Glazing · Glaze problems, causes and fixes
Crawling, pinholes, crazing, shivering and blisters.
What each one looks like, what caused it, and which are worth refiring. Most glaze problems are application problems and refire; two are fit problems between the glaze and the clay, and no second firing changes fit. The diagnostic below names them.
Four steps with a tick list at each: naming the defect, deciding application against fit, fixing the cause for the next pot, and the refire. Six defects in the diagnostic with causes and verdicts, six videos.
This is the short version: name it, application or fit, 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.
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 show you a crazed surface next to a shivered one at fired scale, 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
Application problems refire. Fit problems do not.
Crawling, pinholes and dry glaze come from a dusty bisque or a wrong coat, and they refire. Crazing and shivering come from a glaze and a clay that shrink by different amounts, and no refire changes that. Name the defect, read its cause, let the verdict 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 and with the places they fail marked.
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 under a lamp, a green scouring pad and a diamond pad, the same glaze, a damp sponge, the tile drawer, and the refire fee.
Step one
Name the defect. Six of them cover nearly every pot.
Hold it under a lamp. Beads with bare clay between: crawling. Pits: pinholes. A fine network of cracks: crazing. Slivers off the rim and edges: shivering. Bubbles or craters: blisters. Dry, rough or paler than the jar: thin. The diagnostic below has each with its cause and its verdict.
Two 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 you can feel an edge with a fingernail.
The Washington Street Studios video below is thirty-seven minutes of fired examples of every defect on this page, and if the picture in your head does not match the pot in your hand, that is where to look.
Step two
Ask which kind it is: application, or fit.
This is the question that decides everything. Application problems come from how the glaze went on: a dusty bisque, a coat too thick or thin, coats not dried between, wax in the wrong place. Crawling, pinholes, dry glaze and most blisters are application, and application refires. Fit problems come from that glaze on that clay shrinking by different amounts. Crazing and shivering are fit, and fit does not refire.
Application
Crawling, pinholes, thin and dry glaze, and blisters once. The glaze is fine and the clay is fine; the coat was wrong. Add glaze where it is missing, wipe the excess, fire again at the normal fee. The Goods Pottery and Annie Gietz videos below are the two most common of these, solved.
Fit
Crazing and shivering. The glaze and the clay disagree about how much to shrink, and a second firing closes the cracks and reopens them on cooling. The fix is a different pairing: that glaze on another body, or another glaze on that body. The Seattle Pottery Supply chalkboard is the explanation.
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 do-not-use list and, if you tell the studio team, on theirs.
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 prevents more glaze defects than everything else on this page put together. 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. A pot that pinholed in that firing pinholed because of what was on it, not because of the kiln. A glaze at the very edge of its range can blister; the jar says the range, and the test tile says the rest.
And the clay. A groggy, iron-bearing body outgasses more than a smooth white one and pinholes and blisters more under thick glaze; a smooth body crazes some glazes that a groggy one does not. Our glaze brushing lesson is the coat method, and the test tiles lesson is how a pairing is checked before a pot pays for it.
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 with a green pad, brush glaze into them and over the island edges. Pinholed: rub glaze into the holes, wipe the surface. Dry: one more coat. Blistered: grind it flat, wet, and a thin coat over the scar, once. Same glaze every time.
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. A note on the ticket if there is something the studio team should know.
Crazed and shivered pots do not refire, and the honest verdict is worth saying plainly. A crazed pot on a vitrified body is a hygiene question, 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 the last word on what a refire can and cannot do.
Watch it done
Now watch six people name six defects.
Watch this one with the pot in your hand.
None of these potters work here or know we exist. Between them they cover every defect with fired examples, pinholes chased honestly, fit at a chalkboard, whether a crazed pot is safe, crawling solved, and what a refire can and cannot do. Nothing but the still frame loads until you press play.
Thirty-seven minutes of fired examples of every defect on this page. The other five videos are in the comprehensive version.
When it goes wrong
The six defects: what it is, what caused it, and whether it refires.
The three defects you will meet first.
Pick the one that looks like your pot. Each panel is the cause, the fix for next time, and the honest refire verdict.
The glaze pulled back into beads and left bare clay
Crawling. The glaze did not stick to the bisque before it melted, so surface tension pulled it into islands. Dust, oil from fingers, wax where it should not be, a coat put on too thick, or a second glaze laid over a first that had dried and cracked.
Wipe the bisque with a damp sponge before glazing and keep fingers off it afterwards. Three coats, not five. Let each coat dry before the next. Wax only where you mean to.
Refires well. Rough the bare patches with a green scouring pad, brush glaze into the gaps and over the edges of the islands, and fire again. Most crawled pots come back.
Tiny holes and pits all over the glaze
Pinholes. Gas from the clay or the glaze bubbled through the melt and the glaze did not heal over the hole before it stiffened. Underfired bisque, a dusty or greasy surface, glaze put on thick, or a firing that got to temperature and stopped.
Wipe the bisque. Thinner coats, dried between. A groggy or dark clay pinholes more than a smooth light one; know your body. A hold at the top of the firing heals most pinholes, and our cone 5½ firing has that hold built in.
Refires often. Rub a little glaze into the holes with a fingertip, wipe the excess off the surface, and fire again. Pinholes in a thin clear usually heal; pinholes in a thick matte usually do not.
A fine network of cracks in the glaze
Crazing. The glaze shrank more than the clay as the pot cooled and cracked under the tension. It is a fit problem between that glaze and that clay body, not an application problem, and it sometimes shows up weeks later with a ping.
A different glaze on that body, or that glaze on a different body. Thinner helps a little. Nothing about how you brushed it changes fit.
Does not refire out; the cracks close in the kiln and reopen on cooling. On a vitrified body at cone 5½ a crazed pot holds water and is a hygiene question rather than a safety one; the pot is decorative if you want to be careful, and the glaze goes on the do-not-use list for that clay.
Slivers of glaze flaked off the rim and edges
Shivering. The opposite of crazing: the clay shrank more than the glaze, the glaze was left under compression, and it popped off at the edges where there was nothing holding it. Rare on our bodies and worth taking seriously, because the flakes are sharp as glass.
A different glaze on that body. Shivering is a fit problem in the other direction, and a glaze that shivers on one clay is often perfect on another.
Does not refire. Grind the sharp edges off with a diamond pad, wet, and retire the pot from the table. Tell the studio team which glaze and which clay, because that pairing goes on a list.
Bubbles, blisters and craters in the surface
Blisters. Gas that came through the glaze late and big, or a glaze that boiled: too thick, overfired for that glaze, or a body with something in it that outgasses at the top of the firing. Craters are blisters that burst.
Thinner. Check the glaze’s cone range on the jar; a glaze at the very top or bottom of its range at cone 5½ blisters more easily. A clean bisque fired properly.
Sometimes. Grind the blister flat with a diamond pad, wet, brush a thin coat of the same glaze over the scar, and refire once. If it blisters again, it is the glaze on that clay, and the pot is done.
The glaze came out dry, rough or a different colour
Underfired or too thin. A coat that was one brushload short fires dry and pale, and a glaze that needed three coats to develop its colour gives a different colour at two. On a shelf near the door of a big kiln it can be a cooler spot as well.
Three coats, each fully covering, checked against the light for thin patches before it goes to the drop-off table.
Refires beautifully. One more coat of the same glaze over the whole surface and fire again. This is the happiest refire on the page.
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, so you know before you start.
Three things to know here
Our glaze firing is cone 5½ on the same schedule every load, with a hold at the top that heals most pinholes; 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.
The firing
Cone 5½, the half being a twelve-minute hold at the top, the same every load, with our staff loading. The hold heals bubbles and settles glazes, which is why pinholes are rarer here than the internet says. The kiln is not the variable; what went on the pot is.
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, per the sanding and grinding lesson.
Questions people actually ask
The ones that come up every week.
Which problems can be fixed by refiring?
Crawling, pinholes, dry and thin glaze, and sometimes blisters: anything that is an application problem. Add glaze where it is missing, fire again, and most of them 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.
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. Add the glaze, let it dry, and put it on the drop-off table with a note if the studio team should know something about it.
Is a crazed pot food safe?
It is a hygiene question more than a poison question. 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. Most potters keep crazed pots for dry things or flowers. The Earth Nation video on this page argues the whole thing out.
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 the tile drawer 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. It is the reason pinholes are rarer here than the internet suggests.
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.
That is the whole method
Name it. Application or fit. 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. Both are the University working, and the kiln was never 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.
Written for potters everywhere. Where our studio does things differently, the page says so.
Which version would you like to print?
Both print clean, with the videos turned into labelled boxes you can search for later.