Glazing · Glaze problems, causes and fixes

Crawling, pinholes, crazing, shivering, blisters and dunting.

What each one looks like, what caused it, and which are worth refiring. Most of them are application problems, which means the coat was wrong, and many of those can be refired. Two are fit problems between the glaze and the clay, and no second firing changes fit. One, dunting, is the pot itself cracking on the way down from temperature. The diagnostic below names all of them. Running onto the shelf has its own lesson, Making a Glaze Catcher.

Four steps with a tick list at each: naming the defect, deciding which of the three kinds it is, fixing the cause for the next pot, and the refire. Seven defects in the diagnostic with causes and verdicts, a picture of six of them, and seven videos.

This is the short version: name it, decide which of the three kinds it is, the bisque and the coat, the refire, each with a tick list. The reasoning, the clay-body notes and the questions are in the comprehensive version.

x7Defects that cover nearly every pot
x3Kinds, and the kind decides the refire
x7Videos, from makers with no link to us
x4Steps, each with a tick list you can work off
Jump to a section
Why this page has words and video The whole idea in three lines The method in six moves What you need on the table 1. Name it 2. Which of the three kinds 3. The cause, next time 4. The refire Seven videos on glaze faults The one video to watch The seven defects How it works at The Clay Hole Questions people actually ask Related lessons
An illustration of a bearded potter in round glasses and a clay stained apron holding up a grey mug with glaze crawled away from one bare patch, in a workshop with glaze tiles on the wall and a kiln behind him

Every defect in full, the reasoning, the clay-body notes and seven videos.

Read it, then watch it

Why this is written down and filmed, both.

Glaze problems are a diagnosis. You are holding a pot with something wrong on it, and the job is to name it, find the cause, and decide whether a refire is worth a second firing fee. That is a table with three columns, and a table is what a page holds. What a page cannot do is talk you through each defect one at a time, and that is the videos, one of which is thirty-seven minutes of exactly that.

Use both, in this order

Hold the pot. Find its picture in the diagnostic below. Read the cause and the refire verdict. Watch the video for that defect if the picture is not enough.

The whole idea

Three kinds of problem. Only one of them refires.

Every defect on this page is one of three things, and the three are worth learning in this order.

The coat, the pairing, or the pot

Application is the coat. How the glaze went on. Crawling, pinholes, blisters and dry glaze. Many of these can be refired, once an instructor has looked.

Fit is the pairing. This glaze against this clay, and how much each one shrinks while it cools. Crazing and shivering. These do not refire.

Dunting is the pot. The clay cracked as the kiln cooled, usually because the pot was built with uneven walls, and cooling too fast made it worse. This does not refire either, and it is the only one that leaves you holding two pieces.

Name the defect, read its kind, let the kind decide.

The five-minute version

The whole method in six moves.

If you already glaze and you just want the shape of it, this is the shape of it. Everything below is these six moves, slowed down, with the places they fail marked.

1Name itBeads and bare clay: crawling. Pits: pinholes. Fine cracks in the glaze: crazing. Flakes off the edges: shivering. Bubbles: blisters. Dry and pale: thin. A crack through the clay: dunting.
2Ask which kind it isThe coat, the pairing, or the pot. Only the coat refires. A cracked pot stops the process here.
3Wipe the bisque, next timeA damp sponge before glazing stops a lot of the crawling and pinholes on this page.
4Three coats, dried betweenThick crawls and pinholes. Thin fires dry. Use about three coats, and let each lose its shine before the next.
5Refire what refiresCrawling, dry glaze, sometimes pinholes and blisters. Show an instructor, add glaze where it is missing, fire again, same fee.
6List the bad pairingA glaze that crazes or shivers on a body goes on your do-not-use list for that body. Tell the studio team the pairing.

Before you decide

What you need on the table.

Six things, and one of them is the fee. The damp sponge is the one that prevents the most.

The pot, and a good lightMost defects are named in ten seconds under a lamp. A magnifier for crazing on a dark glaze, and for reading the edges of a crack.
A green scouring pad and a diamond padFor roughing a crawled patch before reglazing, and grinding a blister or a shivered edge flat, wet.
The same glazeA refire adds the glaze that is missing. A different glaze over a defect is a second experiment on a pot that already failed one.
A damp spongeThe habit that prevents the most problems: wrung nearly dry, and used on the bisque before the next pot is glazed.
The test tileA tile on your own clay asks the question first, though a tile is a small pot and not the pot.
The refire feeFive cents a cubic inch, $4.80 minimum. The verdict on each defect below is whether the pot is worth it.

The pot under a lamp, a green scouring pad and a diamond pad, the same glaze, a damp sponge, a test tile, and the refire fee.

1

Step one

Name the defect. Seven of them cover nearly every pot.

Hold it under a lamp. Beads with bare clay between: crawling. Pits: pinholes. A fine network of cracks in the glaze: crazing. Slivers off the rim and edges: shivering. Bubbles or craters: blisters. Dry, rough or paler than the jar: thin. A crack running through the wall of the pot, or a pot in two pieces: dunting.

The first question is whether the damage is in the glaze or in the clay. Glaze damage sits on the surface and the pot underneath is whole. A dunt goes through the wall, and you can see it from both sides.

Two of the glaze faults look alike and matter most to tell apart. Crazing is cracks in a glaze that is still stuck down, sometimes heard as a ping days later. Shivering is glaze that has come off, leaving a bare edge with a sharp lip of glass beside it. One is a curiosity. The other is a pot that should not be on a table, and the difference is whether a fingernail catches on an edge.

A crack tells you when it happened

Run a fingernail along it and look at the edges under the lamp. If the crack cuts straight through the glaze and the edges are sharp, it opened on the way down, after the glaze had already set hard. That is a dunt, and it is a firing-stage crack.

If the glaze has rolled over into the crack and the inside of it is smooth and glossy, the crack was already there when the kiln got hot, and the melting glaze poured into it. That one came from drying or from a joint that never welded, weeks before the pot was glazed.

Neither one refires. The difference is which habit to change next.

The Washington Street Studios video below is thirty-seven minutes of a teacher at a whiteboard taking the glaze faults one at a time, and if the picture in your head does not match the pot in your hand, its chapters are where to look.

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2

Step two

Ask which kind it is. There are three, and the kind decides the verdict.

This is the question that decides everything. Three questions, asked in this order, sort every pot on the drop-off table.

Ask them in this order

1. Is the pot cracked through, or in pieces? Then it is a crack in the clay, and a dunt if the edges are sharp with no glaze run into them. Nothing else on this page applies to that pot.

2. Is the glaze cracked in a fine net, or flaked off the edges? Then it is fit. The net is crazing. The flakes are shivering.

3. Anything else. Application, which means the coat. Crawling, pinholes, blisters, dry glaze. Most of them can go back in, blisters only sometimes. Show an instructor before you add glaze.

Two of those three verdicts are disappointing, and it is better to know which one you are holding before you pay for a second firing.

Refires

Application: the coat

Crawling, pinholes, thin and dry glaze, and blisters once. The glaze is fine and the clay is fine. The coat was wrong: a dusty bisque, a layer too thick or too thin, coats not dried between, wax where it should not be. Show an instructor, then add glaze where it is missing, wipe the excess and fire again at the normal fee.

Does not refire

Fit: the pairing

Crazing and shivering. Everything shrinks as it cools, and the glaze and the clay under it shrink by different amounts. When the glaze shrinks more it is stretched and it cracks. When the clay shrinks more the glaze is squeezed and it buckles off at the edges. The fix is a different pairing, not a second firing.

Does not refire

Dunting: the pot

A crack through the clay itself, opened while the kiln cooled. Clay does not shrink smoothly on the way down. Around 1,063 degrees Fahrenheit the silica in it contracts all at once, and a pot with a thick base and thin walls reaches that moment with one part still moving and another already still. It splits along the stress.

Eight clay bodies and 650+ glazes is a lot of pairings, and most of them are fine at cone 5½. The ones that are not get found on a test tile if you are careful and on a pot if you are not, and either way the pairing goes on your own do-not-use list, and you can tell the studio team the pairing. Dunting is not on that list, because it is not about the glaze at all. It is about how the pot was built.

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3

Step three

Fix the cause for the next pot, which is usually the bisque or the coat.

A damp sponge on the bisque and three coats dried between. That sentence is the habit that prevents the most glaze defects. Damp means wrung nearly dry, or a quick flash rinse at the wash pit, never a soak. Dust and finger oil on bisque make glaze crawl and pinhole; a coat too thick crawls and blisters; a coat too thin fires dry; coats laid wet on wet slide and crack and crawl.

Then the things people blame the kiln for. Our kilns fire the same cone 5½ schedule every load, the half being a twelve-minute hold at the top, and the hold is there to let a glaze heal over its bubbles. Most pinholes come from what was on the pot or in the clay, and the hold at the top gives the ones that start a chance to heal. A glaze at the very edge of its range can blister; the jar says the range, and the test tile says the rest.

Dunting is prevented weeks earlier, at the wheel and at the table. Even walls, and a base no thicker than the walls beside it. A floor that got compressed before the pot came off the wheel. Joints scored and welded rather than pressed together. A soft inside curve where the wall meets the floor instead of a sharp corner. A pot that is twice as thick in one place as another is a pot asking to crack, and no amount of careful glazing changes that.

And the clay. A groggy, iron-bearing body outgasses more than a smooth white one and pinholes and blisters more under thick glaze. Our glaze brushing lesson is the coat method, and the test tiles lesson is how to ask the kiln a question on your own clay before a pot pays for it.

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Step four

Refire what refires, and let the rest go.

A refire is adding what is missing and firing again. Crawled: rough the bare patches wet with a green pad, brush glaze into them and over the island edges. Pinholed: ask an instructor first, then rub glaze into the holes and wipe the surface. Dry: one more coat. Blistered: grind it flat, wet, in safety glasses and an N95 or better, and a thin coat over the scar, once. Same glaze every time. Keep the new glaze thin, and stop well above the foot, because two layers run more than one.

A refired pot goes to the drop-off table like any other, on a cookie with a naked bottom, at five cents a cubic inch with the $4.80 minimum. Glaze added to a fired surface does not soak in the way it does on bisque, so let it dry completely and handle it gently; it flakes if knocked. Tell the studio team it is a refire when you drop it off, with a note on the ticket.

A dunted pot is the shortest verdict on the page. The crack goes through the clay, heat does not close a crack in clay, and a second firing usually opens it further. Keep the pieces if the pot was worth keeping, glue them for a shelf, and read the crack for what it says about the next one.

Crazed and shivered pots do not refire either. A crazed pot should not hold food, and most potters keep it for dry things. A shivered pot has edges like glass, gets ground with a diamond pad, wet, and does not go back on a table. Pottery by Kent’s video below is a batch of old failures reglazed, with the results shown, and it is one potter’s try at what a refire can and cannot do.

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Watch it done

Now watch seven videos on glaze faults.

Watch this one with the pot in your hand.

None of these makers work here. Between them they cover the glaze faults chapter by chapter, one potter’s pinhole experience, fit at a chalkboard, one maker’s view on crazing and food safety, one potter’s crawling fix, cracks sorted by cause, and one potter’s try at reglazing. Only the still frame loads until you press play.

Thirty-seven minutes of a teacher at a whiteboard taking the glaze faults one at a time. The other six videos are in the comprehensive version.

Start here: the glaze faults, chapter by chapter Understanding Pottery Chapter 19 Glaze Defects Washington Street Studios
Thirty-seven minutes of a teacher at a whiteboard taking the glaze faults one at a time: blistering, runs, pinholes, crawling, crazing and shivering, thin application, color and spitting. It has no dunting chapter. Long, so use the chapters. Watch on YouTube
Pinholes, one potter’s way Pinhole Problems- How I Try to Fix and Prevent Pinholes Stories N' Stoneware (Annie Gietz)
Fourteen minutes of one potter’s own experience with pinholes, in chapters on clay and bisque, fit, firing schedule and refiring. Watch on YouTube
Crazing and shivering explained Pottery Chalkboard: Thermal Expansion, Glaze Fit, and Common Glaze Issues Seattle Pottery Supply
Seven minutes at a chalkboard on why a glaze cracks or flakes: the clay and the glaze shrink by different amounts, and one of them loses. A short, plain start on fit. Watch on YouTube
Is a crazed pot safe to eat from Crazing (food safety in glaze) Earth Nation Ceramics
Twenty-two minutes on how crazing bears on food safety, with chapters on thermal expansion, how to fix it and clay bodies. The description promises reassurance and busting misinformation, so read it beside our verdict here: a crazed pot is for dry things. Watch on YouTube
One potter’s crawling fix No more trash! Crawling glazes solved The Goods Pottery
Five minutes from a potter who stopped losing pots to crawling by adding CMC, a gum, to the glaze so it grips the bisque and can go on thicker without coming off, says the description. We do not add anything to the jars ourselves, so take the idea, and ask an instructor about a jar that seems off. Watch on YouTube
What a refire can and cannot do Reglazing pots - can we fix some old mistakes? Pottery by Kent
Four minutes of one potter’s first try at reglazing old failures, with the results shown. The description says the glaze came off the fired pots easily at first. Watch on YouTube
Telling one crack from another Stop cracks in your pottery! // Different kinds of ceramic cracks and how to prevent them. Pottery to the People
Eighteen minutes sorting cracks by where they came from, which is the skill this page needs for dunting. Its chapters run from core causes of stress through S cracks, an argument about compressing the bottom, thermal shock, glaze fit and crazing. Watch on YouTube

When it goes wrong

The seven defects: what it is, what caused it, and whether it refires.

The four defects you will meet first.

Start with the picture. Find the tile that looks like your pot, then find its heading below for the cause, the fix for next time, and the refire verdict.

Six fired stoneware test tiles laid out on a wood bench, each labeled: crawling with the glaze beaded into islands, pinholes, crazing, shivering with glaze flaked off the edges, blisters, and dunting with a crack straight through the tile

Six of the seven, fired on test tiles. Top row, left to right: crawling, pinholes, crazing. Bottom row, left to right: shivering, blisters, dunting. The seventh is a glaze that came out dry and pale, and it has no tile here because it looks like a duller version of the right color, which is exactly what makes it the one people miss.

How it works here

At The Clay Hole, the kiln is the same every time, so the pot is the variable.

This page is written for potters everywhere. A few things are specific to our building.

Four things to know here

Our glaze firing is cone 5½ on the same schedule every load, with a hold at the top that gives pinholes a chance to heal. A pot that comes out wrong nearly always went in wrong. A refire is a normal glaze firing at the normal fee. Tell the studio team about any glaze that crazes or shivers on one of our bodies. And a crack through the clay is not something the studio team can fire out, so even walls and a rounded inside corner are the prevention on your side, and we never open a warm kiln.

The firing

Cone 5½, the half being a twelve-minute hold at the top, the same every load, with our staff loading. The hold gives a glaze time to heal over bubbles and settle. Most problems still trace to what went on the pot.

Cracks

A crack through the clay is not a glaze fault, and nobody here can fire it out. Every pot on a shelf gets the same schedule, so when one dunts and the pot beside it does not, look at the pots first. Even walls and a rounded inside corner are the prevention on your side, and both happen weeks before the glaze goes on. We never open a warm kiln.

After it leaves

A finished pot can still crack, from the same physics in a smaller way. Boiling water into a cold mug, a cold dish into a hot oven, a hot dish onto a wet counter. Stoneware fired to cone 5½ takes more of that than earthenware does, and it does not take all of it. Warm a piece up gradually and most of them last decades.

Refires

A refire is a glaze firing: five cents a cubic inch, $4.80 minimum, on a cookie with a naked bottom, paid before it goes on the drop-off table. A note on the ticket if the studio team should know something. Bisque is cone 05 and free, and a refire never needs one.

Bad pairings

Eight clay bodies, 650+ brush-on glazes and seven dipping buckets. Nearly every pairing is fine. Tell the studio team about one that is not, glaze and clay both, because a crazing or shivering pairing is worth every member knowing.

Tools

A green scouring pad and a diamond pad are worth owning; the diamond pad is the best small purchase in pottery. Grinding is wet, over a bucket, away from shared tables, in safety glasses and an N95 or better, and the grindings go in the bin, per the sanding and grinding lesson.

Questions people actually ask

The ones people ask most.

Which problems can be fixed by refiring?

Crawling, pinholes, dry and thin glaze, and sometimes blisters: anything that is an application problem. Show an instructor, add glaze where it is missing, fire again, and many come back. Crazing and shivering are fit problems between the glaze and the clay, and no amount of refiring changes fit. The cracks close in the kiln and reopen on cooling. Dunting does not refire either, because the crack is in the clay and heat does not close it.

Can I refire at The Clay Hole?

Yes. A refire is a glaze firing, paid at the same five cents a cubic inch with the $4.80 minimum, on a cookie with a naked bottom like any other pot. Show an instructor, add the glaze, let it dry, and put it on the drop-off table with a note.

What is dunting?

Dunting is a crack that opens in the clay itself while a pot cools in the kiln. Clay does not contract smoothly on the way down. Around 1,063 degrees Fahrenheit the silica in it shrinks all at once, and a pot whose walls and base are wildly different thicknesses reaches that moment with one part still moving and another already still. The pot splits along the stress. It is a making problem rather than a glazing problem, and it does not refire.

My pot came out of the kiln cracked. Was it the firing?

Usually not, and the crack itself will tell you. Look at the edges under a lamp. A crack that cuts clean through the glaze and leaves sharp edges opened on the way down, after the glaze had set, and that is a dunt, usually from uneven wall thickness and made worse by cooling too fast, which is why we never open a warm kiln. A crack with glaze melted smoothly into it was already there before the kiln got hot, from drying or from a joint that never welded. Every pot on that shelf got the same schedule, so the difference was in the pots.

Can a dunted pot be saved?

Not by firing it. Heat does not close a crack in clay, and a second firing usually opens it further. Glue is honest work if the pot meant something, and a mended pot is a shelf pot rather than a mug. A dunted mug should not be drunk from either way, because the crack holds water inside the wall where nothing can clean it.

Is a crazed pot food safe?

No, it should not hold food. Our clay bodies fire vitrified at cone 5½, so a crazed pot does not weep, but the cracks hold coffee and bacteria and are hard to clean. Keep crazed pots for dry things or flowers, and for any glaze you do want on a food dish, check the Food Safe Lookup. The Earth Nation video on this page gives one maker’s view.

Why do some glazes craze on one clay and not another?

Because fit is a relationship. Every clay body shrinks by its own amount as it cools, and every glaze does too. When the glaze shrinks more it cracks; when the clay shrinks more the glaze flakes. Eight clay bodies and 650+ glazes is a lot of pairings, and a test tile on your own clay is where the bad ones are found before a pot pays for it.

What causes most problems?

Application. A dusty bisque, a coat too thick, coats not dried between, wax where it should not be, a thin patch missed. The kiln gets blamed, and our kilns fire the same cone 5½ schedule every time; the pots that come out wrong are nearly always the pots that went in wrong.

Does the glaze firing hold at the top?

Yes. Our glaze firing is cone 5½, the half being a twelve-minute hold at the top, and the hold is there to let the glaze heal over bubbles and settle.

What do I do with a pot that cannot be saved?

Keep it as a test tile with a lesson on it, or let it go. A shivered pot with sharp edges should not be on a table. Everything else is a plant pot. The fused and sharp pottery lesson covers the grinding.

Related lessons

Six lessons that sit next to this one.

Three to read next.

A glaze defect is a coat, a pairing or a firing. These are the pages for each.

GlazingGlaze Brushing TechniquesThe three coats, dried between, that prevent most of this page. Firing and fixingFixing Fused and Sharp PotteryHand grinding, wet, for sharp spots and drips. Power grinding stays at home. GlazingTest TilesHow to ask the kiln a question on your own clay before a pot pays for it. GlazingMaking a Glaze CatcherRunning is the defect this page leaves to its own lesson. Firing and fixingWhat a Firing Actually DoesThe hold at the top, and why it gives pinholes a chance to heal. Firing and fixingScouring, Sanding and GrindingWet, over a bucket, with the right tool for the stage.

That is the whole method

Name it. Pick the kind. Wipe the bisque, three coats. Refire what refires.

Your first crawled mug will get roughed, reglazed and refired into a mug you like more than the one you meant to make. Your first crazed one will teach you a pairing you never repeat. Your first dunted one will teach you to feel the base against the wall before a pot ever leaves the wheel. All three are the University working, and the kiln was rarely the problem.

If you want the reasoning, the clay-body notes, the food-safety argument and the answers to the questions everybody asks, switch to the comprehensive version at the top. Otherwise, go hold the pot under a lamp.

Back to Clay Hole University The three coats that prevent most of this

Written for potters everywhere. Where our studio does things differently, the page says so.