Warm Glass

[Norm Stuart]

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"

https://www.amazon.com/Clay-Molds-Warm-Glass-Burton-ebook/dp/B00FBP...

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?

[Robert Coyle]

The argon/CO2 is mostly inert gas... It is NOT either and oxidizing or a reducing atmosphere. It inhibited any sort of surface chemical reaction from taking place. So metals do not take on an oxidation layer. Or if you had an already oxidized metal... say CuO, it would not reduce it down to Cu metal. Nor would it remove the oxidation layer off of your kiln elements. 

So ( in theory) you could have shiny copper, or silver metal fused into the glass Without causing discoloration due to  the creation of soluble oxides in the presence of air... or have a stainless steel inside mold that would not leave an oxidation ring on the slumped glass.

This is all "theory" Karen, and until some body tries it, it remains such. As a chemist, I ran a good number if reactions under Argon gas to prevent oxidation, but in a much more controlled environment and at much lower temperatures.

[Norm Stuart]

Karen,  If you have not misted fused glass with a solution of Borax, I think you'd be impressed with the results. With some experience now I've found after a tacking fire followed by a slump firing many transluscent colors lose their sheen. Misting them first with water with as much Borax as will dissolve adds sodium and boron flux to the surface of the glass.

This very inexpensive mixture is sold at a hefty cost as "Devitrification Spray" and "Super Spray". Boron is a glass as well as a flux and the Sodium and Boron in the Borax is fully melted and bonded as part of the Bulkseye glass, likely well below our speak slumping temperature of 1,050.

It's likely our 1,050 F is actually a true 1,250 or 1,350. I needed to adust our temperatures down to acieve the various tack, fuse and full-fuse results and also slow the heating ramp to avoid cracks. It's likely this could be the result of using a spacer which leaves only 3/4 inch of our platinum thermocouple tube exposed in our kiln to make loading and unloading the kiln easier. The drawback is the thermocouple perhaps responds to temperature changes slower than it would if a full 1.5 inches were exposed to the heat.

I plan to try this spray on cone 6 glazes when they are subsequently refired to cone 06 after adding low-fire glaze.  This second firing often leaves the cone 6 glaze somewhat dull from devitrification.

I'm also impressed with the shockingly costly ZYP boron nitride release spray. While it's $54 per 13 oz aeresol can. I tried another brand Slide Hi-Temp 1800 for half the price which really doesn't work. They're both boron nitride with 1% talc and a butane gas, but somehow the implementation is quite different.

I've also tried three other shelf and mold resists:

1.)  Calcium carbonate on flat shelves as a resist but i found I needed to wet it first to tool it flat.

2.)  Alumina, suggested by a clerk at a Bullseye store, but I can only guess he had never tried that himself as it embeds into the glass, frosting it completely.

3.) Also the commercially sold kalin with pottery plaster resist, but did not like repairing the flaking after each firing. I mixed my own using calcined kaolin but the Glomax brand calcined kaolin left too much texture on the glass due to granularity.

I was interested in a video on YouTube showing the lost wax casting of a three inch thick globe.  Likely to release bubbles and annealing time the video showed the artist starting the kiln then unloading it, with a caption "Three Weeks Later" ! !  Wow. Dicroic clear does help prevent bubble trapping as the thin layer of metal resists fusing for a short period of time after the glass is plastic.

Thanks for your tips when we were first getting started.

KarenB said:

Norm, 

>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. 
http://www.clearwaterglass.com/Tutorials/Devitrification.html
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!