Glass cracks on fused glass oil lamps I made

[Bharr]

I slumped a 3mm piecs of 90 Bullseye irr. into a 5 in square bowl 1" deep. I put a piece of this glass on top. It has a 1/4" hole in the center for a wick and glass wick holder.. I filled the bowl with paraffin lamp oil. They have burned for entire days, but a few have cracked apart. I am using a very long annealing schedule. Is it the single layer that is the problem? I am stumped. Will cold air outside cause this?

[Brad Walker]

If the hot flame comes close to the glass or touches it, or even if it burns long enough for the glass to get really hot, then the glass will probably crack due to thermal shock.

Do you have a photo of the setup and the crack? That would help confirm the cause of the breakage.

[Bharr]

Thanks Brad.
Sorry, threw away the pieces. I figured that it was thermal shock. Actually mine looks very similar to a pic I have attached. I don't see anything different in them than like those from Charlton or others I have seen. The glass wick holder touches the glass top. I don't see anything between the holder and the glass top for insulation on any I have seen. These oil lamps are a main part of Charlton and some other peoples lines. I am stumped.

[Bharr]

They basically cracked in half right through the center hole.

[Rick Wilton]

If the hole was drilled but not either refired or "smoothed" out it will easily break from the heat. When a hole is drilled and not either fire polished nor cleaned up there are dozens of "micro starts" these are microscopic flaws in the glass just waiting to "run" like a score mark. Think about how strong the glass is until you put a simple scratch (score mark) and suddenly you can break the glass.

[Bharr]

That makes sense. I will try it. Thanks