Has anyone else fused frit into a glass bowl?

We've been asked to do a glass project, which is apparently called "warm glass" being fired only to 1,250 to 1,450 F with an hour long annealing hold at 970 as it cools.  I'm curious if anyone else has experience with this.

¿ Perhaps I should post this on Cone 016 Pots ?

I've tried different COE glass and settled on COE 90. After reading the booklet "Clay Molds for Warm Glass"


I've learned the author, an experienced ceramicist, finally settled on using Paper Clay with Grog to make the glass molds.

Glass, like most glaze, has a higher COE than clay - so you can melt glass in a bisque container and the glass cools smaller than the ceramic so lifts out, particularly if the ceramic is coated with glass kiln wash. I've found calcium carbonate (whiting) is the best glass kiln wash - much better than the commercial glass shelf wash, which is kaolin with a little bit of plaster.  I've also discovered glass is quite a bit more expensive than ceramic and making your own "batch glass" ceramic glaze.

The problem with fusing a bowl, is the interior.  Since glass shrinks more than ceramic as it cools, it separates from the ceramic bowl it sits in, but if I place a ceramic insert to form the interior of the glass bowl - the glass, like glaze, will shrink around the clay making extraction impossible.

The author of the booklet Ava Burton said she uses wire mesh covered in a refractory fabric to form the interior, but I think there has to be a more elegant solution. Metal has a higher COE than glass, but it has to be a metal which can withstand corrosion at 1,450 F temperature and not transfer metal coloring to the glass.

Barton simply makes the observation that glass sticks to some things but not to other things without taking the observation further. I understand how to slump a bowl from a glass sheet, but we want to make a thicker fused bowl.

Has anyone else made a fused glass bowl in your kiln, and how did you form the interior?

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Mr. Stuart, 

I have no experience with this, but fellow member Denise Mendez may be able to help, if you can get a hold of her.  She knows her glass.  Also, thank you for the Cone 016 Pots joke!  That, sir, was funny.  Good luck.

Hi Norm, 

Has anyone else made a fused glass bowl in your kiln, and how did you form the interior?

It depends somewhat on what you want the interior to be formed into.
And how many layers of glass you want to work with.  When fired, glass wants to make itself 6mm thick. So, if it is thicker or thinner, you will probably get movement (spreading or contracting) of the glass as it tries to get to 6mm thickness. 

Also, you will need to use a product similar to kiln wash to keep the glass from sticking to the kiln shelf and/or the mold you slump it into.

Typically making a fused glass bowl is a two firings process if using more than one layer or adding frit, design elements, etc. 

1 - to "full fuse" the layers of glass and/or the embellishments, designs, etc., permanently together and soften the edges,of the piece you will usually need 1450 F .
If you just want a "tack fuse" which means they are stuck together but not melted together and edges are still sharp.
use a lower temp. See this link for more detailed info https://www.bullseyeglass.com/images/stories/bullseye/PDF/TechNotes... 

2 - to shape the already fused piece into a bowl form, you would of course need a bowl shaped mold. This step is called slumping and done at a lower temp than fusing. Here is a link to more info on slumping.

By the way... I am not affiliated with Bullseye glass in any way. They do have some very helpful info on their website though. 

Hope this is helpful... please let us know how it goes. 
And, if you have any more questions, I'd be happy to try to help. 


I have fired fused/slumped glass in my small kiln. As far as I can see, it worked as well as that done in the dedicated kiln.

The ramp is very different and fast. you can make your own slump molds from bisqued clay. The biggest mistake I made on fusing the glass, was to try to fuse too thin... bought the thinnest sheet from Bullseye... had to double it. The glass needs to be close to 3/16 " thick to get good fusion and surface tension roll on the edges.

I fooled around with the cycle a couple of times and got what I wanted. The stuff I made was simple two layer fusions. Probably doing big fancy stuff  would do better with the correct element placement.

Threw some simple slump molds on the wheel from cone 5 B-mix and they worked fine for an outside concave slump.

I Used stainless steel cylinders for convex with some of the expensive spray on release.  Worked OK but after a few uses it left a dark ring where it touched the glass.

Robert, that's interesting about the dark ring problem you had. 
I'm curious, where was it on the glass and was it consistently in the same place?
Did it seem to matter how much release you put on?


The dark ring was due to oxidation of the stainless steel. Even stainless will oxidize at slump temperatures and leave an iron oxide ring on the inside where it touches the glass.  Tried coating the top edge with kiln wash or a wrap of kiln paper, worked pretty good but still happened some times. Tried sanding back down to metal...this also helped.

I did not get too involved in this for long, since we were just seeing what we could do with glass without too much expense, in the existing ceramic kiln. Made some slumped glass candle holders for sale at the local senior center using this method. Not a big problem since the candle covered most of the bottom where the ring was.

Thank you Karen and Robert.  I've lots of questions.

When do you need to add the borax (aka Devit Spray) to maintain a clear glass surface?

I already discovered the metal oxide transfer, even with kiln wash.

I've used commercial glass wash, which I'm told is 2/3 kaolin and 1/3 pottery plaster.

I've also used calcium carbonate which is more work but seems to provide a smoother surface.

I use white wash (calcium hydroxide) on the shelf to provide a base layer, then apply whiting mixed with water with a plastic rib to a super-smooth surface. With some initial application like the white wash, the calcium carbonate really doesn't like to adhere to cordeirite shelves.

The fact that Bullseye glass likes to be 6.8 mm thick is pretty clear, so we'll be using 3 mm sheets for most projects.

Personally I wanted to try using nuggets / pebbles to makes a bowl more like this one on the cover of this book, but in reading the description it mentions "cold working" which I assume must mean a lot of grinding which we're not set-up for.

I understand in concept larger "nuggets" will create fewer bubbles than frit or smaller glass pieces, but I'm quickly seeing how it might be even better to work with Billets - the 5" x 10" x 3/4" pieces.

So in theory I suppose something like this bowl to the left is made by placing the glass (billet or nuggets) into a ceramic bowl and forming the interior, necessarily due to the COE, with a piece of metal weighted down so it doesn't float on the molten glass.  Finally grind off the metal oxidation transfer, and polish?

Unless someone can think of a more direct method.

In my imagination I assume firing the glass to a temperature above the typical 1,450 fusing temperature would lower the glass viscosity and help any bubbles to migrate up and out of the glass. But it's perhaps likely the glass might bond to the ceramic at higher temperatures like 1,900 or so, and certainly start to incorporate the kiln wash rather than be separate from it - something to try for sure, unless someone else here has already.

In making my own slump molds for 6 mm glass sheets, I'm also discovering there's limits to the plasticity of the glass, where the glass sheet separates into several pieces if the angle of the form is too great, which may mean I need to lower the temperature of the pre-programmed firings, rather than change the shape of the mold.


>The fact that Bullseye glass likes to be 6.8 mm thick is pretty clear, so we'll be using 3 mm sheets for most projects.

It's not just Bullseye glass that wants to be 6mm thick.
Bullseye glass is great, I personally am fond of Uroboros, but have used both.

>When do you need to add the borax (aka Devit Spray) to maintain a clear glass surface?

Devit is applied before fuse firing.
Here's a great link to info about devitrification and how to avoid it. 
It's very important that the glass is super clean before firing. No fingerprints.  
And that your kiln is clean, the atmosphere inside can cause devit too.

Just to clarify - there's really no reason you can'/shouldn't use a ceramic kiln for glass.
It's just that glass kilns usually have a lower temp range and they often have elements in the lid which helps to heat evenly.  A drawback being that kiln bits from the lid can sometimes fall onto the glass pieces and ruin them.
I use a Skutt kiln that is actually a ceramic kiln, no elements in the lid and it works just fine with a fairly slow ramp. 

>I'm quickly seeing how it might be even better to work with Billets - the 5" x 10" x 3/4" pieces.

I have not tried this, but, it seems you could get some great results by fusing some billets together (edge to edge) to form a larger than 5"x10" thick sheet, then slump it into your mold. 
When fusing you would need to use a dam to keep the 3/4" thickness which is basically done by building a "container" on the perimeter to hold the glass' outer edge from spreading. This can be done by using kiln shelf stilts horizontally on the shelf and lining the edge with kiln shelf paper (made for glass). You would need to be sure the perimeter is heavy enough (adding a second 'reinforcement perimeter is a good idea) to keep the glass from spreading and that it meets the kiln shelf squarely, otherwise the glass is likely to ooze a bit at the bottom. 

Regarding bubbles, long soak is one way to reduce bubbles.

>limits to the plasticity of the glass, where the glass sheet separates into several pieces if the angle of the form is too great, which may mean I need to lower the temperature of the pre-programmed firings, rather than change the shape of the mold.

I would agree, lowering the temp would be a good idea. You don't need full fuse temp in order to slump into a mold.

Hope this helps!

You have gone far beyond what I tried Norm. The people at Bullseye here were very helpful. I think you need some advice from people who have already done some of these things and know about  slump time/temperature and annealing time for thicker glass, which becomes very important.

Excellent point Robert... thicker glass annealing is another important factor.
And, Bullseye folks are a great resource!

Thanks. Our glass supplier told me we're going to need to "feel our way through the learning process" which seemed sensible advice.

With many failures along the way I'm making only one example of each variation of method and material per firing. Fortunately the low firing temperatures mean firing costs are extremely low, comparable to a gilding firing.

The pre-programmed annealing in our Cress E23 is one hour at 1,000 F and an additional hour at 970 F, while the annealing program for glass beads is eight hours at 960 F. I'll have to find out from Bullseye if these times need to be adjusted for their glass Billets.

It's interesting to note that the process of making float glass on molten tin was first patented in the United States in 1902 and again in 1925. But the process of producing float glass on molten tin was never perfected and used commercially until 1958.

After millions of Pounds of investment and six years of work by Sir Alastair Pilkington and Kenneth Bickerstaff the manufacture of float glass finally became practical.

The process depends on an atmosphere of 90% nitrogen / 10% hydrogen, with the hydrogen as an oxygen scavenger, to prevent the tin from oxidizing and transferring tin oxide to the glass. In metal, the hydrogen would lead to faults, but it's apparently fine for glass.

Short of changing the atmosphere of our kiln it seems this transfer of metal oxide to glass greatly limits the use of metal molds to slump glass over. My understanding is due to the COE difference glass can only be slumped into ceramic.

So it was interesting to read many reviews of Delphi "glass button molds".

Users were complaining the prongs which form the button holes break after the first use. There may be a solution, but it seems an obvious COE problem upon reflection.

I've read ceramic molds should be repaired with pottery plaster prior to the next use, but it doesn't seem it would be strong enough in this type of mold.

I'd like to try the ZYP type Boron Nitride spray as a release, but the $50 price for a spray can puts me off when a 50 pound bag of calcium carbonate is just $12. For some reason they all use 3% talc in addition to the 97% boron nitride - there must be a reason but I can't think of what it would be.

It's interesting too in the Pilkington Float glass process, the thickness of the glass, after the molten glass flows out of the gate onto the molten tin, is controlled by width restriction of the tin bath and also rotation speed of the gear-toothed wheels which dig into the glass on either side of the tin bed - speeding up to produce thinner glass.

The float glass process for very thin and strong glass for computers and phones relies upon a different glass recipe which naturally flows out to a thickness of less than one millimeter. The "hardening process" of Corning Gorilla Glass floats the glass over molten Potassium, where through ion exchange the much larger potassium atoms take the place of sodium atoms on the surface of the glass. These potassium atoms in the surface glass matrix place the glass under a high stress temper without the rapid cooling used to temper Pyrex borosilicate glass.

The sulfur in the sodium sulfide used in the glass melt apparently acts as a clarifying agent, making metallic impurities transparent and I think does something to help expel bubbles from the glass melt. Manganese and lead seem to act similarly, but sulfur is far less toxic.

I bought some sodium sulfide to try out in making Cone 6 clear glazes less yellow.  There may be other glass technology concepts as well which could proved useful in glaze making.

Float glass lines operate 24 hours a day for 16 years or so until the plant is rebuilt.

Quite an amazing process!

Since I also do mig welding, I have often thought of the trouble it would eliminate if I were to seal my kiln a little better and introduce argon/CO2 during the run. All you would need is enough to displace the air throughout the run. Not much really.

I will do this down the line, as I do see it as a viable approach, especially as I would like to incorporate metal into the clay sculpture.

Anyway, this probably would help the oxide ring problem.

Thanks for the great technical info guys!
Way beyond my chemical knowledge.... but very interesting!

Would you be willing to help me understand how that would be different than firing in reduction?

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