Kiln firing problems and how to fix them

Most kiln firing faults are schedule faults. Bloating, pinholing, crawling, dunting, crazing, shivering, and warping each begin in a known temperature band, and each usually answers to a change in ramp rate, hold, or cooling rather than a new recipe. Find the band, change one segment, and refire a test tile.

Publie le 22 juillet 2026 · Source : Ceramic Arts Network, Techno File: Defects From Firing

A firing fault is a message about a segment. Clay and glaze both pass through a sequence of chemical and physical changes on the way up and again on the way down, and every one of those changes has a temperature band and a pace it will tolerate. Miss the pace and the ware reports it in a specific, repeatable way.

The seven faults below cover most of what comes out of a studio kiln looking wrong. For each one, look at what it looks like, the band where it starts, and the segment to change first. Change one variable per firing and keep notes, because a fault with two possible causes will never tell you which one you fixed.

What causes most kiln firing problems?

Three things cause most of them: heating too fast through a band where the clay is still releasing gas, landing the wrong amount of heat-work at the top, and cooling too fast through the silica inversions. Nearly every fault traces back to one of those three, which is why the fix is usually a segment change rather than a new glaze.

Ceramic Arts Network makes the first point plainly in its guidance on firing defects: the kiln should be fired in complete oxidation from about 573 to 1291 F (300 to 700 C) so carbonaceous material is driven out of the body. Carbon left behind after that window is the seed of several later problems, including black coring and bloating.

• Bloating: raised blisters in the clay wall. Carbon left in the body gasses out after the surface has begun to seal. Fix it in the bisque, with more oxygen and more time from roughly 1250 to 1750 F (677 to 954 C).
• Pinholing: needle holes through the glaze. Gas breaks the surface after the melt has stiffened. Add a hold at peak, or drop about 100 F below peak and hold there.
• Crawling: bare clay islands with the glaze beaded back. The bond failed before the kiln was hot. Slow the first 500 F and check the bisque surface.
• Dunting: a sharp, clean crack found after unloading. Free-fall cooling through a silica inversion. Program a cooling ramp across 1150 to 950 F and again from about 550 to 350 F.
• Crazing: a fine web of cracks in the glaze only. The glaze contracts more than the body. A schedule can only fix the underfired version of it.
• Shivering: glaze flakes off rims in sharp slivers. The body contracts more than the glaze. A fit problem in the recipe first.
• Warping: oval rims, rocking plates, slumped feet. Too much heat-work at the top, or uneven heating in a tight load.

Why does clay bloat, and can the schedule stop it?

Clay bloats when carbon or sulfur still trapped in the body turns to gas after the surface has started to seal. The pressure pushes the softening wall out into a blister. The schedule can stop it, but the work happens in the bisque firing, not at the top of the glaze firing.

Burnout needs three things at once: oxygen, time, and temperature. A slow bisque cycle near 108 F (42 C) per hour from roughly 1250 to 1750 F (677 to 954 C) gives chemically combined sulfur and organic material room to leave, and a running vent gives it somewhere to go. Dark, heavily grogged, or thick bodies need that window most, because they carry more carbon and take longer to release it.

On the glaze side, easing the last 200 F helps the body finish reacting before it vitrifies, but no glaze program rescues a body that arrived carrying carbon. If bloating repeats on the same clay, bisque a little hotter and a little slower before you change anything at cone 6. The bisque firing schedule guide sets out the full burnout sequence.

How do I stop pinholes and blisters?

Give the melt time to heal. Pinholes are craters left when a gas bubble breaks the glaze surface too late for the melt to flow back over it. Blisters are the same bubble caught a step earlier, frozen in place before it burst. Both respond to a hold that keeps the glaze fluid while the gassing tails off.

Two moves are worth testing, one at a time. A hold of 15 to 30 minutes at peak keeps the surface mobile at the moment most gas is finishing. A drop-and-hold does the same job lower down: after the cone bends, drop roughly 100 F below peak and hold 20 to 60 minutes. The lower hold often works when a top hold alone does not, because fewer new bubbles are being generated down there while the glaze is still soft enough to close.

Slowing the final approach helps too. A vitrification ramp of about 108 to 125 F (42 to 52 C) per hour from around 2050 F to peak lets feldspars finish off-gassing at a pace the glaze can keep up with. If pinholes survive all of that, the gas is coming from the clay, and the answer moves back to the bisque again.

Why did my glaze crawl?

Because the glaze layer never bonded to the surface underneath it, or it cracked as it dried and lifted. Crawling starts before the kiln gets hot. Firing cannot repair a bond that was never made, though a calmer early ramp stops the firing from making a weak bond worse.

Look at the bisque surface first. Dust, oil from hands, a glaze applied too thick, or a bisque so smooth that nothing keys into it all produce the same rolled bead of glaze pulled back from bare clay. Wipe or rinse the ware before glazing, thin the application, and let glazed pieces dry fully before loading.

The schedule contribution sits in the first 500 F. A fast early climb drives the remaining water out of the glaze layer quickly and can lift a fragile coat off the wall. Slowing that stretch, at the pace you would use to candle bisque, gives the layer a gentler start. Bisquing slightly hotter also helps on very thirsty clays, since a less absorbent surface pulls water out of the glaze less violently during dipping.

Is it a cooling crack or a heating crack?

Read the edges. A crack that opened while the kiln was heating has soft, rounded edges, because glaze melted over it afterwards. A cooling crack, called a dunt, is sharp and clean, cuts straight through the glaze, and often runs through a foot or a wall. Sharp edges point at the cooling segments.

Dunting comes from silica changing volume. Quartz inverts near 1063 F (573 C) on the way down as well as on the way up. In ware fired hot enough to form cristobalite, which begins as low as 1634 F (890 C) and forms mainly above 1922 F (1050 C), a second inversion waits on the way down, reported near 473 F (245 C) by Ceramic Arts Network and across a wider 410 to 536 F (210 to 280 C) band by Digitalfire, with roughly double the volume change of quartz. Free-falling through either band, or cracking the lid to speed things up, is the classic cause.

A programmed cooling ramp of roughly 100 to 200 F per hour across 1150 to 950 F, then patience again from about 550 down to 350 F, costs an hour or two and removes most dunting. Heating cracks belong to the other end of the program. They are a candling and early-ramp problem, covered in the candling and preheating guide.

Can a schedule fix crazing and shivering?

Only partly. Crazing and shivering are fit faults, meaning the glaze and the body contract by different amounts as they cool, and a schedule cannot rewrite thermal expansion. What it can fix is the version of crazing caused by underfiring, and it can lower the shock that makes a tense glaze let go.

Crazing shows as a web of fine cracks in the glaze layer alone, sometimes appearing days or weeks later once the piece has taken up moisture. An underfired body stays more porous and develops less of a clay-glaze interface, and both of those make crazing likelier. Fire a witness cone. If you are landing short, slow the last 200 F rather than raising the target temperature, because the cone measures heat-work and time counts as much as degrees.

Shivering is the mirror image. The body contracts more, the glaze ends up in compression, and it flakes off rims and edges in sharp slivers. Shivered ware is not safe to handle or use, and the answer lives in the recipe or the clay pairing rather than the controller. Slowing the cool below about 1000 F reduces the mechanical shock that pops an already stressed glaze, but treat that as a symptom check, not a cure.

Why is my work warping?

Warping is a heat-work fault plus a loading fault. Rims go oval, plates rock, and feet slump when the body spends too long above the point where it starts to soften, or when one part of the kiln runs hotter than another. Read the cones before you touch the recipe.

If witness cones at the top, middle, and bottom of the load are all over-bent, the load took more heat-work than the program suggests, so shorten the hold at peak or slow the final ramp until the controller and the ware agree. If the cones disagree with each other, the problem is distribution: elements of different ages, a tight stack, or heavy flat work sitting directly over a hot element.

Wide flat forms want even heating as much as they want the right peak. Leave room between shelves and posts for air to move, avoid crowding a shelf against one wall, and give large platters a shelf of their own where you can. A controlled cool through the first 300 F below peak also lets a softened body settle evenly instead of freezing into a shape it was pushed into.

How do I test a schedule change without wasting a load?

Fire small and change one thing. A shelf of test tiles from the same clay and the same glaze batch, with a witness cone at the top, middle, and bottom, will teach you more in two firings than a season of guessing. Write down the segment you changed and what the cones read.

1. Write down the current program segment by segment, including the cooling, before you change anything.
2. Change one variable: one ramp rate, one hold, or one cooling segment. Two changes at once hide which one worked.
3. Load a small test: tiles of the same clay, the same glazes, and witness cones at three heights.
4. Record what the cones read at each height, not only what the controller reported at shutoff.
5. Keep the sheet. Three of these build a picture of your own kiln that no published schedule can give you.

None of this makes a schedule safe for a particular kiln or clay body. Elements age, controllers drift, thermocouples read a little high or low, and suppliers reformulate clay. The witness cone and the manufacturer manual decide. Everything above is a starting point to test, and the firing schedule builder lays the segments out so you can see exactly which one you moved.