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Approaching off-topicivity here, but it's been slow so I'll go for it:

In a glass discussion on Facebook some folks were discussing occasional breakage when firing sandblasted flashed glass. Particularly thick green. Particularly when the flash side is down on the shelf. I understand the contributing factors to breakage in this scenario: dark glass next to light glass and etched unevenivity. To make matters worse they use Hoaf kilns which go from 70 to 1300 in 11 minutes, then they cool to handling temperature in about an hour. I get it that the glass is prone to break. I imagine the thickness of the flashed layer is a greater contributor than the color of the flash: it's pretty much a black as far as absorbing light in the kiln. But why is the flash side down, more of a problem than is up side? I would think the kiln shelf next to the dark glass would help it rather than hurt. Is there an equivalent scenario in layering-up 3mm glass like y'all do?

Speed always kills glass.
Must be slow learners too if they continue to have the problem.
That of course is what you can get if you try sourcing knowledge from there instead of here.

You already know the answer, dark colours absorb heat faster, there is uneven heating of the piece due to having chewed away half the thickness of the material with the blasting and to which you add in the possible coe differences in the flashed layers. Their match would have been good enough to work in creating the material but not good enough to endure uneven high speed heating after the blasting.
The flash layer is reflecting heat upwards into the dark colour.
If you want to high speed heat normal glass layups with differing colours, you too may experience negative results.
Speed always kills glass.

Don,
I'd want to know if they are firing as fast as they can; what kind of breaks - tears in the flash, complete splits, shatters

I don't imagine the flash is really thick, although I have seen some flashed Lamberts green where the flash was thicker than the clear, so I don't imagine the difference in thickness is great, but it would be good to know.

I often slump 6mm Bullseye pieces in my Hoaf and have not ventured beyond 600C per hour as yet. I do anneal for the same times and rate as in an electric kiln though.

I don't really feel like slumping a piece of flash to test my questions, so I will need to wait to see if my assumptions are correct.

Kevin Midgley wrote:Speed always kills glass.
Must be slow learners too if they continue to have the problem.
That of course is what you can get if you try sourcing knowledge from there instead of here.

You already know the answer, dark colours absorb heat faster, there is uneven heating of the piece due to having chewed away half the thickness of the material with the blasting and to which you add in the possible coe differences in the flashed layers. Their match would have been good enough to work in creating the material but not good enough to endure uneven high speed heating after the blasting.
The flash layer is reflecting heat upwards into the dark colour.
If you want to high speed heat normal glass layups with differing colours, you too may experience negative results.
Speed always kills glass.

Kevin, yes and no. Unevenivity is what kills glass. In a tempering furnace, 10mm thick glass can be heated from room temp to 600ºC in 5 minutes without breaking it. The glass is being heated from 2 sides and is being rolled back and forth.

The Hoaf kilns are gas fired radiant heat. Electric kilns are also radiant heat. Like a body in a room with a radiant heater, the heater heats the body not the air. Radiant heat goes right through clear glass and heats whatever is on the other side. I discovered this when fusing 2 layers of clear float on a sand bed with depressions. The bottom sheet slumps before the top sheet and leaves a little air bubble inside. Since the elements are on the roof, you might think the top sheet would heat first, but clearly the sandbed beneath heats first, which is in contact with the bottom sheet of glass which heats first.

I have heard that sandblasted sheets of glass tend to break in tempering furnaces. I have also been told that if you heat and anneal that sandblasted glass before you temper it, it has a decent chance at survival. I believe sandblasting changes the stress patterns in the sheet. So, it is not surprising that if you fire sandblasted flashed glass with color down, it might break in the process.

Another strange thing with speed and unevenness. I was making some oval door handles out of 19mm float glass. These were about 9" x 6"

I could fire these in my kiln from room temp to 1500F in an hour and half easily. That was until I drilled the two holes into them for mounting. Once there were two 10mm holes in the glass I had to slow WAY down. I wouldn't have thought two holes would make much difference but it really did.

Rick