Frit casting, thermal shock and bubbles

[Jenna]

Since bubbles are an inherent part of frit casting, is there any point in doing a long bubble squeeze? From my experiments, it doesn't seem that I get any significant decrease in bubbles even with a long hold at 1250, so can I shorten this part of my firing schedule? Has anyone eliminated all bubbles with a really long soak, say 3 plus hours? Haven't tried that yet. Wouldn't the bubbles have a better chance of rising when the glass is at its most liquid state/top temp?

Also, is frit subject to thermal shock on a rapid ramp up? (400 deg/hr)
If firing to full fuse, would it even be noticeable?
Insights much appreciated. I have been wondering about this for a while.
Cheers, Jen

[Morganica]

What a bubble squeeze does is slow down the sagging process, so that the glass has time to gently compact at the heaviest/closest points, pushing out air. The more time you spend squeezing, the more bubbles you'll remove...to a point. Once the glass has dropped down and sealed the edges, a longer soak isn't going to do much good.

You might be able to eliminate more bubbles buy soaking at a lower temp with a slower up-ramp once you're past the strain point. I've never tested it; I suspect it would help a little bit.

The best way to remove more bubbles in a frit cast is to do a good bubble squeeze, then ramp to your process temp (+25 degrees or so) and stay there for awhile. "Awhile" could be 4 hours, 12 hours, or more. The hotter, much longer soak allows the bubbles to meander up through the mass of glass to the surface, pop, and settle down. It will (probably) have several different effects:

-Increased transparency
-Reduced number of bubbles
-Bigger bubbles (they have more chance to combine)
-Increased chance of holes in the top surface
-Remaining bubbles can start gathering in some areas (such as at undercuts), and move away from others, forming clusters/patterns
-You're more likely to get surface devit (or at least I am)
-The longer you keep the glass that hot, the more likely the glass will do something undesirable, like opacify when it's supposed to be transparent, change color or change compatibility

The bubbles will NOT go away entirely. Some will be too small or not have enough time to overcome surface tension/combine/rise to the top.