Melting Glass Into Pottery, and Why Our Kilns Change the Answer
Drop a marble in a bowl, fire it, and you get a pool of colored glass. It is one of the most satisfying tricks in ceramics and one of the most commonly botched, because almost every guide online assumes a kiln far cooler than ours. Here is what glass does at our temperature, how to contain it, and what it can never be.
Glass melts thousands of degrees below where we fire. At cone 5½ it is not soft, it is liquid, so it needs a sealed well with walls to sit in. It cracks, it is sharp, and it is never food safe.
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At our temperature, glass is a liquid
Bottle glass and marbles soften somewhere around 1,300 to 1,400 F. Our glaze firings go to cone 5½, which is well over 2,200 F.
That is not a little hotter. It is roughly 900 degrees past the point where glass gives up being a solid. By the time our kiln reaches temperature, glass is not slumping gently into a shape. It is as runny as syrup and it goes wherever gravity sends it.
Which is why the whole technique here is containment. Give it a sealed well with walls, or give it a shelf you do not mind replacing.
The route that works
- Make a well with walls. A flat-bottomed bowl, a dish, a carved recess. Something that can hold liquid, because that is what it will be holding.
- Glaze the pot as normal and let it dry. Glass sits on glaze, not on bare bisque.
- Use far less than you think. A single marble or a small handful of crushed glass. It spreads much further than it looks.
- Keep it well away from the rim and further still from the foot.
- Put a catch underneath it in the kiln. Not optional on a first attempt.
- Fire it in a normal glaze load at cone 5½.
- Let it cool completely. Slower than usual, and no early peeking. Glass and clay shrink at different rates and a draft cracks it.
Decorative only. Never on a food surface and never on a rim.
Three ways it goes wrong
It ran off the pot
No walls, or too much glass, or both. At cone 5½ glass flows like syrup. A gentle dip in the surface is not a well; it needs a real lip to hold against.
It cracked, or popped off entirely
Glass and clay contract at different rates on the way down. Thick pools crack most, and cooling too fast cracks everything. Use less, and let the kiln cool right out.
It came out cloudy instead of clear
Overheated glass devitrifies: crystals grow through it and it goes scummy and dull. Most glass-in-pottery guides are written for cone 06, which is far cooler than we fire.
Our kilns are the whole problem
This is the section that makes the difference between a pot and a mess, and it is the part nearly every tutorial leaves out.
- Most guides assume cone 06. That is roughly 1,830 F, and it is where a lot of glass-in-pottery work is done because glass behaves nicely there: soft enough to pool, stiff enough to stay put.
- We fire to cone 5½, which is over 2,200 F. That is not a small difference. Advice written for cone 06 does not transfer.
- Hotter means far more fluid. Glass that would sit in a shallow dimple at cone 06 will find the lowest point of your pot at our temperature, and then keep going.
- Hotter also means thinner. The same marble that gives a deep glossy pool at low fire spreads into a thin film here, so the color reads paler and the effect is less dramatic.
- And it can go cloudy. Held hot for long enough, glass devitrifies: crystals grow through it and the clarity turns to a dull scum. That is not a firing fault, it is glass being taken past where it wants to go.
- Which is the honest summary. Glass in our kilns works, and it works differently from what you have seen online. Expect thinner, paler, runnier and more fragile than the pictures.
If you want the low-fire look
That effect belongs to a cooler firing than ours, and we do not run one for this. The version available here is a thinner, more spread out pool in a properly walled well. Worth knowing before you build a piece around a picture you saw.
The rules that keep it on the pot
Break any of these and the cost lands on a kiln shelf, which is expensive, or on somebody else's work, which is worse.
It must sit in a well with walls
Flat bottom, real sides, and a route to nowhere. A plate with a slight curve is not a well. If a spoonful of water would run off it, so will the glass.
Never on a vertical surface
The outside of a mug, the shoulder of a vase, the rim of anything. Gravity wins at 2,200 F and it wins completely.
Far less than you think
One marble in a small bowl is plenty. Glass spreads a long way at our temperature, and the usual first attempt uses three or four times too much.
A catch underneath, always
A cookie, a waster slab, something sacrificial. The whole point is that you cannot be certain, and a kiln shelf costs real money to replace.
Tell whoever loads the kiln
It changes where the piece goes on a shelf and what sits next to it. A run that reaches a neighbor's pot is the outcome nobody wants.
Nowhere near food or a mouth
Not the inside of a mug, not a serving dish, not a rim. This is a decorative technique and it does not have a functional version.
Which glass, and how much
- Marbles are the easy start. Cheap, consistent in size, and they come in colors. One marble is a surprisingly large pool once it has spread.
- Bottle glass works and is unpredictable. Green and brown bottles give lovely colors. They also carry whatever the manufacturer put in them, so results vary between bottles.
- Crushed glass gives an even layer. Easier to distribute in a shallow well than a single lump, and it melts more evenly. Crush it inside a bag, wearing eye protection.
- Avoid tempered glass. Car windows and oven doors. It is designed to shatter into thousands of pieces under stress, which is not a property you want in a kiln.
- Avoid anything with a coating or a print. Mirrors, painted bottles and anything with a metallic finish put things into the kiln atmosphere that nobody chose.
- Amount, honestly. Start with one marble in a bowl the size of your palm. If that looks thin, use two next time. Going the other way, from too much to just right, usually costs a shelf.
Step by step
- Design the well when you make the pot, not afterward. A deliberate flat-bottomed recess with a real lip, thrown or carved in at the leather-hard stage.
- Bisque as normal.
- Glaze the piece, including the well. Glass on top of glaze behaves better than glass on bare bisque, and the glaze underneath changes the color you get.
- Let the glaze dry completely before the glass goes in.
- Place the glass in the center of the well, not against a wall. It needs room to spread evenly.
- Set the piece on a catch and hand it in with a note that it contains glass.
- Fire it in a normal cone 5½ load. No special schedule going up.
- Cool slowly and completely. This is where pieces are lost. The kiln should be genuinely cold, not merely cool enough to touch.
The glaze underneath matters
Clear glass over a dark glaze reads dark and glossy. The same glass over white reads as its own color. It is worth deciding that on purpose rather than discovering it, because the glass is largely transparent once it has spread thin.
Why it cracks, and what to do about it
Cracked glass is not a mistake you made. It is two materials that disagree about how much to shrink, and the best you can do is manage it.
- Glass and clay contract at different rates. Exactly the same physics as glaze crazing, on a much thicker and more visible scale.
- Thickness is the main variable you control. A thin spread flexes with the pot. A deep pool has enough stiffness to fight it, and loses.
- Cooling speed is the other one. Fast cooling cracks glass reliably. A slow cool down to room temperature is the single best thing you can do.
- Peeking is how pieces die. Opening the kiln to look, even briefly, puts a draft of cold air onto glass that is still contracting. Wait it out.
- Some crazing in the glass is normal and can look good. A fine web inside a clear pool catches the light. It is only a problem when it turns into a piece lifting out.
- If it lifts or flakes, stop using that combination. The glass and that clay body do not get along, and no firing schedule fixes a genuine mismatch.
Sharp edges are a real hazard
Cracked or lifted glass leaves edges that will cut. Handle a failed piece the way you would handle a broken bottle, and do not put it somewhere a hand will reach in later.
What a glass piece can never be
This is a short list and none of it bends.
- Not food safe. Melted glass on a pot is not a tested glaze surface, and the fine cracks that develop in it hold onto whatever gets into them.
- Not dishwasher safe. Repeated heating and cooling is exactly the stress that cracks it.
- Not microwave safe. Uneven heating on a material that is already in tension.
- Never on a rim, and never anywhere a mouth or a hand lands by accident.
- Say so when it changes hands. A bowl looks like a bowl, and somebody will put snacks in it unless you tell them not to.
What it is genuinely good for
Trinket dishes, decorative bowls, wall pieces, spoon rests that hold a spoon rather than food, garden pieces, anything that is looked at. Within that, it does something no glaze can do, and that is the whole reason to bother.
What goes wrong, and what caused it
It ran out of the well
Too much glass, or walls that were not high enough. At our temperature it behaves like water, not like honey. Halve the amount and raise the lip.
A thin film instead of a deep pool
That is cone 5½ doing what it does. The deep glossy pools in most photographs are low-fire results. More glass makes it thicker and also more likely to crack.
Cloudy, dull or scummy
Devitrification, from being held too hot for too long. There is no fix in the schedule we run; a different glass sometimes behaves better.
Cracked all the way through
Too thick, cooled too fast, or both. Thinner next time, and let the kiln go completely cold before it is opened.
The glass lifted off the glaze
A genuine mismatch between that glass and that clay. Try a different glass rather than adjusting anything else.
Bubbles trapped in the pool
Usually air escaping from the glaze underneath as the glass sealed over it. A thinner glaze coat under the well helps. Some people like the bubbles.
The ones that come up at the tables
Can I put glass in a piece fired here?
Yes, with a properly walled well, a small amount of glass, and a catch underneath. Tell whoever is loading the kiln, because it changes where the piece goes.
Why does mine look thinner than the photos online?
Because most of those photos are low-fire results at around cone 06. We fire to cone 5½, which is several hundred degrees hotter, so the same glass spreads further and reads thinner and paler.
Can I use broken bottles or jars?
Yes. Green and brown bottle glass gives good colors, and results vary between bottles because the recipes do. Avoid tempered glass entirely, and avoid anything printed, painted or mirrored.
Why did my glass crack?
Glass and clay contract at different rates as they cool, so thick pools crack and fast cooling cracks everything. Use less glass and let the kiln cool right down. Expect some fine crazing inside the pool as normal.
Is a bowl with glass in it safe to eat from?
No. It is not a tested glaze surface, the cracks that develop hold onto what gets into them, and cracked glass has edges. Decorative only, and worth saying out loud when the piece changes hands.
Does the glaze underneath matter?
A lot. Once glass spreads thin it is largely transparent, so what it lies over is most of what you see. Clear glass over dark glaze reads dark and glossy; the same glass over white shows its own color.
Can I put glass on the outside of a mug?
No. On any vertical surface it runs, and at our temperature it runs a long way. The technique only works in something that could hold liquid.
Why does it come out cloudy?
Devitrification. Crystals grow through glass that has been held hot for a long time, and the clarity turns to a dull scum. It is a consequence of our firing temperature rather than a mistake, and some glass resists it better than others.
Where to go next
What pottery glaze actually is
Glaze is glass too, made to melt at the right temperature and to fit the clay. This explains why that matters.
Read it →Food safe, dishwasher safe, what you can claim
Why a decorative surface stays decorative, and how to describe a piece honestly.
Read it →The risks every potter should know
Crushing glass and handling cracked pieces are both jobs for eye protection.
Read it →Reglazing and refiring
What can and cannot be rescued when a firing does not go your way.
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 →Worth doing, with your eyes open
A pool of melted glass in the bottom of a bowl does something no glaze does. It has depth, it catches light from underneath, and it looks like an accident that went well. People pick those pieces up.
The reason this page spends so long on temperature is that almost everything written about the technique assumes a cooler kiln than ours, and following it here produces a thin pale film, a cracked pool, or a shelf that has to be replaced. Build a real well, use half what you think, put a catch under it, and let the kiln go cold.
Glass is liquid at cone 5½, so it needs a walled well and a catch underneath. Use far less than you think, cool it slowly, and never put it near food or a rim.
If you want the full picture, including which glass to use and why it cracks, switch to the Comprehensive Version at the top of the page.
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