Glaze understanding

Why the Same Glaze Looks Different on Two Pots

Two pots, one jar, and they could have come off different shelves. The glaze did not change its mind. Something between the jar and the kiln changed: how much went on, the clay underneath, the shape of the pot, or the firing it got. This page puts each of those side by side, so the question stops being what the glaze did to you and becomes what changed.

One jar can fire like two different glazes. Four things change between the jar and the finished pot: how much glaze went on, the clay under it, the shape of the pot, and the firing it got. Read your two pots for each one, then test the likeliest by changing one thing.

Four variables, side by side Ten questions for any two pots Three videos, none of them ours
The whole idea in two minutes What to look at first How to test the answer
A puzzled potter compares a dark bowl with thick, dripping blue glaze in one hand and a smooth pale blue vase in the other, a glaze bucket on the table
Jump to a section
The jar holds a recipe, not a result How much glaze went on The clay underneath The shape of the pot Where it fired, and how it cooled Ten questions for any two pots What the difference usually means The whole idea, quickly Read the two pots in this order Three things worth knowing Change one thing, then look again Questions we get asked Keep going

HINT_C

The whole idea

One jar, four things that change

A glaze in the jar is a recipe. It becomes glass on your pot, and how it looks depends on four things that travel with the pot, not the jar: how much went on, the clay under it, the shape it sits on, and the firing it got.

They are not equally likely. Thickness explains most surprises, then the clay, then the shape. The firing matters most for matte, satin and variegated glazes, and it is the one variable you do not set here.

So when two pots disagree, ask what was different about this pot rather than what the glaze did. A variegated glaze always varies a little. Everything else leaves clues on the pot.

What to look at

Read the two pots in this order

Side by side, in daylight. Most answers turn up at the first or second step.

  1. Thickness. Pale rims, pale raised marks and clay showing through mean thin. Deep color, glossy pools and runs mean thick. Thin is the usual answer.
  2. The clay. Look at the bare feet. A different clay underneath makes a different glaze on top: white clay brightens it, speckled clay spots it, darker clay darkens it.
  3. The shape. Floors and grooves collect glaze, and rims and ridges shed it. A bowl and a cylinder glazed from the same cup will not match.
  4. The firing. A glossier or more feathered surface on a pot from a different load points to the cooling. A pink blush on a white glaze points to a neighbor in the kiln. Tell the studio team about either.
Worth knowing

Three things worth knowing

The wall tile is one answer

One clay, a flat face and, for most of the wall, three coats. A pot that differs in any of those will look different. The clay display by the information center shows one glaze on every clay we stock.

Three coats is not one thickness

A thickened jar, a loaded fan brush or soaked bisque all change how much each coat lays down. Stir the jar, spoon it into a cup, and take a thick jar to an instructor rather than adding water.

A thin pot can often go back in

Thin and dry is a good refire candidate. Show an instructor first, then add a coat and refire it on a kiln cookie.

The mechanism

The jar holds a recipe, not a result

A glaze is finished on your pot, in the kiln, and everything about that pot has a say in how it turns out.

54. Same Glaze Different Results Mikkel deMib - Pottery Tips Forty-two minutes, with chapters. He sets out three variables, the glaze, the firing and the clay, then fires three glazes on five clays and goes through every result. His kiln is not ours, so read the pattern rather than the colors.
  1. Glaze turns into glass on the pot. In the jar it is powdered minerals in water. It only becomes glass at the top of the firing, in a layer as thick as you laid it, over the clay you chose, on the shape you made, in the heat and cooling that load got. Change any one of those and you have changed the glass.
  2. Four things travel with every pot. How much glaze went on. The clay underneath. The shape of the pot. The firing, meaning the spot it sat in and how fast the load cooled. Each one can make a single glaze look like two.
  3. Some glazes barely notice. An opaque gloss looks much the same across all four. Glazes that pool, break, float or grow crystals answer to every one of them, which is why celadons, rutile blues and soft mattes give the biggest surprises.
  4. The wall tile is one answer. Every glaze on our wall is fired onto a numbered tile, and each tile is one set of choices: one clay, one flat face and, for most of the wall, three coats. Your pot almost certainly differs from it in at least one of the four.
  5. A difference is evidence. Two pots from one jar that came out differently are a free experiment. Read them the way you would read two test tiles, and they tell you what that glaze responds to.
  6. Some variation is the glaze being itself. A variegated glaze rarely fires exactly the same twice, even on two pots made alike. The useful skill is telling that apart from a cause you set yourself.

The question worth asking

Not why the glaze did it, which has no useful answer, but what was different about this pot. That question always has one, and the four sections below are where to look for it.

Variable one

How much glaze went on

The first suspect and the most common one. Thickness changes the color, the depth, the surface and how far a glaze moves.

Glazing with PC-20 Blue Rutile: Applications Thickness Testing AMACO Brent About fifteen minutes from the glaze maker on applying its Blue Rutile, a glaze in our collection, and on how thick it should go on.

The illustration holds everything else still: one glaze, one clay, one tile shape. Only the number of coats changes, from one on the left to four on the right.

Illustration: four identical test tiles with one, two, three and four coats of one blue glaze, from tan clay under a blue haze to deep blue with runs
Illustration, not a photograph of our tiles: one glaze on one clay, one to four coats.
  1. One coat, far left. A haze. The clay shows through almost everywhere and the raised texture is close to bare. Too little glaze to make a proper layer of glass, which is why a single coat so often looks dry.
  2. Two coats. A veil of color with the clay still reading through it. The low parts of the texture have started to fill.
  3. Three coats. The coat count on most tiles on our wall. Deep color in the grooves, the raised marks breaking lighter, a little gathered in the inside corner.
  4. Four coats, far right. The deepest and glossiest, and moving. Runs have started down the face and a pool has formed in the corner. On a tall pot that movement keeps going.
  1. Color is a depth of glass. A glaze darkens with thickness the way a pond looks darker where it is deep. Thin glaze lets the clay show through, and thick glaze hides it.
  2. Thin is the usual culprit. Most disappointing results are on the thin side. A pot that came out paler, drier or browner than the tile almost always got less glaze than the tile did.
  3. Thick has its own problems. Past a point, extra glaze runs, crawls or clouds. A see-through color laid on too heavily can turn cloudy and fire a different shade from the one on the tile.
  4. The same coat count is not the same thickness. A jar that has thickened lays down more glaze per coat. A loaded fan brush lays down more than a small round one. Bisque that soaked in water drinks less glaze than bisque given a quick rinse. Three coats means three full coats of a well mixed glaze.
  5. A dip is a coat too. In a dipping bucket, the coat comes from how long the pot stays in, and anywhere two dips overlap carries a double coat.

At our tables

Three coats, each dry before the next and each laid in a different direction, is the brush-on default, and fan and hake brushes often cover in two. Spoon glaze into a cup rather than dipping a brush in the jar, and if a jar looks thick, take it to an instructor rather than adding water. Every glazed piece goes on a kiln cookie, which matters most when the coat is a generous one.

Variable two

The clay underneath

A glaze is not a lid on the clay. It melts into it, and the clay sends some of itself back up.

Clay Comparison - 5 Clays, 1 Glaze Tony Lockhart Seven and a half minutes. Five mid-fire stonewares from one maker under the same glaze, fired in an electric kiln.

Here the glaze and the coat count stay the same, and only the clay changes. The bare foot of each tile shows which clay it is.

Illustration: one blue glaze on four clays, white porcelain, buff stoneware, speckled buff with dark spots bleeding through, and gray stoneware reading slate
Illustration, not a photograph of our tiles: one glaze, three coats, four clays.
  1. White porcelain, far left. Nothing in the body to tint the glaze, so the color is at its brightest and the thin breaks go pale instead of brown.
  2. Buff stoneware. The warm body shows through every thin spot, and the breaks over the texture read tan.
  3. Speckled buff. Each dark speck in the clay bleeds up through the glaze and leaves a dark spot with a soft halo.
  4. Gray stoneware, far right. A darker body under the same glaze pulls the whole color darker and smokier.

Why that happens, drawn as a slice through the surface of two fired pots:

Glaze and clay body, labeled section sketch: glaze melted into a white body beside a speckled body where a dark speck bleeds color up into the glaze
  1. The glaze is a layer of glass. A fired glaze is a thin sheet of glass, and a see-through one lets you look straight down to the clay.
  2. Glaze and clay melt together here. At temperature the bottom of the glaze and the top of the clay react and blend into a layer that belongs to both. That is why a good glaze cannot be peeled off like paint.
  3. White clay keeps color bright. Nothing in a white body tints the glaze, so the color reads clean.
  4. Color bleeds up from below. Metals in the clay, iron and manganese above all, move up into the melt and color it from underneath.
  5. A dark speck in the clay. One speck under the glaze becomes one dark spot in it. A speckled clay makes a speckled glaze, whatever the jar looks like.
  1. See-through glazes show the most. Celadons, clears and other translucent glazes are a window onto the clay. Opaque glazes hide more of it, though even they break toward the body color at their thin edges.
  2. A denser clay can make a glaze run. A dense, glassy body such as porcelain lets the same glaze melt more fluidly than a more open stoneware does, so a glaze that behaved on one clay can travel further on another.
  3. Fit changes with the clay too. A glaze that sits happily on one clay can craze on another, because the two shrink by different amounts as they cool. Fine cracks on one clay and not the other are a fit problem, not a glazing one.
  4. Our wall tiles are all on one clay. So a glaze on any other clay is, in effect, untested until you have seen it on that clay.

Where to check it here

The clay display by the information center shows one glaze across every clay we stock, side by side. Look at it before you pair a clay with a glaze you care about, and look at the bare feet of your two pots: if the clay under them differs, you may already have your answer. Dipping works the same way. One dipping glaze on Berlin Gray and on porcelain fires as two different-looking glazes.

Variable three

The shape of the pot

Glaze is liquid at the top of the firing. It thins where the surface rises and gathers where it falls, so the shape of a pot redraws the glaze on it.

The drawing shows where one even coat ends up after it melts on a simple thrown cylinder. The glaze layer is drawn thicker than life so the differences are easy to see.

Same glaze thick and thin on one pot, labeled section sketch: thin at the rim and over ridges, thick in grooves, sliding down the wall, pooling in the floor
  1. Thinnest at the rim. A rounded edge sheds glaze as it melts, so rims fire thin, and a glaze that breaks shows the color of the clay there.
  2. Thin over every ridge. Raised marks lose glaze to the lower ground around them.
  3. Thick in every groove. The lower ground collects what the ridges lost, so throwing lines, carving and stamps all come out striped.
  4. Slides down the wall in the firing. On an upright wall gravity pulls the melt downward, so the lower wall carries more than the upper one, and a fluid glaze runs.
  5. Pools deep in the floor. A bowl's floor and the inside corner of a cylinder collect glaze from the walls above, and fire deepest and glossiest.
  6. Bare foot on a kiln cookie. The glaze stops above the foot so a run has room to stop, and the cookie catches anything that does not.

The same thing on two finished pots, one glaze, one clay and three coats on both:

Illustration: one blue glaze on a small bowl, pooled dark and glassy in the floor, and a ringed cylinder striped tan and blue with drips stopping above the foot
Illustration, not a photograph of our pots: one glaze, one clay, three coats inside and out.
  1. The bowl, left. The floor holds a deep, glassy pool, darker than anything on the walls, and the rim reads tan all the way round where the glaze ran thin.
  2. The cylinder, right. The throwing rings stripe it tan and blue, the lower wall is heavier, and two drips stopped short of the foot.
  1. A flat tile hides all of this. Our wall tiles are flat, so they show a glaze without the help or the harm of texture. A tile with your own texture on it shows what the glaze will do on your pot.
  2. Height changes how far a glaze travels. A test tile is a wall a few inches tall, and a vase can be four times that. A glaze that stopped short on the tile can run a long way down a tall pot.
  3. Inside and outside can differ. The inside of a bowl collects glaze in its floor. The outside of the same bowl sheds it toward the foot. One coat can fire two ways on one piece.
  4. For some glazes the shape is the point. Celadons and other glazes that break are chosen for exactly this, and on a smooth wall they have nothing to work with.
Variable four

Where it fired, and how it cooled

The one variable you do not set here. It is real, and it sits further down the list than most people put it.

The same glaze on the same clay at the same three coats, with one difference: how fast each tile cooled after the firing.

Illustration: two identical test tiles in one blue glaze, the left cooled fast and glossy and even, the right cooled slowly with pale feathered variegation
Illustration, not a photograph of our tiles: one glaze, one clay, three coats, cooled at two speeds.
  1. Cooled fast, left. Glossier, smoother and more even. The crystals that give a glaze its feathering had less time to grow.
  2. Cooled slowly, right. A softer sheen, far more variegation and crisper breaks over the texture. Same glaze, same clay, same coats.
  1. Our glaze firing is electric, to cone 5½. That is cone 5 with a twelve minute hold, which lands the heat work between cone 5 and cone 6, in a clean, oxygen-rich firing. The studio team loads and unloads every kiln and decides where each piece goes.
  2. A kiln is not one temperature. Kilns lose most of their heat through the lid and the floor, so kiln makers such as Skutt put hotter elements at the top and bottom to even it out. Potters still set witness cones on more than one shelf, because a cone reports the heat work at its own spot.
  3. Cooling decides much of the surface. Many glazes grow microscopic crystals as they cool. Cool fast and the crystals have less time, so the surface comes out glossier and calmer. Cool slowly and matte, satin and variegated glazes show more of their character. How much it matters depends on the glaze, and a stable gloss barely changes.
  4. How full the kiln is changes the cooling. A densely packed load holds its heat differently from a loose one. One potter writing for Ceramic Arts Network found his matte glazes coming out glossy in his small electric kiln when it was loosely packed. Kinley’s Corner, our kiln log, shows how much member work went into each load.
  5. Occasionally, a neighbor. Some colorants travel through the kiln air. Chrome is the best known: it can leave the glaze it came in and turn a nearby white or pale glaze that contains tin a shade of pink. A pink blush on a glaze that should not be pink is worth mentioning to the studio team.

What you can do about it

Firing is the one variable on this page you do not choose, so make it visible instead. Two pots from different loads had two different firings: note when each went to the glaze shelf and when it came back, and the difference between them stops being a mystery. If a glaze you love depends on its cooling, expect it to vary a little from load to load, and pick it for the pieces where that variety is welcome.

Before you blame the glaze

Ten questions for any two pots

Put both pots side by side in daylight and go down the list. Every yes is a lead, and the first card usually finds the answer before the second one is needed.

Look at the pots

  • Is the bare clay at the foot a different color? Then the clay is your first suspect.
  • Are the rims, edges and raised marks paler on one pot? That pot got less glaze, or has a shape that sheds it.
  • Is the color deeper in the low spots of one pot? More glaze, or a shape that collects it.
  • Is one glossier, or more feathered, than the other? Look at the coats first, then at the firing and the cooling.
  • Does one have spots or a pink blush the other lacks? Spots point to a speckled clay. A pink blush points to a neighbor in the kiln.

Look at the record

  • Same jar, well stirred, in the same session? A jar that thickened between sessions lays down a different coat.
  • Same number of coats, with the same brush? A loaded fan brush and a small round brush do not lay down the same amount.
  • Same way of applying it? Brushed, dipped and sprayed coats come out at different thicknesses.
  • Fired in the same load? Two pots fired weeks apart had two different firings.
  • Did you write any of it down? The kiln ticket already holds the clay and every glaze. Add the coats and the dates in your own log, and next time this list answers itself.
Read this after an unload

What the difference usually means

Six differences, the likeliest cause behind each one, and what to change on the next pot.

Paler and browner than the wall tile

Thin, by a long way the likeliest answer. Check the coat count, the brush and whether the jar was stirred. Next pot: full coats, three of them, each dry before the next.

Darker, glossier and running

Thick, or a tall wall that gave a fluid glaze room to travel. Check the coats against the tile, and remember the tile is only a few inches tall. Next pot: lighter coats on the lower third.

Spotted or smoky on one pot only

The clay. Specks bleed up through a glaze, and a darker body darkens a see-through one. Check the bare feet. Next pot: the clay you wanted the glaze on, chosen from the clay display.

Glossy where you expected satin

A thin coat of a glaze that needs thickness to develop, or a faster cool. Check the coats first, because the firing is the rarer cause.

Fine cracks on one pot only

Crazing. The glaze and that clay shrink by different amounts as they cool, and no amount of care at the table fixes it. Tell the studio team which glaze and which clay.

A pink blush on a white glaze

Chrome from a nearby glaze, carried in the kiln air onto a glaze that contains tin. Nothing you did. Tell the studio team which load it came out of.

From a guess to an answer

Change one thing, then look again

Two pots that differ in four ways cannot tell you which one mattered. A test that differs in one way can.

  1. Pick the likeliest suspect. Thickness first, then the clay, then the shape. The firing comes last, because it is the one you cannot change.
  2. Make tiles that change only that. Same glaze, same jar, same brush, and one difference between them: the coat count, or the clay, or the texture. Our test tile lesson shows a tile that stands up, takes a number and carries texture on half its face.
  3. Use your own clay and your own texture. The wall tile already shows the glaze alone. Your tile should show it on what you actually make.
  4. Log it. The tile number, the clay, the glaze, the coats, the dates, and one line on what you thought. A photo of the fired tiles laid out on the page takes seconds.
  5. Glaze a small pot exactly like the winning tile before a big one. A tile is a short wall. A tall pot gives the same glaze far more room to move.

Half the log already exists

Every glaze firing here needs a kiln ticket with your name, the date, the clay and every glaze you used. Keep the same four things in your own notes, add the coat count and the brush, and the next two pots that disagree come with their own explanation.

Questions we get asked

The ones that come up at the glaze table

Why does my pot look nothing like the tile on the wall?

Because the tile is one set of choices: one clay, a flat face, most of them three coats, fired in one load. Your pot differs from it in at least one of those, usually more. Thickness is the likeliest, so check the coats before anything else.

Is it the kiln’s fault?

Sometimes, and rarely first. When a fired color is nothing like the tile, the cause is usually thickness, sometimes the clay, and only occasionally a neighbor in the kiln. Where the firing itself matters most is with matte, satin, variegated and crystal glazes, whose surface depends on the cooling.

Will one glaze always look the same on one clay?

Close to it, for a stable opaque glaze applied the same way. A variegated glaze will differ a little every time, even on two pots made alike, and that variety is a large part of why people choose it.

Do more coats always mean more color?

Up to a point. More glaze gives more depth and color, and past that point it runs, crawls or clouds. Most glazes have a thickness they are happiest at, and most disappointing results sit on the thin side of it.

Why is the glaze darker inside my bowl than outside it?

The inside of a bowl collects. Glaze runs down the inside walls during the firing and gathers in the floor, where it fires deepest and glossiest. The outside sheds glaze toward the foot instead, so the same coat ends up two thicknesses.

Why did my speckled clay put spots in the glaze?

The specks are dark particles in the clay body. In the firing, metals from them move up into the melting glaze and color it from below, so each speck under the glaze becomes a spot in it. On a white clay the same glaze has nothing to pick up.

Can I fix a pot that came out too thin?

Often. A thin, dry result is a good refire candidate. Show an instructor first, then add a coat and refire it on a kiln cookie. Our reglazing and refiring lesson covers getting new glaze to stick to a fired one.

Do I need to test every glaze on my own clay?

Not for choosing a glaze: the wall tiles and the clay display do that job. For anything you care about on a clay other than the wall tile’s, at a coat count other than three, or over your own texture, one tile is cheaper than one disappointing pot.

One last thing

The answer is written on the pot

A glaze jar promises a recipe, and it keeps that promise every time. What it cannot promise is your thickness, your clay, your shape or your firing, because none of those are in the jar. They are in the pot, and the pot shows them: at the rim, in the floor, at the bare foot.

So the next time two pots disagree, set them side by side and read them before you judge the glaze. Then change one thing and look again. That is how a surprise turns into a result you can repeat. Come make something, and bring the two pots to the glaze table.

Thickness, clay, shape and firing. Read your two pots for each, in that order, test the likeliest by changing one thing, and write down what you did.

For the illustrations side by side, the section drawings and the ten questions, switch to the Comprehensive Version at the top of the page.