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Published Feb 5, 2024

Ceramic glaze blisters are small craters or bubbles that form on the surface of a glazed ceramic piece during the firing process. Unless you are going for a craggy surface on a non-utilitarian form, blisters can ruin an otherwise successful pot.

There are several factors that can contribute to the formation of ceramic glaze blisters, and Jeff Zamek does a deep dive into them in the February 2024 issue of Ceramics Monthly. Today I thought I would share an excerpt from that article with a great Q&A designed to help diagnose the cause of glaze blisters. It's a great way to help build an understanding of this annoying glaze fault. –Jennifer Poellot Harnetty, editor

PS. If the Q&A doesn't solve your blistering problems, be sure to read the entire article, which goes into greater depth on the causes and cures for glaze blistering. Or you can browse the rest of the February 2024 issue of Ceramics Monthly.

Asking Questions Can Yield Answers

When confronted with any kind of defect, it is important to determine the point of origin and then apply the appropriate adjustment(s). It is essential to have a systemic approach to isolate the actual factor(s) causing blistering. Specifically, there are several questions the potter can ask to isolate glaze blistering:

Does the blister glaze heal when fired again? Generally, if the glaze can be re-fired successfully, it should have been fired longer (more heat work) during the first glaze firing.

Are different glaze formulas in the same kiln blistered? The problem probably originates in the firing procedures, glaze-mixing errors, or from a common raw glaze material.

Are the blisters only on one side of the pot? If so, direct flame impingement might cause an over-fired area and/or an over-reduced area in hydrocarbon-fueled kilns.

Are the blisters only on overlapping glaze surfaces? Incompatible glazes when overlapped can have a eutectic effect, which can result in over-fluxed areas and blisters.

Are the blisters only on horizontal surfaces? High surface tension glazes with high viscosity are stiff and do not move when molten. Gravity on the vertical molten glaze pulls down causing the formed blister to heal. Another possible cause occurs when flat pots are placed directly on the kiln shelf. If the glaze is not formulated or fired correctly the radiant heat from the shelf during cooling can cause it to remain in its maturity range longer causing a blister.

Are the blisters only on the edges or high areas of the pots? Fast cooling of the kiln and/or pottery loosely stacked can freeze the glaze in its maturation process.

Are blisters present only in one kiln and not in others? This could be an indication of an error in kiln firing.

Are blisters present in only one part of the kiln? Check for direct heat source impingement or kiln atmosphere irregularities.

Are blisters present on one clay body and not another? Check the level of organic material in the clay body causing the blisters.

the author Jeff Zamek started his career 48 years ago. He obtained BFA/MFA degrees in ceramics from Alfred University, College of Ceramics, New York. In 1980, he started Ceramics Consulting Services, a ceramics-consulting firm developing clay body and glaze formulas for ceramics supply companies throughout the US. His books, The Potter’s Studio Clay & Glaze Handbook, What Every Potter Should Know, Safety in the Ceramics Studio, and The Potters Health & Safety Questionnaire are available from Jeff Zamek/Ceramics Consulting Services. For technical information, visit www.jeffzamek.com.

Unfamiliar with any terms in this article? Browse our glossary of pottery terms!
Topics: Glaze Chemistry

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