What Pottery Glaze Actually Is
Glaze is glass, made on purpose, stuck to a pot. Everything confusing about it comes from the fact that glass does not want to melt at a temperature clay survives, and it does not want to stay attached once it has. Here is how the problem gets solved, and what that means for the bucket you dipped in last week.
Glaze is a thin layer of glass fired onto clay. It needs three things: something that becomes glass, something that makes it melt low enough, and something that stops it running off. Change the balance and you change the gloss, the color and whether it fits the pot.
It is glass, and it is glued on by melting
Sand is the raw material of glass, and sand on its own melts somewhere near 3100 F. No pot survives that, so a glaze has to be tricked into melting far lower.
That trick is the whole subject. You add other materials that lower the melting point, then add something else to stop the result from running off the pot like syrup. Balance those three and you have a glaze. Get the balance wrong and you have a puddle, a dry crust, or a surface full of cracks.
Everything else, the color, the gloss, whether it is food safe, comes out of that same balancing act.
The three jobs
- Silica becomes the glass. It is the only ingredient that actually turns into the glassy layer. Everything else is there to make silica behave.
- Flux makes it melt low enough. Feldspar, whiting, boron and the rest drop the melting point from something no pot survives to something a kiln can reach.
- Alumina stops it running off. It thickens the melt so the glaze stays where you dipped it instead of pooling on the kiln shelf.
Then, on top of those three, a colorant if you want color and an opacifier if you want it opaque. Those are the decoration. The three above are the glaze.
Three things people get wrong
Glaze is not paint
Paint dries. Glaze melts. Nothing about the bucket tells you what the fired surface looks like, and thickness matters more than brushwork, because you are laying down a depth of glass rather than a color.
Crazing is usually the glaze, not you
Those fine cracks mean the glaze shrank more than the clay as they cooled. It is a mismatch between two materials, not a sign you dipped badly. Some glazes craze on some clays no matter who applies them.
Matte does not mean unfired
A properly matte glaze is fully melted. It is matte because of what is in it and how it cooled, not because it needed longer. A glaze that is matte because it under-fired is a different thing, and it feels dry and rough rather than smooth.
Glaze is glass, and that is the whole problem
Six facts. Every other section on this page is a consequence of one of them.
- The glass part is silica. The same material as window glass and beach sand. It is the only ingredient in a glaze that becomes the glassy layer, and everything else in the bucket exists to make it possible.
- Silica alone melts far too high. Around 3100 F. No clay body survives that, and no studio kiln reaches it. So a usable glaze can never be silica on its own.
- So you cheat, with flux. Certain materials break silica's structure apart and let it melt hundreds of degrees lower. Which flux you use is most of what decides what temperature a glaze wants.
- Then it melts too well. A glaze that melts freely runs straight off the pot. Alumina is added to thicken the melt so it grips the surface and stops moving.
- It is not a coat sitting on top. At temperature, the glaze and the clay surface react and form a layer that belongs to both. That is why a well fitted glaze cannot be chipped off in flakes the way paint can.
- Nothing about the bucket predicts the result. The wet color, the smell, the texture on the brush, none of it survives. What comes out is decided by the recipe, the thickness and the firing.
The sentence worth keeping
A glaze recipe is a compromise between three things that want different temperatures. Almost every glaze problem people bring to us is that compromise slipping in one direction or the other.
The three ingredients
Open any glaze recipe and every line is doing one of these three jobs, plus color at the end.
Silica, the glass former
The backbone. Usually somewhere around half the recipe. More silica gives a harder, glossier, more durable surface and pushes the melting point up. Too much and the glaze stays dry and unmelted at your temperature.
Flux, the melter
Whatever brings the melting point down into reach. Feldspar is the workhorse and brings its own silica and alumina with it. Whiting, dolomite, talc and boron sources each melt differently and each leave a different surface behind. Which fluxes a recipe uses is most of its personality.
Alumina, the stiffener
Usually arriving as clay in the recipe. It thickens the melt so the glaze stays put, and it also helps the raw glaze stick to the pot before firing. Too little and it runs; too much and it goes stiff, dry and matte.
Boron, the useful cheat
Worth naming separately, because it is why mid-range glazes exist at all. Boron melts very early and makes a glassy surface possible in the middle of the range rather than only up at cone 10. Most commercial mid-fire glazes lean on it heavily.
Colorants
Metal oxides, carbonates or stains, usually a few percent. They ride along in the melt and do not change whether it works. A glaze without them is not unfinished; it is clear or white.
Opacifiers
Tin, zircon or titanium, added to make a transparent glaze white and solid rather than see-through. They also change what every colorant in the same glaze looks like, usually softening it.
Why the same three make thousands of different glazes
Because the ratios move, and because each flux behaves differently at temperature. Shift the silica up and the surface goes glassier. Swap one flux for another and the color of the same cobalt changes. That is the entire field, and it is why people spend decades on it.
Why a glaze crazes
Those fine cracks in a glaze surface are not a mistake you made dipping. They are two materials disagreeing about how much to shrink.
- Everything shrinks as it cools. The clay shrinks by one amount and the glass on top of it shrinks by another. If those amounts are close, the glaze fits. If they are not, something has to give.
- Crazing is the glaze shrinking more than the clay. It ends up stretched tight over the pot, and it relieves that by cracking into a web of fine lines.
- Shivering is the opposite, and it is worse. The glaze shrank less than the clay, so it is squeezed. Eventually it pops off in flakes, usually at a rim or an edge. A crazed pot is a nuisance. A shivering pot is sharp.
- A crazed pot is not sealed. Water gets into the cracks and into the clay beneath. On anything meant to hold liquid that is a real problem, not a cosmetic one, and it will eventually smell.
- Some crazing is deliberate. Crackle glazes are crazing, sold as a feature, often rubbed with ink to show the lines. Beautiful on a vase. Not something to put on a mug.
- It is fixed at the recipe, not at the pot. You cannot dip your way out of crazing. The fix is a different glaze, a different clay body, or a chemistry change. Which is why, if a glaze crazes on our clay, it is our problem to sort out rather than yours.
What to do if you see it
Tell us which glaze and which clay. That pairing is information worth having, and it is the only way a crazing glaze gets found and dealt with rather than quietly ruining pots for months.
Gloss, satin and matte
Shine is not a finish applied to a glaze. It is what the glaze did as it cooled.
- Gloss is a smooth, fully melted glass surface. Nothing interrupting it, so light bounces off cleanly. Generally the most silica and the most complete melt.
- Matte is a surface full of tiny crystals. As a matte glaze cools slowly, crystals grow in it and break the surface up at a scale you cannot see. Light scatters instead of reflecting. The glaze is still fully melted.
- Satin sits between them. Fewer crystals, a softer scatter, and the finish most people reach for on functional work because it hides fingerprints without feeling dry.
- Cooling speed matters as much as the recipe. Those crystals need time to grow. The same glaze cooled fast can come out glossier than it does cooled slowly, which is one reason results shift between kiln loads.
- An under-fired glaze is a different thing. It looks dull because it never fully melted, and it feels dry, rough or chalky. A real matte feels smooth. If a surface feels like sandpaper, that is not matte, that is unfinished.
- The surface changes the color. The same colorant reads deeper and richer under gloss and paler and softer under matte. This is why a test tile in one base tells you very little about another.
What makes a glaze food safe
Food safe is not a property a glaze has in the bucket. It is a property of a finished pot, and it depends on the glaze, the clay, the thickness and the firing together.
- It has to be fully melted. An under-fired glaze is a porous, chemically unfinished surface. Whatever is in it is far more available to whatever you put in the pot.
- It has to fit the clay. A crazed glaze is a cracked surface with clay exposed underneath. However good the recipe, a crazed mug is not a sealed one.
- It has to be the right thickness. Too thin and it does not seal. Too thick and it can craze or crawl. Both undo the first two points.
- The colorants have to be locked in. In a properly melted, well balanced glaze they are. In an unbalanced or under-fired one, some can leach.
- Manufacturer testing is worth something. A commercial glaze labeled dinnerware safe has been tested to a standard on a specific firing. That claim assumes you fired it correctly.
- Surface matters in practice. A very matte or heavily textured surface holds onto food and is harder to clean, whatever the leach test says. For the inside of anything, a smooth surface is the practical answer.
The short version
A well made glaze on a vitrified pot, fully melted and uncrazed, is food safe. The failures are almost always under-firing, crazing, or a surface nobody can get clean.
What is in our buckets
We stock 650+ glazes, and none of them are mixed here from raw materials.
- They are commercial glazes. Made by manufacturers who tested them, published a firing range and stand behind the result. That is the whole reason the shelf is that big and that reliable.
- They are made for our temperature. Everything on the wall is chosen to work at cone 5½, which is cone 5 with a twelve minute hold. A glaze rated for a different range is not going to behave.
- Mixing from raw materials is a different craft. Weighing silica, feldspar and clay yourself gives you total control and a real learning curve, and it means owning every problem on this page personally.
- Commercial does not mean lesser. Almost every piece Dan sells is finished with glazes off this shelf. What you lose in bragging rights you gain in knowing that a glaze which worked last month will work again.
- The manufacturer's range is a real constraint. A mid-fire glaze fired to cone 10 runs onto the shelf. One rated for cone 10 and fired at our temperature comes out dry and unmelted. Check the range before you buy anything to bring in.
- Test tiles still matter. The same glaze behaves differently on different clay bodies and at different thicknesses. A tile in the clay you actually use costs one firing and saves whole pots.
What goes wrong, and what it means
It ran onto the shelf
Too much flux for the temperature, fired too hot, or applied far too thick. On a commercial glaze it is nearly always thickness or an over-fire rather than the recipe.
It came out dry and rough
Under-melted. Either the glaze wanted a higher temperature than it got, or it went on too thin to form a proper layer of glass. It feels like sandpaper, which is how you tell it apart from a real matte.
Fine cracks all over it
Crazing. The glaze shrank more than the clay. Not a dipping fault and not fixable on the finished pot. Worth telling us, because it is the glaze and clay pairing that needs changing.
It flaked off at the rim
Shivering, the opposite of crazing, and the one that can leave sharp edges. Take that pot out of use and tell us which glaze and which clay it was.
It pulled away and left bare patches
Crawling. The glaze did not grip the bisque, usually because of dust, oil from hands, or a coat so thick it cracked while drying. Wipe bisque before glazing and handle it less.
Little holes across the surface
Pinholes. Gas escaped through the glaze after it had already melted and sealed. Comes from the clay, from a heavy colorant underneath, or from a firing that climbed too fast at the end.
The ones that come up at the tables
Why does the bucket look nothing like the fired glaze?
Because none of what you see in the bucket survives. The wet color comes from raw powders suspended in water. The fired color comes from metals reacting inside molten glass. There is no reason for those two to look alike, and mostly they do not.
Does thicker glaze mean stronger color?
Up to a point, and then it means problems. More thickness gives more depth and more color, and past a certain point it crazes, crawls or runs. Most glazes have a thickness they are happiest at, and most disappointing results are on the thin side of it rather than the thick.
Can I mix two glazes together?
You can layer them, and that is where a lot of the best surfaces come from. Stirring two buckets together is a different matter and can produce something that runs badly, because you have changed the flux balance without knowing what you changed it to. Test layering on a tile before a pot.
Why did the same glaze look different this time?
Thickness, the clay underneath, where it sat in the kiln, and how fast that load cooled. Matte and crystalline glazes are the most sensitive, because their surface depends on crystals having time to grow. Some variation is the material being itself rather than something going wrong.
Is a crazed mug safe to use?
It is not sealed, which is the real issue. Liquid works into the cracks and into the clay, and over time it holds onto what it absorbs. Fine on a vase or a piece that lives on a shelf. Not what you want for daily drinking.
What is cone 5 and a half?
Cone 5 with a twelve minute hold at the top. Cones measure heat work, meaning temperature and time together, not temperature alone. Holding at cone 5 does more melting than racing to it, which is why a held cone 5 behaves like something between cone 5 and cone 6.
Why are some glazes more expensive?
What is in them. Cobalt, tin and some of the manufactured stains cost many times what iron and feldspar cost, and a few colors rely on genuinely scarce materials. The price of a glaze is mostly the price of its color.
Do I need to understand any of this to make good pots?
No. People made beautiful work for centuries without a word of it. What this page buys you is the ability to read a bad result instead of guessing at it, and to know which problems are yours to fix and which belong to the glaze.
Where to go next
Oxides, carbonates and ceramic stains
Where the color in all of this comes from, with the percentages that actually work.
Read it →Glaze problems, causes and fixes
The troubleshooting version of the last section, in far more detail.
Read it →Food safe, dishwasher safe, what you can claim
The four claims and what each one actually promises about a finished pot.
Read it →Mixing your own glazes
If this page made you want to weigh silica yourself, start here.
Read it →Making and logging glaze test tiles
One firing of tiles teaches more about a glaze than any amount of reading, including this.
Read it →Clay Hole University
Every lesson and guide in one place, from first day at the wheel through firing and glaze chemistry.
Start there →Sand, persuaded
Strip away the recipes and the arguing, and a glaze is sand that has been persuaded to melt at a temperature a pot can survive, and then persuaded to stay put once it has. Two problems, solved thousands of different ways over thousands of years.
Knowing that will not make your next dip better. What it does is change what you see when a pot comes out wrong. A crazed surface stops being a personal failure and becomes a fact about two materials. A dry one stops being bad luck and becomes a temperature. That is worth the ten minutes this page took.
Silica becomes the glass, flux makes it melt low enough, alumina stops it running off. Crazing is a shrinkage mismatch, matte is crystals, and food safe means fully melted, well fitted and easy to clean.
If you want the full explanation, why glazes craze, what makes one food safe and what is in our buckets, switch to the Comprehensive Version at the top of the page.
Which version would you like to print?
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