I need castable mould materials to melt plate glass into a disc. - Page 2 - WarmGlass.com

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

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Aaron Solt
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Re: I need castable mould materials to melt plate glass into a disc.

Postby Aaron Solt » Wed Feb 13, 2019 2:43 pm

jolly wrote: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.



Thank you for posting about the shards. I came back to post my idea of a bar going across the edge of the bottom piece, propping up the edge of the too piece till it slumps over, giving air space to escape.

Your shard idea is simpler, except I don't have shards. I'll use a strip of stainless steel instead, sticking 1mm in.

Shards make sense though for people who want to fuse not melt.

As for the manufacturers, 2900 sounds believable, and likely necessary for the small pieces. I wonder if they use the incoming pieces to absorb some of the heat of the outgoing melt or if it is worth it.

Aaron Solt
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Re: I need castable mould materials to melt plate glass into a disc.

Postby Aaron Solt » Wed Feb 13, 2019 2:52 pm

Also, old glass may have contamination that can seed devitrification. Brand new glass is cheap. I can get 1/2" for $10 per square foot. Vastly cheaper than $70 for a 10" disk that has to be ground to the curvature I want. Coarse grinding is noisy, and the machines likely cost as much to build as the kiln does.

Aaron Solt
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Re: I need castable mould materials to melt plate glass into a disc.

Postby Aaron Solt » Wed Feb 13, 2019 5:03 pm

I found the name for dental stone. Hydrocal.
Melting temp is 2650 F, but upper working temp is 900 F. I wonder why it does not work higher.

Bert Weiss
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Re: I need castable mould materials to melt plate glass into a disc.

Postby Bert Weiss » Wed Feb 13, 2019 8:59 pm

Aaron Solt wrote:I found the name for dental stone. Hydrocal.
Melting temp is 2650 F, but upper working temp is 900 F. I wonder why it does not work higher.


Plaster has 2 kinds of water in it, the water you mix it with and chemically bonded water. Both need to exit the mold. Solid Hydrocal, will just crack. So you need other stuff in the refractory mix. You can buy Ransome and Randolph glass casting investment. Or, you can formulate your own.

Remember that mold and glass must be compatible. That means that the glass would need a higher expansion coefficient so it will shrink more on cooling. Otherwise you won’t be able to unmold. Every material has a different coefficient of expansion.
Bert

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Aaron Solt
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Re: I need castable mould materials to melt plate glass into a disc.

Postby Aaron Solt » Wed Feb 13, 2019 9:55 pm

Bert Weiss wrote:
Aaron Solt wrote:I found the name for dental stone. Hydrocal.
Melting temp is 2650 F, but upper working temp is 900 F. I wonder why it does not work higher.


Plaster has 2 kinds of water in it, the water you mix it with and chemically bonded water. Both need to exit the mold. Solid Hydrocal, will just crack. So you need other stuff in the refractory mix. You can buy Ransome and Randolph glass casting investment. Or, you can formulate your own.
.


I really want to learn what makes these castables bond together, and how strong the bond is. Is it just hydrogen bonding? Water ice is held together by just hydrogen bonds.
Last edited by Aaron Solt on Wed Feb 13, 2019 10:03 pm, edited 3 times in total.

Aaron Solt
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Re: I need castable mould materials to melt plate glass into a disc.

Postby Aaron Solt » Wed Feb 13, 2019 10:01 pm

Bert Weiss wrote:
Aaron Solt wrote:I found the name for dental stone. Hydrocal.
Melting temp is 2650 F, but upper working temp is 900 F. I wonder why it does not work higher.


Remember that mold and glass must be compatible. That means that the glass would need a higher expansion coefficient so it will shrink more on cooling. Otherwise you won’t be able to unmold. Every material has a different coefficient of expansion.


Thank you for the reminder. I stated that when saying why plate glass is good, but then forgot about it when I greedily contemplated Borofloat 33 for its low CTE. But I guess that means it might not case we'll, or at least not as well, even if I can conquer the extra 500 deg.

Borofloat 33 stores 83% as much heat as plate, because of lower density and lower specific heat. I think I'll just start out with the cheaper glass and see if I even need the fancier stuff.

Aaron Solt
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Re: I need castable mould materials to melt plate glass into a disc.

Postby Aaron Solt » Thu Feb 14, 2019 12:43 am

Well, since stainless steel has a melting point of 2550 and max working temp of 1600, I guess that means working temps are half of melting temps on average, but depending on the loads.


I wonder how reflective titanium is in the infrared, compared to stainless steel. Titanium is not what shiny. I'll look up chromium too, which might cost less.

...

Well, the high melting point metals seem to be $100 per square foot for even thin sheet. Amazing. Titanium is the cheapest by a long shot at $40 per square foot.

I don't know if it is reflective when red hot. I also read that any metal must be polished to reflect infrared. So I'll probably abandon my reflective walls design.


...

Stainless steel at home improvement stored is way less, $40 for a 24x30 back splash. I'm going to get all my materials at Home Depot.
It does not need to reflect infrared at 1000 F. It just needs to do so at most of the climb there.
Last edited by Aaron Solt on Thu Feb 14, 2019 5:49 pm, edited 2 times in total.

Aaron Solt
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Re: I need castable mould materials to melt plate glass into a disc.

Postby Aaron Solt » Thu Feb 14, 2019 2:20 am

I just learned that clay pottery is hardened with mineral fusing. There is no mould, so water initially holds it together till other crystal bonds can form. It is a very gradual, well controlled process.

Aaron Solt
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Re: I need castable mould materials to melt plate glass into a disc.

Postby Aaron Solt » Thu Feb 14, 2019 2:48 am

Schott likely has heavier tougher bricks that stay about the same temp in their flow through set up.

As for the temperature their shelf/mould can handle, their shelf is molten tin.

I can not copy them.

Bert Weiss
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Re: I need castable mould materials to melt plate glass into a disc.

Postby Bert Weiss » Thu Feb 14, 2019 12:13 pm

Aaron Solt wrote:
Bert Weiss wrote:
Aaron Solt wrote:I found the name for dental stone. Hydrocal.
Melting temp is 2650 F, but upper working temp is 900 F. I wonder why it does not work higher.


Remember that mold and glass must be compatible. That means that the glass would need a higher expansion coefficient so it will shrink more on cooling. Otherwise you won’t be able to unmold. Every material has a different coefficient of expansion.


Thank you for the reminder. I stated that when saying why plate glass is good, but then forgot about it when I greedily contemplated Borofloat 33 for its low CTE. But I guess that means it might not case we'll, or at least not as well, even if I can conquer the extra 500 deg.

Borofloat 33 stores 83% as much heat as plate, because of lower density and lower specific heat. I think I'll just start out with the cheaper glass and see if I even need the fancier stuff.


I once quoted a large cylinder casting made of borosilicate glass. What was required was first, a furnace capable of melting the boro. Then it would be poured in to a mold, however the beginning and end of the pour would not just flow together, so the mold would then need to be reheated quite hot to get to flow. In order to make the job, my guy would have had to build both the furnace and the furnace/annealer. Needless to say, it was too expensive.
Bert



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

Postby Bert Weiss » Thu Feb 14, 2019 12:15 pm

Aaron Solt wrote:Schott likely has heavier tougher bricks that stay about the same temp in their flow through set up.

As for the temperature their shelf/mould can handle, their shelf is molten tin.

I can not copy them.

While SCHOTT does make float glass, their optical casting glasses are not float. I believe HIS glassworks is still their US distributor. These high lead glasses are the easiest to cast, but not cheap.
Bert



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Aaron Solt
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Re: I need castable mould materials to melt plate glass into a disc.

Postby Aaron Solt » Thu Feb 14, 2019 3:52 pm

I'm going to level with everyone. I'm making a front surface mirror. Bubbles won't block the light, but they can change size with temperature pressure and deform the surface, which needs to be exact.

The reason I'm against crystal is not the price so much as the density: 3 vs 2.5. That can make the glass sag under its own weight more and move the balance point of the scope.



I found that the coefficient of thermal expansion of plate glass is about the same as that of crystal. No issues there. But I doubt the leaded glass is stiffer. And even if it is, polishing would take longer. Also polishing lead sounds hazardous. Plate glass is edible if atomized and not sharp.



I can get 3/4" plate glass, but slumping that is heavier than I want. Grinding it down is noisy.

What I want is 0.65" in the middle half and then tapered to 1/2" at the edge, where the thermal issues are. That requires the extra molding and casting.

I'm now thinking of slumping 1/2" across the whole diameter. That would dramatically simplify the kiln, easing many requirements with casting gone.
The problem is the thin mirror might flex too much with only 6 support points. So I think I'll fuse flanges and reinforced points on the back, though that might run into buying bubbles again and possible a double firing.

I've got ideas around that.

Aaron Solt
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Re: I need castable mould materials to melt plate glass into a disc.

Postby Aaron Solt » Thu Feb 14, 2019 4:05 pm

I also just found out that chalk is calcium carbonate, not magnesium oxide. Also magnesium oxide has high conductivity, as do most metal oxides. It is the carbonates that are thermal insulators. And the CO2 is driven off at very low temp. So that scratches my MgO thermal shock protectant idea.

I still would like one good reflector under my heating element to protect the brick.

Aaron Solt
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Re: I need castable mould materials to melt plate glass into a disc.

Postby Aaron Solt » Thu Feb 14, 2019 4:21 pm

I doubt I can cut tempered glass. But I can split it. I also saw videos of splitting circles from squares. I think that is much less difficult with 1/4" glass than with 1/2".


So, my task is to fuse two 1/4" pieces of glass together and slump them, hopefully in the same cycle.

Freeing a half inch circle would be a home run. I'll check YouTube.

Ed Cantarella
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Re: I need castable mould materials to melt plate glass into a disc.

Postby Ed Cantarella » Thu Feb 14, 2019 7:39 pm

Aaron Solt wrote:I doubt I can cut tempered glass. But I can split it. I also saw videos of splitting circles from squares. I think that is much less difficult with 1/4" glass than with 1/2".


So, my task is to fuse two 1/4" pieces of glass together and slump them, hopefully in the same cycle.

Freeing a half inch circle would be a home run. I'll check YouTube.
You can de-temper the tempered glass by taking it up to >1200f and then giving it a more normal anneal. I'm shooting from the hip there, I haven't done it but I know some of these folks have.
HER last words were, "I'm melting, melting . . . " Dissenting opinions generally welcome for comic relief or personal edification. Sometimes both.

Kevin Midgley
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Re: I need castable mould materials to melt plate glass into a disc.

Postby Kevin Midgley » Fri Feb 15, 2019 12:49 pm

Contact https://sandfire.com/
They can probably flame lathe a precision curved surface for you out of boro or quartz glass.
I'd guess no or little grinding would be needed.

Bert Weiss
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Re: I need castable mould materials to melt plate glass into a disc.

Postby Bert Weiss » Tue Feb 19, 2019 1:30 pm

Aaron, your knowledge of glass is sorely lacking. You continually make wrong statements. Answering them all is book length. If you can purchase what you want for $75, I advise you to do so, unless you would rather spend $75,000 learning about glass.

Tempered glass never breaks in 2 pieces. 2,000 is closer to the mark.

With the proper tools in hand, ½” annealed glass is not more difficult to cut in half than ¼” glass, unless the size is small. It does require a different skill set.

Aaron Solt wrote:I doubt I can cut tempered glass. But I can split it. I also saw videos of splitting circles from squares. I think that is much less difficult with 1/4" glass than with 1/2".


So, my task is to fuse two 1/4" pieces of glass together and slump them, hopefully in the same cycle.

Freeing a half inch circle would be a home run. I'll check YouTube.
Bert



Bert Weiss Art Glass*

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