-1250°C. However, in low-temperature firing, the glaze and the body are not well "interlocked" with each other. It is better to use a special, calcareous earthenware body, such as Item 143 (grogged) or 173 (ungrogged). This promotes the formation of interlayers (crystalline interlocking of the body and glaze).

With glaze cracks in porous earthenware, the phenomenon sometimes occurs that the ceramic emerges from the kiln without cracks, yet after days, weeks, or months, a coarse-meshed craquelure network forms. In this case, the thermal expansion of the body and glaze generally matches. The cause of this is what is known as moisture swelling of the body. Through unglazed areas (e.g., floors), the earthenware absorbs moisture during washing or from the air. This causes the body to expand slightly and crack over time. Therefore, it is recommended to glaze stoneware ware over its entire surface (using triangular glazes with a small contact area, e.g., 2917). The calcareous stoneware bodies described above also exhibit lower moisture expansion than a universal body with a wide firing range!

There is no such thing as perfect stoneware ware

Nevertheless: If you look at professionally produced stoneware ware in a museum, you will always see that glaze cracks appear after 10 years at the latest – no matter how good the manufacturing technique! This is a peculiarity that unfortunately cannot be completely eliminated. The only alternative is the production of dense stoneware ware. This is traditionally fired between 1200 and 1280°C. As an alternative, we also supply early-sintering stoneware bodies, which usually allow you to continue using your existing stoneware glazes.

Sometimes glaze cracks can also look quite decorative (e.g., with our craquelé glazes). If you stain them with ink, strong black tea, or manganese dioxide after firing, the crack network will become deliberately visible. However, from a functional and perhaps also hygienic perspective, this is not recommended for everyday ceramics.

And if all else fails, you can subsequently seal porous ceramics with cracked glaze using our Sealant 1128.