Would air trapped between layers of glass cause the glass to break when heated to high temperatures?

When heating, you’re essentially increasing the pressure exerted by the air on the glass in which it’s trapped. As the air has no means of escape if enough heat is added the increased pressure will eventually exceed the strength of the glass leading to fractures. If you’re really interested you could probably figure out just how much pressure is building via the ideal gas law.

It could also be you’re heating the glass itself too quickly, that’s easy enough to test by just putting in a single sheet and seeing if you get the same results without the air pressure.

Your problem may be the bits and pieces of glass lying between the two layers.

House windows are built on the same principle as you have described and they do not break in hot summer sunshine. They do not have bits and pieces of glass lying between the two layers.

Try this one more time without the bits and pieces and see what happens. jp

Doesn’t seem likely. If the glass has already begun to fuse so as to entrap the air, then it will be too plastic to break. It would just deform with the expanding air.

It’s possible that you’re using glasses with different coefficients of thermal expansion. Different formulations of glass expand and contract more with temperature changes. If your glasses are incompatible, they will create stresses as they cool.

Also, you’re doing this in the microwave, right? Are you allowing the kiln to completely cool before opening?

Thanks for your answers so far.

@thorninmud The pieces are breaking before the glass is hot enough to be “liquidy” so I’m thinking that’s why it’s breaking rather than deforming? I use only glass with the same COE (coefficient of expansion) – 90 – so that’s not it. It is in the microwave. Yes, I let the kiln cool before opening. But the glass is breaking while it’s heating up, not after I open the kiln.

@Mariah You are heating it in a microwave? ? ?

Are you doing a pre-heat cycle on “defrost” before cranking it at full power?

If the glass is not yet soft, then the air should have no trouble escaping. I still don’t think it’s the air.

@Tropical_Willie I have a kiln that goes in the microwave, yes.

@thorninmud I haven’t tried using “defrost” but I do start on a lower power setting. Unfortunately we have a really strong microwave (1500 watts) so I usually start at 50% or lower. Do you think it would help to use defrost instead? Thanks for all your help.

That is a strong microwave. Defrost is typically 30% power. I’d give that a shot, 5–6 minutes, before letting ‘er rip. How big are the pieces you’re firing?

Okay, thanks! My pieces are usually no bigger than 1” by 1” and ¼” thick.

Alright, I’m here to brag about some success! Yesterday I broke a beautiful, beautiful piece. I’ll admit I shed a few tears over it. Last night I glued it back together and let it dry overnight. Tonight I put it back in the kiln, fully expecting complete failure. I started the microwave at 20% and slowly worked my way up to 50%, then repeated at 50% for 30 second intervals until the piece was glowing bright red. Let cool. Opened up…the two broken halves have fused together and it’s like it never broke in the first place. HELL YEAH.

Thanks for the help, jellies.

Yay!