Glaze crawling, pinholes, and running are three of the most common glaze faults, usually caused by dust or grease on the bisque surface, trapped gas escaping during firing, or glaze applied too thickly or fired too hot for its recipe. Most cases are preventable once you know what to look for before the next firing.
Opening the kiln to a glaze fault after waiting days for a firing is a genuinely deflating moment every potter has been through at some point. Here at Le Studio Art Space, helping a member work through exactly what went wrong after a disappointing firing is a regular part of studio life, so here’s an honest, practical breakdown of glaze crawling pinholes and what usually causes each one.
Why Glaze Crawls
Crawling happens when glaze pulls away from the clay surface during firing, leaving bare, unglazed patches, sometimes with the glaze bunched up in thick, cracked-looking ridges around the exposed area. It’s one of the more visually obvious glaze faults, and it’s almost always related to how well the glaze was actually bonded to the bisque before firing.
Dust and grease are the most common culprits. Even a light layer of dust, or oils from handling a bisque piece with bare hands before glazing, can stop glaze from adhering properly to the surface underneath. Under the heat of firing, that weak bond gives way, and the glaze contracts and pulls back rather than staying put and melting evenly.
Uneven or overly thick application is another frequent cause. A glaze layer applied too thickly can develop small cracks as it dries before firing even begins, and those tiny cracks are often exactly where crawling starts once the kiln reaches temperature, since the glaze has already lost its even, continuous bond to the clay.
Glazing over cracks, dust, or existing surface flaws in the bisque itself can also trigger crawling, even with a careful, even application on top. If the bisque surface has a hairline crack or a rough patch, the glaze layer sitting over it inherits that same weak point, and it’s often exactly where crawling shows up once the piece is fired.
Why Glaze Gets Pinholes
Pinholes are small, pin-sized holes left in the fired glaze surface, usually caused by trapped air or gas trying to escape the clay body or the glaze layer during firing. If that gas doesn’t fully escape and the glaze surface seals over before it does, it leaves behind these tiny craters once the piece cools.
Air trapped in the clay body itself, from incomplete wedging or small air pockets left during forming, is one common source. Organic material or moisture still present in the bisque at glazing time can also off-gas during the glaze firing, pushing up through the glaze layer as it melts and leaving pinholes behind.
Firing speed plays a role too. A glaze firing that climbs to peak temperature too quickly can seal the glaze surface before trapped gas has fully worked its way out, which is part of why a slower approach to peak temperature, particularly through the range where glaze starts to melt, often reduces pinholing on pieces that struggle with it.
Thicker application can worsen pinholing as well, since a heavier glaze layer takes longer to fully mature and seal in the kiln, giving trapped gas even less time to escape cleanly before the surface sets. That’s another reason a moderate, even coat tends to perform more reliably than a thick one across most of these fault categories.
Why Glaze Runs Or Drips
Running happens when glaze becomes too fluid in the kiln and drips or pools down the piece, sometimes fusing it to the kiln shelf if it reaches the base. It’s usually a combination of two factors, an overly thick application and a glaze fired hotter than its formulated cone range actually calls for.
Every glaze recipe is balanced for a specific firing temperature, and firing even slightly hotter than that range can push a glaze from its intended melt into genuine excess fluidity. Combined with a thick application, that extra fluidity has nowhere to go but down the piece, which is why runs so often show up as drips pooling toward the base or foot.
Some glazes are simply more prone to running by design, particularly reactive or heavily fluxed glazes formulated for dramatic breaking and pooling effects. Those recipes genuinely call for a bit more caution with application thickness and kiln shelf spacing than a more stable, predictable glaze needs.
Placement inside the kiln matters too, since temperature can vary slightly from shelf to shelf and even across a single shelf depending on the kiln and its element wear. A glaze that behaves perfectly on one shelf can run slightly more on a hotter spot elsewhere in the same load, which is worth keeping in mind if a normally stable glaze suddenly runs unexpectedly.
How To Prevent These Issues Before Your Next Firing
Cleaning bisqueware thoroughly before glazing genuinely prevents a large share of crawling issues on its own. A quick wipe with a clean, slightly damp sponge to remove dust, followed by handling the piece by its edges or base rather than the glazing surface, keeps oils and residue from interfering with adhesion.
Getting application thickness right takes a bit of practice, but most glazes have a recommended number of coats or a target thickness stated on the label, and sticking close to that guidance genuinely reduces both crawling and running. When in doubt, a slightly thinner, even coat is almost always safer than an overly thick one.
Kiln shelf spacing matters more than a lot of beginners realise too. Leaving enough clearance around a piece, especially one glazed with a known runny or reactive glaze, means a small amount of unexpected flow doesn’t end up fusing it to a neighbouring piece or the shelf itself.
What Competing Guides Leave Out
Most glaze fault guides focus purely on the technical causes without mentioning how much studio conditions themselves affect glazing. In our experience at Le Studio Art Space’s Ceramic Studio, humidity genuinely changes how a glaze applies and dries, a glaze that goes on smoothly on a dry day can behave slightly differently in more humid conditions, since drying speed affects how evenly a coat settles before firing.
What to actually do with a piece that already has one of these flaws is another gap most guides skip. A glaze fired piece with a small run or a scattering of pinholes isn’t automatically ruined, in a lot of cases it can be reglazed and refired over the flawed area, particularly for crawling and pinholes, though a run that’s fused to the shelf is a more difficult fix and sometimes means starting over.
Studio etiquette around shared kiln space rarely gets a mention either, but it’s genuinely practical advice. If you know a glaze runs or reacts strongly, leaving generous clearance protects not just your own piece but everyone else’s sharing that same kiln shelf.
When A Flawed Glaze Result Is Actually Still Usable
Not every crawl, pinhole, or minor run needs to be treated as a failure. A small amount of pinholing on a decorative piece, or a light crawl along one edge, can genuinely read as character rather than a flaw, especially on handmade work where a bit of imperfection is already part of the appeal.
For functional ware, the standard is understandably a bit stricter, since exposed bare clay from crawling can affect food safety and cleanability. In those cases, reglazing the affected area rather than accepting the flaw is usually the more sensible path forward.
Many potters find that learning to tell the difference, when a flaw is genuinely a problem versus when it’s just part of a piece’s character, is one of the more useful skills that develops naturally with more time at the wheel and more firings behind you.
Got a tricky glaze result you’re not sure what to do with? Bring it in for honest, practical troubleshooting advice at Le Studio Art Space’s ceramic studio in Mordialloc.
Frequently Asked Questions
Can I fix a piece that already came out of the kiln with crawled glaze?
Often yes, especially for crawling and pinholes. Reglazing the affected area and refiring is usually possible, though a piece with a heavy run fused to the shelf is a more difficult fix and sometimes means starting over.
Is it my fault if glaze pinholes even though I applied it carefully?
Not necessarily. Pinholing can come from trapped gas in the clay body itself or firing speed, not just application technique. In our experience, a slower climb to peak temperature often helps even when application was done well.
How thick is too thick when applying glaze?
Most glazes have a recommended coat count or thickness on the label, and going noticeably beyond that raises the risk of both crawling and running. When unsure, a slightly thinner, even coat is generally the safer choice.
Does humidity really affect how glaze turns out?
Yes, it genuinely can. Humidity affects how a glaze coat dries before firing, which in turn affects how evenly it bonds to the bisque. Most studios notice at least some seasonal variation in glazing results because of this.
Should I throw away a piece with a small glaze flaw?
Not necessarily. Minor pinholing or a light crawl on decorative work can read as character rather than a defect. Functional ware with exposed bare clay is worth reglazing though, mainly for food safety and cleanability.
This article is provided for general educational purposes by Le Studio Art Space and reflects publicly available product information and common studio practice.