Troubleshooting Glaze Issues: Common Problems and Solutions

Ceramic glazing, while a rewarding art, comes with its share of challenges. From pinholes to crazing, each issue often stems from mismatches in thermal expansion between glazes and clay bodies. These problems not only affect the aesthetic quality but can also compromise the safety and functionality of ceramic ware. Understanding these issues and employing the right techniques and materials are crucial steps towards achieving successful firings. In this article, we delve into common glaze defects and their remedies, offering insights that can help ceramic artists and enthusiasts alike achieve optimal results in their pottery creations.

1. Crazing

Crazing occurs when the glaze cools faster than the clay body it is sitting on. This causes the glaze to shrink and expand and it can “shiver” off of the body forming a network of fine lines.

This problem can be fixed by adjusting both the glaze and the clay body. Adding low-expansion oxides like silica and talc or replacing high-expansion oxides in the glaze with lower ones (using INSIGHT to calculate thermal expansions can be helpful here) will decrease the rate of change and make the glaze and body fit better.

Crazed glazes are also a safety concern since bacteria can get trapped in the cracks and be released into food and drink. Ask your local food service inspector to learn more.

2. Cracking

Crazing occurs when a glaze re-absorbs water in the kiln and expands slightly resulting in cracks. It is often a sign that the glaze and clay body do not have compatible expansion coefficients. This can be remedied by a longer soaking and cooler firing cycle, lower peak temperature, the use of calcined kaolin (to minimize moisture expansion), or the addition of calcium carbonate to talc bodies.

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Pitting/pinholing is caused by the same mismatch in expansion coefficients as crazing but the problem is less obvious since the network of small cracks is harder to see. This can be minimized by using a more fluid glaze or a lower silica content (300 mesh works well). Increasing the oxide or manganese content may also help as these materials gas during oxidation.

3. Pitting

If the kiln is not properly venting it can create pitting damage as gasses escape from the ware and redeposit on the surface of the glaze. Incorrectly sized shale or grog in the clay body will cause open pores. This produces large volumes of gases that converge on the surface of the glaze.

Other causes include a rapid cooling schedule as the glaze and clay body approach maturity that can produce bubbles or craters that freeze before the glaze has time to flow out of them, the presence of some oxides (manganese especially) in the body or glaze that change state during firing and generate gases.

It is important to use good mixing techniques (slips) and glazing methods, kiln ventilation, proper kiln temperature control, and good housekeeping for the kiln and workplace. It is also useful to do frequent witness cone testing to see how the kiln is working.

4. Pinholing

Pinholes, or pits, are holes in a fired glaze surface that penetrate all the way down to the body. They are the result of gases that escape from the body and ceramic glaze during firing. These may come from the decomposition of organic materials in clay and glaze raw materials or from escaping crystal water.

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Some glaze ingredients are notorious for creating this problem, such as rutile and zinc. A glaze can be less prone to pinholing if it is thinned down with more liquid and a deflocculant (such as sodium silicate) added to the mix.

Pinholes can also be caused by rapid cooling as the kiln comes off the line and the ware cools in the last 122 degrees F of the cycle. A slower glaze firing cycle and a soaking of the ware before glazing will reduce this problem.

5. Spit-Out

Blisters, craters, and pinholes are surface defects related to gas formation during glaze or decorating firing. This can be caused by the kiln atmosphere, clay body, or glaze materials releasing gases during oxidation.

When these problems occur it is important to understand the root cause and fix it so your glaze-clay bond will be stronger next time. A good place to start is by examining the pouring and application (glaze) techniques. If the chemistry is off you can try changing the recipe or using different frits to match the melting temperature. Keeping the kiln clean and avoiding dust, oils, and soluble salts can also help.

Finding Supplies and Expertise

When troubleshooting glaze issues, accessing quality materials and expert advice is crucial. A local pottery store often provides a wealth of resources, from specialized glazes to kiln maintenance tips. Building relationships with knowledgeable staff can offer invaluable insights into resolving common ceramic challenges effectively.

Mastering ceramic glazing requires not only technical know-how but also a deep understanding of materials and processes. By addressing common glaze defects such as crazing, cracking, and pinholing with targeted adjustments in glaze formulation and firing techniques, ceramic artists can enhance both the beauty and durability of their creations. Continuous learning and experimentation, supported by reliable resources like pottery stores and community workshops, pave the way for achieving consistent and satisfying results in pottery endeavors.

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