Why Did My Pottery Do That?

Small hollow pockets that show up when you cut the block with a wire, or a soft spot that gives under your thumb while you center.

Clay straight out of the bag is usually fine. Bubbles get in when clay is reclaimed, when scraps are pressed together, or when a pot is built from pieces that were joined without pressure.

Wedge every piece of reclaimed clay, and cut a test slice before you start. A bubble found on the wedging table costs a minute. The same bubble found after the bisque costs the pot.

The block resists the wire, crumbles at the edges, and fights back when you try to center it.

It dried out. Either the bag was open, the bag has a hole in it, or the clay sat unused for months.

Close the bag properly every session and store it out of moving air. Clay that is merely stiff is quicker to fix than clay that has gone chalky.

The wall goes up and then sags. The rim droops. A bowl closes in on itself while you watch.

Too much water, either from the bag or from your hands. Soft clay cannot hold its own weight once the wall gets tall and thin.

Use less water while throwing, and sponge the inside dry between pulls. Most sagging is water, not skill.

Marbled streaks in a block that should be one color, or a pot that fires two different shades.

Scraps from different clay bodies went into the same bucket, or a wedging table was not wiped down between bodies.

Keep reclaim separated by clay body, and wipe the table between bodies. Colored clay scrap never goes in the communal reclaim.

A crack across the inside floor of the pot, curved like an S or a lazy spiral. It often shows up after drying, sometimes not until the bisque.

The floor was never compressed. Clay particles line up in a spiral while you open the pot, and if nothing presses them back together, the floor pulls apart along that spiral as it dries. Too much water sitting in the bottom makes it worse.

Compress the floor properly with a rib or your fingertips after opening, working from the center out, and do it more than once. Sponge standing water out of the bottom before you pull the walls.

The top edge rises and falls as the wheel turns. One side is taller than the other.

Usually the pot came off center during a pull, or one pull carried more clay up one side than the other. A rim can also go uneven if you let go of it while the wheel is still moving fast.

Slow the wheel down for the last pull and the last touch on the rim. Speed is what turns a small wobble into a big one.

The pot folds over on itself, usually about two thirds of the way up, often twisting as it goes.

Three usual causes, in order of how often they turn out to be the real one. The wall got too wet. The wheel was going too fast. The wall was already too thin at the point where it folded.

Sponge the inside between pulls, slow the wheel as the pot gets taller, and check wall thickness with a needle or your fingers before the pull you think will be the last one.

The wall thickness changes as it turns. One side of the rim runs out toward you and the other runs away.

Either the clay was never fully centered to begin with, or a hand came off the pot unevenly during a pull and dragged it sideways.

Spend longer centering than feels necessary. A pot is only ever as true as the lump it started as.

A hole in the floor, or a floor so thin you can see light through it or feel it flex.

The pot was opened too deep. It is the most common beginner mistake on the wheel, and it happens because the floor is the one part of the pot you cannot see while you make it.

Measure before you pull. A needle tool pushed through to the bat tells you exactly what is under your hand, and the small hole it leaves closes with one pass of a finger.

Short dry looking cracks radiating across the inside floor, visible while the pot is still on the wheel.

The floor is being stretched faster than the clay wants to move, usually from opening too quickly or from clay that is already too stiff.

Open more slowly and use a wetter hand on the floor. If the clay is stiff enough to crack while it is this wet, it is too stiff for the shape you are making.

Small tears or splits around the top edge, sometimes turning into a frill.

The rim is the thinnest, driest and fastest moving part of the pot, so it dries first and takes the most abuse. Splitting means it dried out mid throw or it got worked too long.

Compress the rim after every pull, keep a wet finger on it, and stop touching it once it looks right.

The pot stays short and thick no matter how many times you pull.

The pulls are not actually moving clay. A pull that does not start at the very bottom of the wall leaves the clay where it is and just polishes the outside.

Start every pull below the clay, with enough pressure that you can feel the wall thin between your fingers, and move up slowly. Height comes from the first inch of each pull, not the last.

A hole in the middle of the foot, usually a clean round one.

There was less clay in the floor than you thought. Either the pot was opened too deep, or the trimming took more off than the floor could spare.

Tap the floor with a fingernail before you start trimming. A high hollow note means thin. Better still, weigh your clay and keep your proportions consistent so you know what is under there.

A regular ripple of small cuts across the surface, like corduroy.

The tool is bouncing. That happens when the clay is too hard, the tool is dull, or the tool is being held loosely and allowed to skip.

Trim at proper leather hard, keep the tool sharp, and brace your trimming hand against your other hand so the tool cannot bounce. Some potters chatter on purpose as a surface. Yours should be a decision, not a surprise.

A dented or broken pot on the floor, usually with a flat spot where it landed.

It was not held down. Trimming clay is stiff and the tool pushes hard, and a pot sitting loose on the wheel head will walk and then launch.

Center the pot, then hold it. Three coils of clay around the base, a chuck, or a proper clamp. Never a hopeful thumb.

A crack running around or across the foot, often showing up a day or two after trimming.

The foot was trimmed when the pot was too dry, or it was left much thicker than the wall above it. Thick and thin dry at different speeds, and the join between them is where the stress lands.

Trim at leather hard, not bone dry, and keep the foot close to the same thickness as the wall. Even thickness is most of what stops cracks.

Cracks along the outside of a curve, or at the corners of a folded form.

The slab was too dry for the bend. Clay stretches happily while it is soft and tears as soon as it stiffens.

Bend slabs at soft leather hard, and roll the slab in more than one direction so the particles are not all lying the same way.

Splits along the seams between coils, either while building or after drying.

The coils were joined without enough scoring, slip, or pressure. Two pieces of clay pressed together are not joined. They are stacked.

Score both sides, slip both sides, and blend the seam on the inside as well as the outside. A join you can still see is a join that will crack.

A crack at the top or bottom attachment, often a fine line that appears during drying.

The handle and the mug were at different moisture levels when they were joined, so they shrank at different rates. A dry handle on a damp mug pulls itself off.

Join at matched moisture, score and slip both faces, and blend a small fillet of clay into the join. Then dry the mug slowly with the handle covered so it does not race ahead.

A clean break at the join, usually with a smooth face on both sides.

A smooth break face means the two pieces were never really one piece. No scoring, no slip, or not enough pressure.

Score deeply enough to see the marks, slip both faces, and press until clay squeezes out of the seam. Then wipe the seam, do not wash it.

A crack that was not there yesterday, usually starting at a rim, a handle, a corner or the base.

Something dried faster than the rest of it. Clay shrinks as it dries, and if one part shrinks while another part is still wet, the boundary between them tears.

Slow the whole thing down. Cover loosely with plastic for the first day or two, keep the piece off drafts and off direct heat, and wrap the parts that dry fastest.

A round rim that has gone egg shaped, usually noticed at the bisque.

The pot was moved while it was too soft, or it dried sitting on one spot so one side shrank first. Wide open shapes have nothing holding the rim round.

Lift wide pieces from the base, not the rim, and dry them upside down on a flat board so the rim is supported. Turn them a quarter turn each day.

A plate that rocks on the table, or one with a rim that lifts on two sides.

Plates have almost no wall to hold them up and a large flat floor that wants to move. Warping comes from uneven drying, from a floor that was never compressed, or simply from the shape being hard.

Compress the floor hard, dry flat between two boards with a light weight on top, and accept that plates are the least forgiving thing in the studio.

Short cracks like spokes, running from the foot outward or from the center of the floor.

The base stayed wet while the walls dried, which happens whenever a pot sits flat on a nonporous surface such as plastic or a sealed bat.

Dry pots on a bare board or a wire rack so air reaches the base, and get them off the plastic bat as soon as they are firm enough to move.

A piece that looks pale and even in color all over, and feels room temperature rather than cold against your cheek.

Damp clay is the single most common reason a pot explodes in the bisque. Water trapped in the wall turns to steam and steam needs about 1,600 times the space the water took.

Give thick work, closed forms and anything with a heavy base longer than you think. A week is not unusual. Nothing about drying can be hurried safely.

Sharp fragments spread across the shelf, sometimes across other people's work.

Water. The piece went in damp, the water turned to steam, and the steam had nowhere to go. A sealed hollow form can do the same thing with trapped air.

Make sure work is properly bone dry, and put a pin hole in any sealed hollow form so trapped air can escape. This is the one failure that costs other people their work too.

A piece in two or three parts with clean edges, rather than scattered fragments.

Usually a crack that was already there and too fine to see. Sometimes the kiln came up too fast through the first part of the firing.

Check greenware in raking light before it goes in the kiln. Fine drying cracks are much easier to see from the side than from above.

A dark band in the middle of a broken edge, with normal colored clay on both faces.

Carbon and organic matter in the clay did not burn out. That happens when the bisque climbed too fast, ran too cool, or did not get enough air.

Bisque slowly through the burnout range and give the kiln air. Black coring is a firing schedule problem, not a making problem.

Pieces that scratch with a fingernail, shed dust, and soak up glaze like a sponge.

The bisque did not get hot enough, or it got there too fast to do any work.

Bisque to cone 05 and let the kiln do the full climb. Underfired bisque also drinks glaze, which is where a lot of glaze problems start.

Glaze that has pulled itself into islands and beads, leaving bare clay between them.

The glaze could not hold on. Dust, oil from hands, glaze that went on too thick, or glaze applied over a surface that was still damp.

Wipe bisque before glazing, handle it by the foot, and keep coats even.

Small sharp holes in an otherwise smooth surface, often in clusters.

Gas escaped through the glaze after it had already started to set, so the hole never healed over. Underfired bisque, glaze on too thick, or a firing that cooled too fast.

Bisque properly, keep coats even, and give the glaze firing a hold at the top so the surface has time to smooth out.

A web of fine lines across the glaze surface. You can sometimes hear it happening as the kiln cools, a faint ticking.

The glaze shrank more than the clay under it as they cooled, so the glaze is under tension and it cracks to relieve it. It is a fit problem between that glaze and that clay body.

Change the pairing. A glaze that crazes on one clay body often sits perfectly on another.

Sharp flakes lifting away, usually at rims and on the edges of handles, taking a thin skin of clay with them.

The opposite of crazing. The glaze shrank less than the clay, so it is under compression and it pops off. It is the dangerous one, because the flakes are sharp and they can come off in food.

Change the pairing. Shivering is a fit problem, and no application change will fix it.

Raised bubbles, or open craters with sharp rims where a bubble burst and never healed.

Gas trapped under a glaze that had already sealed over. Usually overfiring, glaze on far too thick, or a clay body that is still burning out.

Check coat thickness first. Most blistering the studio sees is glaze applied heavily rather than anything wrong with the kiln.

A surface that looks starved, feels like sandpaper, and shows the clay color through it.

Too little glaze, or a glaze that wants a hotter firing than it got. Matte glazes can also go dry and rough if they are applied thin.

Read what the glaze wants and give it that many coats. Most brush on glazes want three.

A glaze that fired a different color, a different depth, or a different finish from the sample you chose it from.

Glaze color depends on thickness, on the clay underneath, on what is next to it in the kiln, and on where it sat on the shelf. Thickness is the biggest of those by a long way.

Match the number of coats the tile was made with. A test tile is a promise about a specific thickness on a specific clay, and nothing else.

Thin patches shaped like fingers, or clean bare clay where a thumb held the piece.

Bisque takes glaze the instant it touches it, so wherever a finger was, the glaze is thinner or missing.

Hold bisque by the foot, or use glazing tongs, and go back over the spots where your fingers were before the glaze dries.

A pot stuck to the shelf, with a pool or a curtain of glaze around the foot.

Glaze applied too thick, applied too close to the foot, or a glaze that moves more than you expected at temperature.

Leave bare clay at the bottom of the pot, keep the last coat light near the foot, and use a catcher under anything you know is a runner.

A clean crack, often straight or gently curved, through a piece that was clearly whole when the glaze melted. The glaze edges at the crack are sharp, not rounded.

Dunting. The kiln cooled too fast through the point where the silica in the clay changes volume, and the piece tore itself.

This is a kiln schedule problem rather than a making one. Thick and uneven walls make a piece much more likely to dunt.

Blisters coming out of the clay body itself rather than the glaze, sometimes with the wall visibly swollen.

The clay was fired hotter than it can take, or it had carbon still trapped inside from a poor bisque. Gas forms inside a wall that has already sealed, and the wall inflates.

Check that the clay body suits the cone it is being fired to, and make sure the bisque burned out properly. Bloating is nearly always a clay and temperature mismatch.

Two pots stuck at the point where they touched, usually with a torn glaze scar when they come apart.

They were loaded too close and the glaze bridged the gap as it melted.

Leave real space between glazed pieces. Glaze moves more than it looks like it should.

A foot that catches on your fingernail, or one that leaves marks on a wooden surface.

The foot sits on the shelf during the glaze firing, so it is bare clay that has been fired hard, and any glaze that reached it has to be ground off.

Wipe the foot properly before the piece goes in, and plan to smooth every foot as part of making the piece rather than as a repair.

A crack that appears in a piece that has been sitting on a shelf or in daily use, often after it has been through hot water.

Delayed crazing, or a piece that was already under stress and finally gave. Thermal shock from a hot drink or a dishwasher can finish something a firing started.

This is almost always a glaze fit problem that was invisible on the day. Plates and wide flat forms are the usual victims.

A vase that leaves a damp ring, or a mug that feels wet on the outside after it has been full for a while.

Either the glaze is crazed and water is getting through the cracks, or the clay body was not fired hot enough to vitrify.

Match the clay body to the cone it is being fired to, and take crazing seriously on anything meant to hold liquid.

A rim that feels gritty, sharp or dragging against your lip.

Either the glaze is a matte or textured one that was never meant for a rim, or the rim was left bare and is fired clay.

Choose a smooth glossy glaze for anything that touches a mouth, and glaze the rim fully. This is a decision made at the glaze wall, not at the sink.