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I took you folks advise and picked up a 50 pounds of spectrum 96 nuggets. I plan to melt my first experimental telescope mirror with the spectrum nuggets into a 12 in stainless mold about 1.5 in thick. Can someone with more experience with this glass suggest a maximum target temperature to start with. The goal would be a full melt with as few bubbles as possible

Did you purchase System 96 billets, which are made for kiln casting, or System 96 nuggets, which are made for glass blowing? I'm not sure how well the nuggets would perform if used in a casting environment without being remelt in a furnace first. Perhaps someone else on the board has experience with this.

In either case, there are guidelines for use on the System 96 website at system96.com.

As a side note, probably the fewest bubbles would come from hot casting, rather than kiln casting. This requires a glassblowing furnace. (The difference between the two is that kiln casting is placing your glass into a mold, then firing in a kiln, while hot casting is ladling molten glass into the mold, then cooling and annealing.)

Brad Walker wrote:Did you purchase System 96 billets, which are made for kiln casting, or System 96 nuggets, which are made for glass blowing? I'm not sure how well the nuggets would perform if used in a casting environment without being remelt in a furnace first. Perhaps someone else on the board has experience with this.

In either case, there are guidelines for use on the System 96 website at system96.com.

As a side note, probably the fewest bubbles would come from hot casting, rather than kiln casting. This requires a glassblowing furnace. (The difference between the two is that kiln casting is placing your glass into a mold, then firing in a kiln, while hot casting is ladling molten glass into the mold, then cooling and annealing.)

Thanks for the info. I was unable to find the info there that I wanted so I sent them an email.
As a side note I don't really understand the difference between hot casting and kiln casting. At face value it would seem that kiln casting or simply fully melting in a kiln would disturb the glass less thus creating less bubbles. Unless it is that more heat is involved in hot casting!
Ron

The difference is significant. In furnace casting you take the glass to @ 2300ºF and let it sit until all the bubbles dissipate. This is like taking the lid off of a carbonated soda and heating it. 2300º glass is quite hard on refractories and elements. Fusing kilns are not made to work long in this range.

You might try flowerpot casting where you set a flower pot above the mold, fill it with the nuggets and heat it hot enough so that the glass flows out of the pot. I am not a practitioner of this technique, so others can comment on it.

Bert Weiss wrote:The difference is significant. In furnace casting you take the glass to @ 2300ºF and let it sit until all the bubbles dissipate. This is like taking the lid off of a carbonated soda and heating it. 2300º glass is quite hard on refractories and elements. Fusing kilns are not made to work long in this range.

You might try flowerpot casting where you set a flower pot above the mold, fill it with the nuggets and heat it hot enough so that the glass flows out of the pot. I am not a practitioner of this technique, so others can comment on it.

Bert
Are we saying that I will need to take the spectrum glass to 2300 degrees to make a decent piece of telescope glass ?
Ron

Ron Behrens wrote:

Bert Weiss wrote:The difference is significant. In furnace casting you take the glass to @ 2300ºF and let it sit until all the bubbles dissipate. This is like taking the lid off of a carbonated soda and heating it. 2300º glass is quite hard on refractories and elements. Fusing kilns are not made to work long in this range.

You might try flowerpot casting where you set a flower pot above the mold, fill it with the nuggets and heat it hot enough so that the glass flows out of the pot. I am not a practitioner of this technique, so others can comment on it.

Bert
Are we saying that I will need to take the spectrum glass to 2300 degrees to make a decent piece of telescope glass ?
Ron

Not exactly, since I still don't really understand what the glass actually does.

The hotter you get the glass, the quicker it will let the bubbles that form rise, break, and level the surface out. At lower temperatures, the bubbles will rise slowly, break, and it takes quite a while for them to level out. It is a matter of viscosity. The hotter and thinner you get the glass, the quicker it will fine out (that means lose the bubbles). The catch is that the hotter you get the kiln or furnace, the more corrosive the glass becomes to both refractories and elements. Furnaces are made with hard bricks and hard crucibles, and large industrial furnaces must be rebuilt at least every 3 years. Electric furnaces are not made with Kanthal elements like kilns are. The Kanthal is capable of taking the kiln to 2300ºF, but it not capable of sustaining this. Neither is the soft brick kilns are made of, and then you have the mold to worry about.

You might be able to pull this off at 1700ºF, but it will take a long time, and probably plenty of trial and error, mostly error. Hot glass likes to stick to stuff, and then it won't anneal. At any rate, I think you are better off with the Spectrum glass than with float glass, because it has better flow and viscosity characteristics, and is easier to grind and polish. Cost of the glass is only part of the paradigm.

Don't the big tele makers start with molten glass, get it in to a hot mold, and then spin it fast to get the glass to go to the far reaches of the mold?

Paul Marioni once told me that he proposed to NASA blowing glass in space to make telescope mirrors. Out there, gravity isn't the problem it is here...

I use nuggets, and what happens when you put them together is; you get marks at the seams where they meet each other. Every shape has these seams where they meet other shapes - if we start with pieces, you'll see evidence of those shapes in the glass at normal full fusing temps. Bigger the pieces, less we see of the seams.

Mark Hall wrote:I use nuggets, and what happens when you put them together is; you get marks at the seams where they meet each other. Every shape has these seams where they meet other shapes - if we start with pieces, you'll see evidence of those shapes in the glass at normal full fusing temps. Bigger the pieces, less we see of the seams.

Mark, have you ever tried flower potting the nuggets? This might eliminate those veils. Flowing melted glass becomes a single particle.

Ron, as I understand it, you're not looking for perfect transparency since these blanks will become mirrors anyway, but a hole on the surface would be a problem, right?

Spectrum's nuggets are for casting and blowing. They don't need pre-melting if you're casting. If you're blowing, the premelt/batching refines the glass for use (it would be kinda hard to gather nuggets on the end of a blowpipe). You should be ok there.

Bubble-wise, you'll get the fewest bubbles if you contain the glass in a reservoir outside the mold, heat slowly to allow the nuggets to settle and push out air, and drip the glass into the mold. If possible, position the hole in your reservoir so that the glass doesn't splash to the bottom of the mold, dead-center--it generates more bubbles. Rather, move the hole so the glass can slide down

However, since you don't need perfect transparency, putting the glass directly in the mold will work. Just wash the nuggets a couple of times to remove dust and let them dry first. When firing, take the temp up to about 30 degrees hotter than Spectrum's standard casting schedule and hold it there for a couple of hours. That allows time for the bubbles to rise and pop.

Hey Bert - I use them in thick fused slabs (color bars) mostly for adding volume as well as creating effects with color. By the time I cut the slabs into square rods & make kiln formed cylinders, then pick them up (warm) on a collared blowpipe & turn them into vessels, the veils have had plenty of movement. Still, you can see the difference between this and ultra clear as from molten glass alone.

Morganica wrote:Ron, as I understand it, you're not looking for perfect transparency since these blanks will become mirrors anyway, but a hole on the surface would be a problem, right?

Spectrum's nuggets are for casting and blowing. They don't need pre-melting if you're casting. If you're blowing, the premelt/batching refines the glass for use (it would be kinda hard to gather nuggets on the end of a blowpipe). You should be ok there.

Bubble-wise, you'll get the fewest bubbles if you contain the glass in a reservoir outside the mold, heat slowly to allow the nuggets to settle and push out air, and drip the glass into the mold. If possible, position the hole in your reservoir so that the glass doesn't splash to the bottom of the mold, dead-center--it generates more bubbles. Rather, move the hole so the glass can slide down

However, since you don't need perfect transparency, putting the glass directly in the mold will work. Just wash the nuggets a couple of times to remove dust and let them dry first. When firing, take the temp up to about 30 degrees hotter than Spectrum's standard casting schedule and hold it there for a couple of hours. That allows time for the bubbles to rise and pop.

Cynthia
I will make myself a fresh stainless ring for a mold and give your recommendations a shot tommorrow
Ron

If you melt nuggets inside a SS ring you WILL see the veils or seams that Mark has mentioned!

Morganica wrote:Ron, as I understand it, you're not looking for perfect transparency since these blanks will become mirrors anyway, but a hole on the surface would be a problem, right?

Spectrum's nuggets are for casting and blowing. They don't need pre-melting if you're casting. If you're blowing, the premelt/batching refines the glass for use (it would be kinda hard to gather nuggets on the end of a blowpipe). You should be ok there.

Bubble-wise, you'll get the fewest bubbles if you contain the glass in a reservoir outside the mold, heat slowly to allow the nuggets to settle and push out air, and drip the glass into the mold. If possible, position the hole in your reservoir so that the glass doesn't splash to the bottom of the mold, dead-center--it generates more bubbles. Rather, move the hole so the glass can slide down

However, since you don't need perfect transparency, putting the glass directly in the mold will work. Just wash the nuggets a couple of times to remove dust and let them dry first. When firing, take the temp up to about 30 degrees hotter than Spectrum's standard casting schedule and hold it there for a couple of hours. That allows time for the bubbles to rise and pop.

Cynthia
I followed your recommendation to go 30 above temperature. I read that to be 1530. I soaked at 1530 for 8 hours and
seem to have lots of bubbles. The 14 in x 1.75 thick piece sits on 1/8 fiber paper and contained in a stainless mold with 1/4 fiber paper. I am interested in your thoughts and ideas to lessen bubble count. Would more time and or heat help?
Ron

Ron

If this stuff was easy, everybody would be doing it... This sort of casting is not my area of expertise, but I can tell you a few basic things. There are 2 places to soak, the first is for a bubble squeeze. The idea is to make the glass hot enough to begin to deform, but not be hot enough to seal and trap air. (I've seen people report using widely different temperatures for this) The second is at the top, allowing what bubbles formed to rise and pop. This is the tricky part. First, simply relying on time is a poor strategy. You have to look and make changes based on what you see. You either need more heat or more time, if the top doesn't look good.

The big problem is that molds do not handle the big heat that will get the glass to fine out. So you have to find the sweet spot where the mold stays stable, the glass flows, and the bubbles rise, pop, and level out. Then there is the glass chemistry. Some glasses devit and then will not flow. Some glasses get soft at lower temperatures.

After all those issues get resolved, you have to figure out how to avoid getting suckers (dents where the glass recedes from the mold) Worry about that one if it becomes a problem.

By the time you figure this out, you will have spent a lot of time and money. This is why master works are quite valuable. It is about the lifetime it took to be able to make the stunning object, not about time and materials for one piece. Enjoy the process!

May I strongly suggest a block of Schott Crystal. So beautiful . NO bubbles , and flows like butter. Les

twin vision glass wrote:May I strongly suggest a block of Schott Crystal. So beautiful . NO bubbles , and flows like butter. Les

Who sells this? And what can I expect to pay for it?
Ron

HIS Glassworks.

And basically, a lot.

http://www.hisglassworks.com/cart/cart. ... list&c=255

http://www.twinvision.fusedglassartists ... les019.jpg
Yes the LF5 is so beautiful and flows like butter and clear as anything ever. His Glass Works is out of it right now. I know that to cut the large bar Mel and I would SCORE/TORCH/DIP IN BUCKET OF ICE WATER/ HIT WITH ANVIL AND HAMMER for super clean cuts. Les