Casting from container glass

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marekpitera
Posts: 3
Joined: Thu May 04, 2023 11:58 am

Casting from container glass

Post by marekpitera »

Hi. Thanks for letting me on board! I have a few questions about an upcoming project. My main material is stoneware ceramic but I am planning new work of ceramic and glass joined when cold. The glass pieces are discs 1'' thick and 1 to 4'' wide to be cast in open plaster/silica molds in my gas fired ceramic kiln at glass temperatures. I am using container glass cullet (recycling is a part of my theme). I don't mind flaws like bubbles and small amounts of devit and minor cracks so firing does not have to be perfect. The pieces are sculptural so there is no functional issue. I have found lots of firing schedules for art glass and float but nothing about container glass. I've read that full fuse of float is 200F above bullseye but what is it for container glass? And what is the anneal tempearture of container glass? Also, I am not sure how fast I can ramp up to peak with plaster molds. If I fire up from ambient to peak AFAP to save time and energy, will I end up cracking the molds? For a 4'' disc, the largest of my molds will have 2'' depth and 7'' diameter. Thanks.
Kevin Midgley
Posts: 773
Joined: Mon Mar 10, 2003 11:36 am
Location: Tofino, British Columbia, Canada

Re: Casting from container glass

Post by Kevin Midgley »

https://www.amazon.com/Firing-Schedules ... 0578054957
That will provide you with schedules but to have an understanding of what you are doing, DO NOT just find a schedule in there without reading the whole book's notes, cover to cover a few times.
Cheat on the study of the book and you may have your glass break on its own months later.
You might be better finding some glass blower's glass batch and use that if you want consistency.
Every bottle is 'different' even from the same factory made pretty much at the same time.
Bullseye glass for one is made using broken bottles and additives are used to modify the glass to be consistent in its rates of expansion and contraction with different colours.
Without the additives and melting the post consumer waste into a homogeneous mixture, chunks of glass will not necessarily melt nicely together and will have internal stresses where the different chunks of different glass bottles touch each other.
If I was attempting this concept I would not be trying for a perfect shape and instead accept organic shaping and textures where the material is piled and partially melted together and use the all the same glass from the same bottle
You will need polarizing sheets to test your glass for stresses.
These books may help you understand glass better, written by one of the founders of Bullseye glass.
https://www.amazon.com/Kiln-Firing-Glas ... _author_dp
Mold making is another art form.
This will be a good start on learning the complexities of it all and why I suggested the "pile it on a shelf and skip the mold making idea".
https://www.amazon.com/Mould-Making-Gla ... 0044&psc=1
marekpitera
Posts: 3
Joined: Thu May 04, 2023 11:58 am

Re: Casting from container glass

Post by marekpitera »

Thanks for the tips about the books and the polariser, I will definitely use that test. Unfortunately, your pile it on a shelf idea would be unsuitable, the discs can have a slightly irregular surface profile, but the edges need to be fairly sharply right angled, the disc perfectly round and of fairly precise diameter and thickness. I can easily make models to this spec for the molds using ceramic techniques. I suspect I know more about glass than you assume I do, my ceramic course covered glass and we did mold work so I know the basics. There is a lot of overlap between glass and ceramic and I have made thousands of plaster molds (we use them in ceramics too) though I do know that there are differnces between working with pure plaster and investment mold mixes. I have a good book on glass, though not the ones you recommended. I only plan to use glass from a single source in each mold (one bottle one mold, second bottle, next mold) so I am fairly confident compatibility won't be an issue. I only need to know the peak and anneal temperatures for container glass, I am happy to take it from there. Thanks.
Kevin Midgley
Posts: 773
Joined: Mon Mar 10, 2003 11:36 am
Location: Tofino, British Columbia, Canada

Re: Casting from container glass

Post by Kevin Midgley »

Glass moves with time and temperature so you are going to have to observe what happens in Your kiln. You may be surprised at just how hot you have to take the glass to do what you are proposing to make and have it come out as a disk or puck. You most likely will not get any liquid flow until you are getting towards pottery temperatures and at which time the glass will devitrify and have surface changes. You have created a difficult challenge for yourself which you will learn from. Thus my suggestion of piling on a kiln shelf and firing to a more organic composition. If you are thinking that you will be melting crushed and powdered bottle/float glass easily you will discover that the air between all the powder acts like an insulating blanket slowing and or preventing full melting.
marekpitera
Posts: 3
Joined: Thu May 04, 2023 11:58 am

Re: Casting from container glass

Post by marekpitera »

I think trial and error is the way forward, nothing teaches like a mistake. First shot will be a few plaster molds and bisque molds at float temperatures and I will try your suggestion of a little pile as well. I could easily put them through a stoneware firing with my sculptures (plus anneal) but at peak 2400F I would be really worried about the integrity of the molds and my kiln shelves. I have fired bits of glass onto my ceramics at that temperature - you get these amazing waterfalls of glass but woe betide if even a single drip of glass falls onto a shelf at cone 10, it will eat though a thick layer of batwash and then more than half way through a 1'' cordierite shelf. If that fails, i'll buy in some bullseye frit, but I would prefer to get this to work with container glass. Thanks again!
Bob Gent
Posts: 7
Joined: Sun Apr 06, 2003 1:56 pm

Re: Casting from container glass

Post by Bob Gent »

the way to load up the shelf and get square edges is to use a stainless steel dam. I get fairly thin stainless, like 22 gage, in a 1.5" wide strip, then coil it in a round shape, then hold the ends together by overlapping them and making little clips out of the same metal. Kiln wash it and fire as many times as you like.
No reason you couldn't bend the metal into a different shape, perhaps hold inside curves or angles by backing it up with a piece of firebrick to keep it from wandering
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