Surface and decoration

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 starts to soften roughly 800 to 900 degrees below where we fire. At cone 5½ it is not soft, it is liquid, so it needs a well with real walls to sit in. It cracks, it is sharp, and it is never food safe.

Why most guides do not apply here Containing it, and cooling it Two videos, neither of them ours
The rules that keep it on the pot What it can never be Three ways it goes wrong
An illustration of a rough clay dish with a pool of teal glass in a walled hollow, and a smaller dish beside it where blue glass has run over the edge.
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Our kilns are the whole problem The rules that keep it on the pot Which glass, and how much Doing it, step by step Why it cracks, and what to do What it can never be What goes wrong The whole idea, quickly The route that works Three ways it goes wrong Questions we get asked Keep going

What glass does at our temperature, how to contain it, and what it can never be.

The whole idea

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 about 2,200 F.

That is not a little hotter. It is roughly 800 to 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 well with real walls and a catch underneath.

Start to finish

The route that works

The Truth about Mixing Glass and Clay - Tips, Tricks, Risks, and Warnings! Jonthepotter
  1. 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.
  2. Glaze the pot as normal and let it dry. Glass behaves better on glaze than on bare bisque.
  3. Use far less than you think. Start with one marble in a bowl the size of your palm. It spreads much further than it looks.
  4. Keep it well away from the rim and further still from the foot.
  5. Hand it in on a catch. Every piece, every time, not only the first.
  6. We fire it in a normal glaze load at cone 5½.
  7. Let it cool completely. Staff unload the kiln, and we never open one early, because glass and clay shrink at different rates and a draft cracks it.

Decorative only. Never on a food surface and never on a rim.

If it went wrong

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 leave the cooling to us.

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.

The heat

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.

Melting Glass Bottles on Pottery - What is the best temperature? Justins Makery One maker fires glass bottles at eight different cones to find the best temperature. The chapters name cone 017 and cone 015 results, far cooler than our firing, so use it to see how glass behaves, not what to expect here.
  1. 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.
  2. We fire to cone 5½, which is about 2,200 F. That is not a small difference. Advice written for cone 06 does not transfer.
  3. 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.
  4. 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.
  5. 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.
  6. Which sums it up. 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.

Non-negotiable

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 kiln cookie goes under every piece we fire, and glass runs, so a glass piece needs a glaze catcher under it as well. The whole point is that you cannot be certain, and a kiln shelf costs real money to replace.

Tell staff when you hand it in

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.

Sourcing

Which glass, and how much

Making "Art" with marbles in a pottery kiln Justins Makery Marbles melted into pottery. The description says the pottery was fired to cone 5 and the marbles to cone 016, which is a different method from ours. The chapters cover fragility and failures after firing.
  1. Marbles are the easy start. Cheap, consistent in size, and they come in colors. Skip the red, orange and yellow ones, which can carry lead or cadmium. One marble is a surprisingly large pool once it has spread.
  2. 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.
  3. 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 thick cloth bag, wearing eye protection and gloves, and an N95 or better for the dust. Wipe up every sliver afterward with a damp cloth, not your fingers, and never dry sweep.
  4. 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.
  5. Avoid anything with a coating or a print. Mirrors, painted bottles and anything with a metallic finish can give off fumes and leave metal on other people's work. Lead crystal, and red, orange and yellow glass, can carry lead or cadmium, so leave those out too.
  6. Amount. 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.
Doing it

Step by step

  1. 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.
  2. Bisque as normal.
  3. 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.
  4. Let the glaze dry completely before the glass goes in.
  5. Place the glass in the center of the well, not against a wall. It needs room to spread evenly.
  6. Set the piece on a catch and hand it in with a note that it contains glass.
  7. We fire it in a normal cone 5½ load. No special schedule going up.
  8. Let it cool completely. This is where pieces are lost, and it is the one step you cannot do yourself. Staff unload the kiln, and we never open a hot kiln early for anyone.

The glaze underneath matters

Colored glass over a dark glaze reads dark and glossy. The same glass over white shows its true color. It is worth deciding that on purpose rather than discovering it, because the glass is largely transparent once it has spread thin.

The failure you should expect

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.

  1. Glass and clay contract at different rates. Exactly the same physics as glaze crazing, on a much thicker and more visible scale.
  2. 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.
  3. Cooling speed is the other one. Fast cooling cracks glass reliably, and the cooling is ours to manage, not yours. A slow, undisturbed cool down is the best protection, which is why staff unload only when the kiln is cool.
  4. An early peek is how pieces die. Opening a kiln to look, even briefly, puts a draft of cold air onto glass that is still contracting. That is why we never open one early, for anyone.
  5. 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.
  6. 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.

Before you gift it or sell it

What a glass piece can never be

This is a short list and none of it bends.

  1. 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.
  2. Not dishwasher safe. Repeated heating and cooling is exactly the stress that cracks it.
  3. Not microwave safe. Uneven heating on a material that is already in tension.
  4. Never on a rim, and never anywhere a mouth or a hand lands by accident.
  5. 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, garden pieces, anything that is looked at and not eaten from. Within that, it does something no glaze can do, and that is the whole reason to bother.

Read this after a bad one

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 runs like syrup, so it keeps going until something stops it. 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 less glass. We never open a hot kiln early.

The glass lifted off the glaze

A genuine mismatch between that glass and the glaze or clay under it. Try a different glass rather than adjusting anything else.

Bubbles trapped in the pool

Usually gas escaping from the glaze and clay underneath as the glass sealed over it. A thinner glaze coat under the well helps. Some people like the bubbles.

Questions we get asked

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 staff when you hand it in, 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, and leave out lead crystal.

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 be patient while the kiln cools. 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 telling whoever receives the piece.

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. Colored glass over dark glaze reads dark and glossy; the same glass over white shows its true 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.

One last thing

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 be patient while the kiln cools.

Glass is liquid at cone 5½, so it needs a walled well and a catch underneath. Use far less than you think, leave the cooling to us, 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.