I need castable mould materials to melt plate glass into a disc.

[Aaron Solt]

Hello.



I want to melt plate glass down to several plate glass disks 11" diameter and 5/8" thick, with no visible bubbles.


I want to use a mould in a kiln to do it. I'm aware that I can purchase glass disks for $75. 90% of the joy is building it myself. I'm reading many posts to educate myself but need a few pointers.


Thank you!


...
I also need a way to smooth the mould so the glass does not get down in the grooves and grab the mould. Glaze? Or just mould to something smooth?

[Aaron Solt]

So far I've found dental stone and its generic, but I don't know what other options there are or how much it will shrink or expand during curing.


I read that construction cement will explode at high temperature.

[Aaron Solt]

I read that plate melts at 1800 F. Is that true?

How long do I need to keep a stack of 4 half inch 5.5x5.5" sheets at 1900 F to get out the bubbles and have the glass smoothly and flatly fill an 11" round mould? Maybe that depends on the thermal mass of the mould and whether it is warmed up.

Will plate glass explode if heated at 2-3x the schedule speed? I don't care if it merely cracks on the way up. I'll cool it much more slowly. Maybe I should slowly soften it, then zoom up.

I read that there is a heat schedule book out there. Does it go into melts of this magnitude?

Thanks.

[Kevin Midgley]

optically bond them and then grind your telescope lens.
You would want to use some optically clear glass as well not float glass.

[Ed Cantarella]

OP - Google glass table tops - Someone will custom cut a piece that size for less than $75 each. Maybe double down and get a few to save on shipping.
Unsolicited advice - don't try this(molding your own). Nobody will 'post' you up the total learning curve required.

[jolly]

You need to tell us what your final outcome is meant to be. If you are looking to make a telescope, it won't work with float glass.

[Aaron Solt]

I've read over 100 topic related forum posts since posting here.

I also plan to anneal it, but that is a different thread perhaps, or just a book page.


I've edited my above posts to be more specific. I'm looking for specific castable refractor sources. Dental stone is the is the main one.

[Aaron Solt]

Unsolicited advice - don't try this(molding your own). Nobody will 'post' you up the total learning curve required.

I'll do most the reading myself. I just need some keywords, leads, and general answers as to what I'm up against.

I have a lot of other knowledge. These are just my current sticking points.

[Aaron Solt]

OP - Google glass table tops - Someone will custom cut a piece that size for less than $75 each. Maybe double down and get a few to save on shipping.

Yes, I know about those. I want to make my own. I love building. I bored by buying the final product. Design is 2/3 the fun.

[Aaron Solt]

optically bond them and then grind your telescope lens.
You would want to use some optically clear glass as well not float glass.

It is not a telescope lens. Plate glass has a lower melting point than other glasses and is cheap. I don't need help with what it will be used for. I just need castable materials to mould squares into circles.
Thanks.

[jolly]

I still don't understand. Are you going to cut out rounds out of thin float glass (windows) and then stack it and fuse it, hopefully without bubbles? What thickness of glass are you starting with? Why do you want to cast it and not just fuse it? Heating window glass that hot to make it more liquid is probably going to change its properties and probably devitrify. You may have to add some kind of flux but you would have to find someone who melts and pours glass because it is beyond my temperature range.

[Aaron Solt]

I still don't understand. Are you going to cut out rounds out of thin float glass (windows) and then stack it and fuse it, hopefully without bubbles? What thickness of glass are you starting with? Why do you want to cast it and not just fuse it? Heating window glass that hot to make it more liquid is probably going to change its properties and probably devitrify. You may have to add some kind of flux but you would have to find someone who melts and pours glass because it is beyond my temperature range.

I plan to buy 1/2 inch plate glass and cut it into squares and stack them in a mould and melt them to spread out over the round mould. I don't want to use a ladle. I'll just slowly cool it till solid.


Would sitting glass with an 1800 deg melt temp at 1900 degree cause the visible bubbles to rise to the surface as it spreads out, or would bubbles still stick to the bottom? Also, would dental stone and similar be a good choice?

[Aaron Solt]

I still don't understand. Are you going to cut out rounds out of thin float glass (windows) and then stack it and fuse it, hopefully without bubbles? What thickness of glass are you starting with? Why do you want to cast it and not just fuse it? Heating window glass that hot to make it more liquid is probably going to change its properties and probably devitrify. You may have to add some kind of flux but you would have to find someone who melts and pours glass because it is beyond my temperature range.

Cutting circles makes more scrap. Also moulding it lets make more shapes.

I get the feeling that 1800-1900 is hot for most kilns, and that y'all are trying to find a cooler way for me to reach my goal. If you do moulding, you buy Crystal or something with flux in it.

I'm not asking you to solve my problem. I'm asking for a few basics that will help me understand what I'm up against.


You said your kiln can't handle 1900 deg. Please state its design and what the primary mode of failure is. Thanks.

Do kiln bricks get soft well below their melting point?

[Aaron Solt]

Well, I read up on devitrification. Basically need to keep the glass and kiln clean and follow firing schedules so the glass does not crystalize.

Also, high temps wear out kilns in several ways.

There are lots of firebrick formulations on the market, each attempting to get improvement.

Artists buy glass that has melting point lowering compound added, as well as color. So melting plate glass exceeds what many are familiar with.


Id like to just find a reliable melting temp for plate glass. So many sources disagree.

[Ed Cantarella]

Devitrification is not about cleanliness, it's about the glass being at a high temp for too long.
No, bubbles won't float up through the glass like magic, the small bubbles could take days to reach the top, if ever.
1900f is not high temp for a ceramic kiln or a mixed media kiln. Those go to 2300f

Thanks for informing us on what we don't understand because we just don't.

[Aaron Solt]

Thanks for informing us on what we don't understand because we just don't.

I was merely listing what I had to go find or read between the lines due to the silence here. I never said you did not know it. Thank you for telling me that clay kilns go up to 2300 F. I also read that Crystal uses lead or other additives and is not what I want.





I see why no one has told me the melting point of plate glass. It is because glass is not a solid and has no melting point. The hotter it gets, the easier it changes shape.
Also, there are many formulations of plate glass, each with different properties. Green does not mean the same as green.

I've been reading lots of posts and educational articles and now know why firing is done in phases. Lots more reading to go.



I found answers to my questions for this thread:
Use soft firebrick for the kiln.
Use rockwool outside that if heat containment is still a concern.
Use fireclay for shelves and moulds.
Use fire grout to bond bricks.
Soft bricks are easy to cut and half the density of water.

As for glass properties, ask the seller to get them, the anneal point, softening point, recommended slump temp, and casting temp. Google is just for ballpark and does not cover the many individual formulas.

[Aaron Solt]

I plan to make the kiln light weight so that the walls don't absorb or store as much heat and cause the air temp to lag behind the set temp.

[Bert Weiss]

WoW You are in for an extremely steep learning curve. First of all, float glass has a relatively high melting point. Second, refractory molds don’t fare well above 1400ºF. Third you will never get bubble free stacking glass. You would do best starting out with a piece of glass thicker than you want at the end, and get it flow in to a mold of some sort.

I’m not sure what your finished size is. Maybe a Blenko dalle will work. It is about an inch thick, but is rectangular in shaoe. Maybe some Schott optical glass would work, it is very low melting point, but will cost you a lot and you will fail many times regardless of what you try and do.

[Aaron Solt]

WoW You are in for an extremely steep learning curve. First of all, float glass has a relatively high melting point. Second, refractory molds don’t fare well above 1400ºF. Third you will never get bubble free stacking glass. You would do best starting out with a piece of glass thicker than you want at the end, and get it flow in to a mold of some sort.

I’m not sure what your finished size is. Maybe a Blenko dalle will work. It is about an inch thick, but is rectangular in shaoe. Maybe some Schott optical glass would work, it is very low melting point, but will cost you a lot and you will fail many times regardless of what you try and do.

Thank you for the feedback.

Definitely true that plate glass has a higher melting temp than art glass. Besides price, I wonder about physical properties of art glass, such as its density and stiffness and coefficient of thermal expansion at room temperature. I'll have to look to see if they'd work.

As for refractory moulds not handling 1400 deg, I read that clay can go higher, especially fire clay. Am I wrong? I googled melting temp.


As for avoiding bubbles of stacked pieces, that sounds like a worrisome point. I'm trying to find a way around that, since I can't get thicker pieces. I'm all for the hopper method. How do glass manufacturers melt small pieces without bubbles?

[jolly]

I wrote a post a couple of days ago but I must have forgotten to hit the submit button. You need to get hold of a kiln and try firing some glass. It doesn't behave the way you think it will. If I were you, I would cut some rounds, stack them and fire them for the first experiment. If you use old windows, it won't cost much. Then I would try putting tiny shards of glass on the very edges of each layer and try fusing that. Unless you figure out some kind of flux or other ingredients to add with the molten glass, I would stay below 1500 F. By the way, Pilkington melts their batch at 2900F and pours the glass for windows at 2000F. But they know what they are doing. Go watch a Youtube video about their process, it might help. Watch videos on working with molten glass, you might get some ideas there. No one I've heard of casts molten window glass for reasons. It is fun to play with old windows though so go for it but, really, fuse some window glass in a kiln first to get started. There are some ring molds that people use for pot melts which you can search for. Have fun.