Bubble free casting

[Ron Behrens]

I came up with an idea in my sleep --My goal is casting or fusing glass for telescope mirrors. I am looking for opinions here
How would casting or fusing in a vacuum chamber work with regard to getting rid of bubbles?
Ron

[Bert Weiss]

The best way to eliminate bubbles is to start with a single piece of glass that is bubble free and cast it. The amount of bubbles that form tends to depend on the amount and particle size of the glass used. Powder makes a lot of bubbles. A single piece is a single particle... Glass blowers eliminate bubbles by holding the pot of molten glass long enough for all the bubbles to rise and leave. This is much like taking the lid off of a soda bottle, and heating it up (or pop bottle if you grew up you where I did). This is very difficult to do while casting because casting investments don't like that high temperature much. When glass is that hot, it has chemical reactions with whatever it is in contact with. The cleanest glass is made with electric melt where the electrode is embedded inside the glass. The most chemical reaction takes place at the hottest point. Pouring molten glass in to a mold is actually a single particle.

[Ron Behrens]

The best way to eliminate bubbles is to start with a single piece of glass that at the hottest point. Pouring molten glass in to a mold is actually a single particle.

The last line is an interesting statement------ It appears to tell me I can melt glass in a furnace to a bubble free condition and the pour into a mold ----bubble free----then anneal in a kiln. CORRECT??
Ron

[Kevin Midgley]

....... cords !!!!!!

[Ron Behrens]

....... cords !!!!!!

So I will wind up with cords? I'm still a learner here! 2 years playing part time!
Ron

[Kevin Midgley]

not necessarily. It just isn't a guaranteed basketball slam dunk with the clarity and non distortion you require.
Now if you had the ribbon of optical glass that used to flow through the Stuben factory, you could have made some mighty fine lenses.

[Ron Behrens]

not necessarily. It just isn't a guaranteed basketball slam dunk with the clarity and non distortion you require.
Now if you had the ribbon of optical glass that used to flow through the Stuben factory, you could have made some mighty fine lenses.

Please note light will not travel thru my glass. It will be ground to a parabolic curve and top coated with aluminum
To work in a reflecting telescope

[Bert Weiss]

Cords form in a crucible of glass where the crucible is hotter than the glass inside it. As I said, the chemical reactions take place at the hottest point. I don't think those cords will effect your ability to grind and polish the surface. If they do, it would be a far sight less than a bubble in the surface. Your challenge as always is the mold. You can make molds as simply as pressing a shape in to a mix of sand, water, and bentonite, coated with carbon. The right sand mix can have a resolution as fine as a finger print.

[Ron Behrens]

Cords form in a crucible of glass where the crucible is hotter than the glass inside it. As I said, the chemical reactions take place at the hottest point. I don't think those cords will effect your ability to grind and polish the surface. If they do, it would be a far sight less than a bubble in the surface. Your challenge as always is the mold. You can make molds as simply as pressing a shape in to a mix of sand, water, and bentonite, coated with carbon. The right sand mix can have a resolution as fine as a finger print.

So are cords tiny air bubbles or a chemical contamination of sorts?
And yes I still have not found a mold I am satisfied with ---- lack of expeience and local help\ mentor sucks

[charlie]

[quote="Ron Behrens"][quote="Bert Weiss"]Cords form in a crucible of glass where the crucible is hotter than the glass inside it. As I said, the chemical reactions take place at the hottest point. I don't think those cords will effect your ability to grind and polish the surface. If they do, it would be a far sight less than a bubble in the surface. Your challenge as always is the mold. You can make molds as simply as pressing a shape in to a mix of sand, water, and bentonite, coated with carbon. The right sand mix can have a resolution as fine as a finger print.[/quote]

So are cords tiny air bubbles or a chemical contamination of sorts?
And yes I still have not found a mold I am satisfied with ---- lack of expeience and local help\ mentor sucks[/quote]

cords are produced by various problems. they're just a line in the glass where the refraction of a portion of the glass is a bit different than that nearby. they are not bubbles. since you're not concerned about light passing through the glass, you don't need to worry about this problem.

http://onlinelibrary.wiley.com/doi/10.1 ... x/abstract

[twin vision glass]

What about a continuous drip into a mold. An amazing artist from California does this I believe, now if I could just remember his name. John

[Brock]

That would be John Lewis. But, you know, I don't think too many casual fusers have a bottom draining hot glass furnace in their home studios . . .

[twin vision glass]

Yes, that is his name. Thank you. He is totally amazing and no , not many have a continous drip for sure but just an idea to perhaps rent time if it is what he truly needs. How about a large piece of Schott Crystal and then cast as you like. How big is the piece you are trying to achieve. Les

[Bert Weiss]

There is more than one way to get molten glass out of a furnace, although to truly do it in a single particle, it needs to be poured or you need a furnace setup like John's. Bertil Vallien makes his giant castings with a crew of people gathering ladles of molten glass and dumping them in a mold, one after the other in a continuous line. This does run the risk of trapping bubbles, but not near the surface, as the bubbles tend to form between ladlefuls. He can also control surface bubbles with a hand held torch during the process.

John Lewis took many years and hundreds of thousands of dollars to invent and refine his furnace. I once got a quote from him for a 5" thick bench. The quote was quite expensive. I called him and asked why. His answer is burned in my consciousness. He told me he needs to charge enough money to make the piece 3 times and still turn a decent profit. The reason for this is that he works a lot for picky clients (architects) They often complain that there is a zig where they wanted a zag. Then, we are working with glass, which sometimes has a way of not exactly cooperating with one's intention.

[Morganica]

Ron, if I'm not mistaken these are mirrors and not transparent, so the problem isn't seeing bubbles through the clear glass, it's not having any bubbles at the surface causing pits and interfering with the mirror. Right?

If that's so, this is a different problem than eliminating bubbles throughout the casting.

I run into something similar with pate de verre--I tend to carve, grind and polish some of my PdV pieces pretty heavily, and since PdV incorporates bubbles on purpose (that's what gives it translucence), it's very difficult to coldwork without exposing pits. If I know I can't have bubbles within 3-4mm of the surface of a pate de verre casting because carving will expose them, I change my processes a bit:

1) If my casting has a back I don't care about, I direct the bubbles there by casting upside down. That means I hold the glass at process temp about 3X as long as needed and about 25F higher, and make sure that the top of the casting exposes the back of the piece. The bubbles will move up, and clear the bottom, i.e., the surface I do care about. This promotes devit, but I'm coldworking, so I don't care.

2) I slump a mold shell, i.e., a sheet of bubble-free glass into the mold, the same thickness as the depth I need to remove (usually I use 6mm sheet). This requires creating two molds, but if your mirror blanks are always the same size, you could conceivably make a ceramic mold for the purpose. I suspend the sheet over the mold, slump it into the shape, then trim off the excess. I'm left with a bubble-free, clear glass shell for what will be the top/front of the casting. I take the second mold, put my trimmed shell into it, lay in my other glass to fill the mold, and then fire as in #1. The bubbles will move away from the shell, toward the top of the mold, so I generally get a bubble-free surface.

3) When I've borrowed space in a hotshop, I've opened the kiln after about 4 hours at process temps and whacked the surface of the glass with a torch for 15-20 minutes, to help speed up bubbles rising and popping. I honestly don't know if it did much good but the two castings I tried this with were much more transparent than normal.

With transparent castings, you can also double-cast, i.e., cast the glass you're going to use into an ingot shape that approximates your final mold. Drip the glass in (going down the side instead of straight into the mold), hold it hotter and longer to get out as many bubbles as possible. Grind the ingot to remove any devit/scum/wrinkling, sandblast the surface lightly (or coldwork it), clean well and place directly in the mold. Idea is that the less the glass moves in the mold, the fewer bubbles you deal with, so you get the glass as bubble-free and conforming as possible before the actual casting.

A few years back Bullseye demoed a casting setup I still want to try: Stack two kilns. The mold goes in the bottom kiln, and goes through preheat/dry cycles as normal. The glass is in the second, smaller crucible kiln on top, and has the bejasus heated out of it to get the glass nice and juicy, and to eliminate as many bubbles as possible. There's a hole in the bottom of the crucible kiln, and the top of the mold kiln. When the mold and glass are ready, you remove the barriers between the kilns, the glass flows from top through the holes into the mold, and Bob's your uncle.

Was very, very cool, intended to increase clarity. If you drip the glass in properly (i.e., send it down a sloped ramp instead of splashing it to the bottom of the mold), you'll keep bubbles to a minimum.

The vacuum casting is a great idea (and I think the R&R techs have played around with that and centrifugal casting, so you might ask them about it). Whether it gets you more than you could achieve without it, I dunno.