Q: Annealing around trapped air

[glowbawl]

Hello, first post and I'm pulling out a big pounderous question...
I'm working with half inch plate and am trapping pockets of air purposely.
The air entrapment is several cubic inches easy. In a couple of instances I can ascertain that the air is completely trapped before annealling. I'm computer ramping the anneal and other than having a inch thick kiln shelf under the piece am pretty sure I hit all the necessary ramp points coming down.

Here's the problem...I'm having stress problems, the glass doesn't show that it is under stress but a couple of months after sitting around a couple have split into two, not along the seams but straight across the whole piece like a stress fracture (but then those split peices didn't show having any other stress???
Could the air entrapment be the problem. Perhaps triggered by a change of atmospheric conditions. I'm familiar with pumping down glass, but not able to be certain that soda-lime glass would take the pressure of a negative atmosphere. I was thinking that the pressure of the air trapped under the contracting glass wouldn't be all that great, but would it throw off the annealing? Thanks...Any feedback would be helpful.

[Tom White]

We would have to see at least the annealing side of your firing schedule to comment.

[Lynne Chappell]

I would think that as the glass cooled, the pressure inside the bubble would be considerably reduced. So I guess if the air pocket is large, it could create some mechanical stress. I don't know if this has anything to do with annealing however. I guess there is no way you can vent the bubble afterwards to equalize the pressure? I find it hard to imagine that 1/2" plate glass is succumbing to this pressure, however perhaps what you are doing is beyond my imagination.

Or another scenario - what you are ending up with is a sheet of glass that is 1/2" thick in some places and 1/4" thick in others (or 1" and 1/2" - not sure what the layup is). This could easily create an annealing issue and an uneven cooling of the glass.

[Lauri Levanto]

In addition to what Lynne said
The air space is an insulator (think insulated double windows.
The top glass anneals essentially one sided,
but the bottom glass cools down one side through the shelf
the other side trough the insulating air and top sheet.

That is a complicated situation asking for very extended annealing scheme.

The only answer is polarisation test.
-lauri

[Bert Weiss]

What is happening is poor annealing due to the existence of thick and thin glass places in the same piece. You have to significantly increase the annealing time, to let the thick glass get to the same temp as the thin. Unless this happens, the glass can not anneal. Common annealing schedules are for glass with consistent thickness. You need to jump up a level or 2.

[lucho/lachezar/]

I'm behind Dave... I had a piece with trapped air around an inclusion which I gave to cover its back with mirror amalgam, and as they use vacuum in these chambers, it broke from the bubble. However, if you are not going to force your pieces to an opposite vacuum extrema, an adequate annealing should help. A latitude difference theoretically might be an issue in this aspect, but I don't really think so. Another trick that could help is to make these same size air pockets but between thicker sheets of glass and then the strength of the vacuum will be not so big compared to the strength of the glass. This will need some testing as thicker glass behaves differently concerning bubbling on purpose than thin.
Lucho