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I've fused tempered glass shards and chunks onto a small (6X8) piece of picture frame glass. It fused well, but when I look at it through my DIY polariscope (glass on top of LED screen, viewing through a polarizing camera filter), I see a very thin band of rainbow right along the edges of each tempered glass piece. There are almost no rainbows within the chunks of tempered glass that are cracked but still together. I can see the rainbows even without the polarizing filter when I hold it over the LED screen - here's a photo: My firing schedule - this is in a manual-control tabletop Olympic so I have extrapolated the dph:

450-1175-:10
AFAP-1400-:10
AFAP-1050-:30
200-850-0
AFAP-70

Do I need to anneal the whole piece longer? Or hold the de-tempering at 1175 longer?

It's almost certainly incompatibility. It's a good practice to only fuse float with itself, rather than fuse one piece of float with another. In time, the piece will probably crack further.

Thank you Brad. I'll not waste further time on tempered fused to other glass.

The piece isn't actually cracked right now, so I would like to hang onto it just to see what happens. Unless... is this incompatibility the type that could lead to the piece actually exploding later on? Or would it just crack?

The tales of exploding glass I've read online really scare me.... would love to know more about when / why that happens.

It could either explode, just crack or just sit. It is a crap shoot what it will do
The other Jim

FusedLightStudio wrote:The tales of exploding glass I've read online really scare me.... would love to know more about when / why that happens.

I wouldn't necessarily believe everything you read about online, but basically there are four kinds of cracks:

1. Incompatibility, where the coefficient of expansion between two glasses is large enough to cause the crack. This can happen right away, or even take years to surface. It can be explosive or just a crack. It all depends on the severity and type of the incompatibility.

2. Thermal shock, caused by heating or cooling the glass too fast. This is usually a simple crack into two or three pieces, but it can also be a slit in the top of the glass (usually during slumping). Rarely is this a partial crack, usually it cracks all the way through.

3. Annealing, caused by cooling too quickly (or not holding long enough) in the temperature range from roughly 1000F to 800F. This is most often an S-shaped crack that does not split the piece in two, but it can take other forms as well.

4. Sticking to the kiln shelf, caused by not enough kiln wash or shelf paper. Usually this is small cracks on the underside of the piece.

Thanks for that Brad and Jim. Appreciate the details on cracks.

In case my post wasn't clear.... there are no cracks at all in this piece. Only lines that show where the tempered glass chunks cracked when I deliberately shattered it. I kept those chunks intact in the design and I am glad that the lines are still visible.

I want to be sure I understood the rainbows. Rainbows showing in a polariscope indicate stress in the glass, so that would be where the glass MIGHT crack in the future. Am I correct about that?

FusedLightStudio wrote:I want to be sure I understood the rainbows. Rainbows showing in a polariscope indicate stress in the glass, so that would be where the glass MIGHT crack in the future. Am I correct about that?

Yes.

I assume by "rainbows" you mean bright white areas of stress, usually around the corners of the pieces that are incompatible.

Brad Walker wrote:
Yes.

I assume by "rainbows" you mean bright white areas of stress, usually around the corners of the pieces that are incompatible.

Well, no, they are rainbow-colored. I think you can see them in the photo that I attached if you blow it up. They are just thin bands of rainbow around the edges of the tempered glass that's fused onto the pane glass.

FusedLightStudio wrote:

Brad Walker wrote:
Yes.

I assume by "rainbows" you mean bright white areas of stress, usually around the corners of the pieces that are incompatible.

Well, no, they are rainbow-colored. I think you can see them in the photo that I attached if you blow it up. They are just thin bands of rainbow around the edges of the tempered glass that's fused onto the pane glass.

I can't see what you're referring to, but I'm wondering if the polarized film isn't quite oriented right. The two lenses need to be turned so that they don't let any light through. Everything should look black except for any areas of stress.

I know this is an old post, but I think those rainbows you're seeing are just the pixels of your screen being magnified/distorted by the edges of the tempered glass pieces.

Glass Fever wrote:I know this is an old post, but I think those rainbows you're seeing are just the pixels of your screen being magnified/distorted by the edges of the tempered glass pieces.

Oh, interesting point. Thanks Glass Fever!