Pinholes in glaze are tiny craters that show up on an otherwise perfect surface, and they are especially annoying on functional ware where they trap dirt and moisture. The good news is that pinholes in glaze have a limited set of causes, and most can be fixed in the glazing room or with a small change to the firing.
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Quick Answer
Pinholes in glaze form when gas escapes from the clay or glaze during firing and the melted glaze does not flow back over the hole. To reduce them, bisque fire a little hotter and with good airflow, wipe dust off bisque ware, apply thinner glaze, and add a short hold at the top of the glaze firing.
What Causes Pinholes in Glaze?
During firing, gases are released from the clay body, from the glaze materials and from any contamination on the surface. If gas bubbles break through the glaze while it is still stiff, they leave small craters. A glaze that stays fluid long enough will smooth over, but one that cools too fast or is too thick will freeze the holes in place.
Gas from the clay body
Organic matter, carbon and sulfur compounds in clay burn out and release gas. A bisque firing that is too low or too short may leave some of this behind to emerge later during the glaze firing. Porous or low-fired bodies also hold more trapped air.
Gas from the glaze
Some glaze ingredients, such as carbonates, release gas as they decompose. A thick glaze layer has more material releasing gas, and a stiff or matte glaze may not flow back together after the bubbles pop.
Surface contamination
Dust, oils from fingers, bisque residue and old slip can all create tiny gas pockets. They also cause the glaze to crawl or go on unevenly, a problem we discuss in our guide to common pottery glazing mistakes.
Pinholes in Glaze: Causes and Fixes at a Glance
Use the table to match what you see with a first fix. Test one change at a time on a few tiles or small pots before committing a full kiln load.
| Cause | What you notice | Try this first |
|---|---|---|
| Dusty bisque | Scattered holes, often on rims and edges | Wipe with a damp sponge before glazing |
| Underfired bisque | Pinholes on many different glazes | Raise the bisque cone slightly, add airflow |
| Thick glaze | Holes and bubbles in heavy areas | Apply a thinner coat |
| Stiff glaze melt | Holes that do not heal | Add a short hold at peak temperature |
| Fast firing or cooling | Holes frozen into the surface | Slow the final climb and cooling |
| Trapped air in clay | Holes on thrown or reclaimed ware | Wedge or de-air the clay better |
Fixing Pinholes in Glaze Before Glazing
Preparation fixes more pinholes in glaze than any program tweak. Start with the pot itself before the glaze goes on.
Clean the bisque
Wipe bisque ware with a barely damp sponge to remove dust, then let it dry before glazing. Avoid touching the surface with oily hands. A clean, slightly damp surface also helps glaze go on evenly, which reduces thick patches that can lead to pinholes in glaze.
Choose a better bisque temperature
Many potters bisque at a cone that leaves the clay porous enough for glaze, but a firing that is too low can leave carbon and other material behind. If pinholes in glaze persist, test a slightly higher bisque with a slow, well-ventilated program. Read more in our article on firing bisque and glaze in the same kiln.
Wedge and compress the clay
Air pockets in clay expand when heated and can force their way through the surface. Wedge thoroughly, or use a de-airing pug mill, and compress the bases of thrown pots. Careful forming makes your later firings far more reliable.
Fixing Pinholes in Glaze with Better Application
How you apply glaze matters a great deal, because a thick layer carries more gas and takes longer to settle. Learn how your glaze should look when it is dry and how to control the layer.
- Stir the glaze well and check its consistency before dipping.
- Keep dipping time consistent, and shake off excess glaze.
- Avoid building up thick coats on edges, corners and textured areas.
- Let the glaze dry fully before loading, so it is not damp in the kiln.
- Fill small pinholes in dry glaze with a fingertip or brush before firing.
For more on application, see our guide on how to apply pottery glaze and our article about pottery glaze layering.
Adjusting the Firing to Heal Pinholes in Glaze
Even a good glaze can show pinholes in glaze if it does not stay fluid long enough. The firing schedule decides how much time the glaze has to flow and close small holes before it freezes.
Add a short hold
A hold at peak temperature, often ten to twenty minutes, can give the glaze time to heal. Too long a hold risks running, so test the change on tiles before using it on finished work.
Slow the last stage
Slowing the final part of the climb lets gas escape before the glaze fully seals. Some potters also slow the first part of the cooling, which keeps the glaze molten a little longer. Our guide to the kiln firing schedule explains how these programs are built.
Confirm the cone
An underfired glaze is stiff and more likely to show pinholes. Verify the actual heat work with witness cones, and compare with the cone range printed on the glaze label. See our post on witness cones for pottery kilns for how to read cones accurately.
Kiln and Loading Factors
The kiln itself can contribute. A kiln that does not get enough air during the early stages of the glaze firing may not burn out all the carbon and organic material. Make sure vents are working and follow the manufacturer manual for ventilation. A licensed electrician should handle any electrical work, and the room should be well ventilated.
Loading matters too. Crowded shelves block airflow and trap gas, while open spacing lets heat and air circulate. Check out our guide to kiln shelf spacing for ideas, and look at our electric kilns if you are considering a kiln with better controls.
Telling Pinholes from Blisters and Crawling
Several glaze faults look alike at first glance, and the fix depends on getting the diagnosis right. Take a close look with a magnifying glass or the flashlight on your phone before changing anything.
Pinholes
True pinholes in glaze are small, sharp-edged holes that expose the clay or a thin spot in the glaze. They are usually round and about the size of a pinprick, scattered across the surface or clustered in one area.
Blisters
Larger bubbles that burst and leave rough, sharp-edged craters. Blisters usually point to overfiring, very thick glaze or a rapid gas release. They often need a lower peak temperature or a slower climb.
Crawling
Glaze that pulls away into beads and leaves bare patches. It is typically caused by dust, grease or a glaze layer that is too thick or has cracked during drying. Dampen the bisque lightly and apply thinner coats.
Special Cases: Matte Glazes, Porcelain and Textured Ware
Some surfaces are more vulnerable to pinholes in glaze than others. Knowing where trouble tends to show helps you decide where to focus your testing.
Matte and stiff glazes
Matte glazes often contain materials that make the melt stiff, and stiff melts struggle to heal over escaping gas. If your matte glaze is pinholing, try a hold at peak temperature, a slightly thinner application, or a different recipe for that surface.
Porcelain
Porcelain is dense and clean, so it usually fires with less gas from the body, but fine surfaces show every defect. Wipe porcelain bisque carefully, because dust and fingerprints stand out on pale glaze.
Textured and carved ware
Deep texture traps dust and collects extra glaze in recesses, which makes gas harder to escape. Brush texture clean before glazing and avoid letting glaze pool in carved lines.
With a few clean habits and some careful tests, the surface problems that frustrate so many potters can be reduced to the occasional exception.
Testing to Solve Persistent Pinholes in Glaze
If the cause is not obvious, run a controlled test. Change just one variable at a time, and keep records so you can see which change actually worked.
A simple test plan
- Fire six tiles of the same clay, cleaned and glazed identically.
- Give two tiles a thinner coat of glaze.
- Give two tiles a normal coat and a longer hold.
- Give two tiles a normal coat and a slower cool.
- Compare the results and repeat with the best option.
Check different glazes on the same clay
If every glaze shows pinholes in glaze on one clay body, the problem is probably in the clay or the bisque. If only one glaze does it, adjust that glaze, its thickness or its firing. Reference sites such as Digitalfire have helpful troubleshooting information on materials.
Frequently Asked Questions About Pinholes in Glaze
Can I fix pinholes in glaze by refiring?
Often yes. Refiring to the same or a slightly lower cone with a hold can let the glaze flow over small holes. Results vary, and a refire can cause running or other problems, so test on spare pieces first.
Why do pinholes appear on only some of my pots?
Variation in glaze thickness, bisque cleanliness or position in the kiln can all matter. Pots that are glazed thicker or sit in a cooler zone are the likeliest to show holes.
Are pinholes in glaze food safe?
Tiny holes can trap food and bacteria and may expose the clay below. For functional ware, aim for a smooth, sealed surface and consult your supplier for glaze safety information.
Does the clay type matter?
Yes. Some clay bodies contain more organic material or are more porous, which makes pinholes in glaze more likely. Try a different body if the problem follows one clay across different glazes.
Final Thoughts
Pinholes in glaze are frustrating but fixable. Clean the bisque, apply thinner coats, fire the bisque properly, and give the glaze enough time to flow back together. Take notes, change one thing at a time, and your surfaces will smooth out.
If you need help choosing equipment for more controlled firings, explore our kiln furniture or ask through our contact page. For cone charts and reference, see Orton Ceramic.
