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Firing schedule for glass on stoneware


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I have been experimenting with melting glass in the bottom of glaze fired bowls. Example attached. I had to do a lot of experimentation to come up with the firing schedule that melted like I wanted it to. 

I modified a very complex glass fusing schedule. My guess is that glass fusing is a much more delicate process than just putting some glaze in the bottom of a glaze fired bowl and re-cooking it to 1850°F. I would like to come up with a much simpler shorter firing schedule.

Has anyone tried this and what firing schedule did you find worked?

DSC05328 (Small).JPG

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16 hours ago, docweathers said:

My guess is that glass fusing is a much more delicate process than just putting some glaze in the bottom of a glaze fired bowl and re-cooking it to 1850°F.

This is a huge understatement.

While glass and glaze are related, they’re not the same thing. If you like a good research rabbit hole, there are lots of good references out there on firing times for glass slumping, and the phase changes that are important for glass. There’s more of them than we usually concern ourselves with in ceramics. If you’re firing glass to 1850F, it’s gone way too hot, and it’s breaking down chemically. You’ve done the equivalent of firing earthenware to cone 10.  If you’re using stained glass fragments, this can be an issue because colours will alter. And because stained glass is made at much lower temperatures than ceramic, they can use pigments like arsenic.

Even in the event you’re using some form of soda glass with no pigment (bottle glass, windows, clear marbles, etc), you’re adding a whole mess of sodium, calcium and silica to a concentrated area on top of the glaze. Because the 2 will be melting together, this’ll throw the glaze way out of balance. The RO:R2O flux balance will be well into non-functional territory, and the lack of alumina is going to further affect COE and durability.


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Very interesting comments.......... but the pictured pot was.

1. Fired to1850F

2. The colors of the stained glass did not change from pre-firing to  post firing.

3. There are no cracks whatsoever in the glass. It has been about a month since firing so I would think any spontaneous shivering would have happened. One other similar bowl fired at the same time with a little different glass colors has some extremely small cracks. I will be tracking down the kind of glass that caused the tiny cracks and not use it again.

4. I am just a beginning hobbyist potter. I don't sell my stuff, so they will never be used as serving bowls.

This is the first time I've tried this kind of stained-glass pottery application. I will be working to refine it further.

Do you think the voodoo dance that I did around the kiln is responsible  for my relative success.:P

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19 minutes ago, docweathers said:

There are no cracks whatsoever in the glass. It has been about a month since firing ...

Have you tried an indian-ink test? Easy enough to wipe off if there are no cracks.

My first crackle glaze was pre internet and I very much had to guess the recipe. Came out without a crack in sight. Thought well I've got the indian ink so lets see, lo and behold lots of crackle.

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21 hours ago, docweathers said:

There are some very small fine cracks.


1 hour ago, docweathers said:

3. There are no cracks whatsoever in the glass.

You lost me here. 

I believe people are just trying to point out the risk you are taking of both  glass splinters coming off the bowls and the probable toxicity of some of the glass. If that orange is melted glass I would add cadmium sulfide and selenium to the ones Callie mentioned.

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On 9/30/2021 at 10:43 PM, docweathers said:

It has been about a month since firing so I would think any spontaneous shivering would have happened.

I'd give it a good six months before declaring them anywhere near safe.  Hot/cold/dry/damp a whole weather and heating systems worth.  

And I still wouldn't do it.  To me it's like mixing petrol and diesel in my car.

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I have no plans of ever considering them food safe. They are only decorations never intended for anything else.


As a side note. I have decided to go back to firing both glaze and glass at ^6.  Comparing ones that I've done at 1850 Fahrenheit and the ones that I've done at ^6 there is only a slight difference in flow and no difference in color. Firing them all at once is a lot simpler.

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Okay. I don’t think you’re going to hear this, or the idea that adding stained glass to a piece that could be misconstrued as functional, even though its for personal use is a bad one. It sets a bad example to people who don’t know better. But I’m putting this out there for any other readers of this forum thread who are seeking information on the subject.

Assuming that someone is looking to do a similar project, and they have access to stained glass either by buying new material, or using up scrap glass from a stained glass artist or school. Here’s some information to consider.

There are stained glass products out there designed specifically for slumping and fusing, and that have the same COE so they can be used together.  System 96 is stained glass with a COE of 96. Lotsa really pretty colours, and some sheets that are even recommended for slumping, because the strike fire brings out more interesting effects. It’s readily available, and all stained glass suppliers have some on hand. So it’s a logical assumption that this would be the product used. The two suppliers that used to make it, Orobouros and Spectrum seem to have been bought up by Oceanside Glass and Tile. These products are intended to be slumped and fused at a top temperature of 1470F (see pages 11-14 of this instructional pdf from Oceanside).  According to the Safety Information Sheet supplied for “warm colours” from the website:

“The chemicals used to produce this product are in a glass matrix and are, therefore, not available to the environment unless the glass is heated to its melting point or ground to an extremely fine particle size” (emphasis mine).

The sheet also states the melting point to be 2000F. It states that there are quantities of selenium (<.25%) and cadmium (<.6%) compounds in those colours that they’re obliged to report on. 

Cone 6 is 2194F at the bottom end, as it measures heat work, not just temperature.

Extra facts: Calculated expansion (which is inaccurate, but the best we have) of the first 2 glazes on the dashboard of my Insight account are in the 60-70 range. As little 3-4 points of difference can effect glaze fit on a clay body, depending.

And one last quote from the FAQ section of Delphi Glass on System 96 glass:

“Can Spectrum Glass be used for dinnerware? 
Spectrum products have been tested for chemical leaching as required by the FDA for food bearing surfaces. All of our products passed and were determined to be suitable.

However, when you use Spectrum glass to produce a product of your own (slump it, fuse it, foil it, lead it, etc.), it’s not Spectrum glass anymore. It’s your product now, and as such, must pass all tests before being sold or used as a food bearing surface. It is possible that the processes you use to make your product alter the composition of the raw materials (the glass) in such a way that they may no longer meet the required standards. Either way, the regulations are clear: You must have your own finished products tested and approved.”





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I do hear you loud and clear.


On the bottom of each pot, written in large letters is "not food safe". My family is only my wife and I. We both our PhD's so I think were smart enough to not use these for food.


Maybe I should take Babs advice " bury your bowls when they bury your boots!"

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On 10/5/2021 at 3:06 PM, docweathers said:

Maybe I should take Babs advice " bury your bowls when they bury your boots!"

That’s often the unpredictable issue. Non PhD folks may find it counterintuitive to look at the bottom of the bowl before they use the top of it. Once out of your direct supervision and control, well ………. It’s just out of control. The label may be ineffective for the new user. Pretty common actually as wares are handed down, gifted or eventually sold.

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Further to @Callie Beller Diesels excellent comments, glass fusers know not to mix COE 96 with COE 90 glass. Because they don't fit together.

And many potters know that using clay and glaze with non-matching COE can cause shivering or crazing.  Because they don't fit together.

Mixing glass and clay is just asking for problems down the line.  I have unjoined many clay forums in the past due to a constant non-belief by some members that the two are just incompatible.

There is a lifetime of testing available with glaze that is designed to work with clay, why introduce an element that is known to be incompatible.


Edited to add, using the "next" button has taken me to a post on shivering glazes.  Just goes to show there are enough "fit" problems with glaze, without adding glass into the mix!

Edited by Chilly
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