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Assuming one puts larger chunks of broken float glass and borax into a crucible and melt. Would someone please explain to me what the process of quenching in water does to the glass? Chemically? Or Physically? I read it is then ready to cast.

What happens when you dump hot glass in water is essentially that it tempers and breaks up. Each individual piece of the broken glass becomes a piece of tempered glass, itself. This results in 2 things. The pieces of glass are harder to break up further, but when you do break them up, they break down resulting in a lot of fines. They will remelt though.

There are numerous reasons to quench and cast float glass. Ending up with optical crystal is not generally one of them. Float glass has a tendency to devitrify, is a relatively stiff glass, and has quite a bit of light blocking color in it. I am not a glass chemist, but I suspect simply adding borax is not a particularly good plan for improving the float cullet. It is possible to smelt glass in a glass fusing style kiln, but it is hard on both the elements and the insulation, be they brick or fiber. The kiln won't last long doing that.

Remember the picture "Morganica" posted of a float glass casting. It had a lot of white throughout the glass. This is devitrification that happens on the surface of the glass pieces. Devitrification means that the material crystallizes and ceases to be glass. You don't want that for a lens. If you were to stack and fuse sheets of glass, your problem will be internal bubbles, but if it devits, it will likely be on the surface, and be removable.

The cleanest castings come from casting a single particle of glass. Melting and flowing melted glass does result in a single particle. Moving flowing glass avoids devitrification. So, if you pile a bunch of glass in a mold and fuse it, you get white veiling around the particles. If you melt and flow the glass, you avoid this. Flower pot casting is designed to accomplish this. However flower pot casting does not give you the opportunity to smelt and mix a pot of glass. You really need a furnace to do that.

On the other hand, high lead optical crystal is the easiest glass to cast with. It has great optical qualities, a low melting point, easy flow characteristics, and is relatively soft making it easier to grind and polish. HIS Glassworks, a board sponsor, sells Schott optical glasses.

On the third hand, somewhere in the back of my mind I think the telescope folks might use borosilicate, a very low expansion glass. This glass is on the other side of float glass when it comes to hardness, stiffness, and temperatures required to soften.

So would breaking a piece of tempered glass accomplish the same desired result?

Ron Behrens wrote:So would breaking a piece of tempered glass accomplish the same desired result?

Yes. I sometimes get my glass fabricator that tempers glass to save some for me that is on the way to the dumpster. I then wrap it in heavy plastic and bang the corner with a hammer. It breaks up. The catch is that breaking it up further in to individual pieces of usable frit takes a lot more work than cutting and cleaning a piece of sheet glass. I fuse the pieces back together in to circles and then slump them in to bowls. This glass behaves quite differently than any other I have encountered. My goal is to tack fuse it so you see the various rounded glass particles. I have learned that I have to go hotter than I originally tried, or it has a big tendency to fall apart, and to crack during the reheat on a bowl mold. In order to avoid cracking on the reheats, I have to slow down to the extreme.

I have, in years past, water quenched float glass and taken it to a glass blower. He mixed in cobalt oxide and or copper oxide, smelted it in a small crucible furnace, and made me blue and turquoise frits compatible with my original glass. This requires stirring the pot, and you can't use too much oxide. Once it gets hot enough to stir to homogenize, he dumps it back in to water. Not cooking it too long, leaves the glasses compatible.

I am asking these questions because I have 400 pounds of tempered glass. Once it is shattered do the pieces need to be made even smaller?

1. What are you trying to do with it? The telescope mirrors?
2. Is it already shattered?
3. How big are the pieces?

Ron Behrens wrote:I am asking these questions because I have 400 pounds of tempered glass. Once it is shattered do the pieces need to be made even smaller?

Only if your goal is very fine particles of glass. The smaller the particles you use, the less clarity there will be in the casting. This is the inverse of getting the most clarity from a single mass of glass.

Companies that make frit have the option to use water quenched or not water quenched glasses. In some ways water quenching is easier to start out with. But breaking up the particles is much harder and then you get a lot of fines. You have more control over particle size outcomes with not quenched glasses.

Cynthia
I am trying to make telescope mirrors---and they don't have to be perfect inside--- just well annealed
And I need to achieve consistent results
I have some broken tempered and 300 pounds unbroken tempered
They are quarter inch in size

Ron Behrens wrote:Cynthia
I am trying to make telescope mirrors---and they don't have to be perfect inside--- just well annealed
And I need to achieve consistent results
I have some broken tempered and 300 pounds unbroken tempered
They are quarter inch in size

Wrap the sheets in heavy plastic. Bang the corner with a hammer. Walk all over it with relatively clean shoes. Put it in a plastic bucket and bang the hell out of it with anything handy.

For annealing, my recommendation is to download the Bullseye chart for annealing thick glass. Add 80º - 100º F to all their temperatures. I like to anneal soak float at 1000. I am pretty sure you can get away with 980, but I never tried it.

I'll be surprised if this makes a viable telescope mirror. let us know.

This is one I made and am grinding. I need to improve my success rate. I am going to try using some broken glass in a flower pot next and see what happens.

Ron- Can't you buy cast blanks specifically for lenses? I understand the DIY part but is it cost-efficient (or doesn't it matter)?

32 and larger can have prohibitive wait times. Larger mirror blanks can become exponentially more expensive. Also when you add the complexity of slumping a meniscus blank it starts to make sense to make ones own. Plus all my life I have enjoyed tackling and completing difficult projects. And when retirement comes around it could offer an opportunity to make some cash