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What causes glass separating and moving in the kiln. The pieces were fully fused

From that description, thermal shock . . .

Well, sometimes two glasses find out they're no longer compatible. Glass grows old and changes, and suddenly they're no longer interested in the same thing. Some glass wants to be optometric glass, while the other wants to be decoration. It's especially difficult on the glass children.

Nevermind the family dynamics- what glass did you use and what did you do to it? (step away from the kiln and nobody gets hurt...)

The glass is 96 coe, there were two different firings. There were other pieces in both that fused correctly. Two were almost exactly the same, one piece was perfect, the other split and then moved.

Pictures

Thermal shock in at least one instance, probably both, i.e., the glass was either not well-annealed in the previous firing and/or the last firing ramped up too fast.

I suspect, since those are both tack-fuses, that you're not annealing long enough. Tack-fuses can require 2-4 times longer annealing cycles than a standard, full flat-fuse. And if you put an under-annealed piece back in the kiln, it frequently will thermal shock unless your upramp is very, very slow (and sometimes even that won't help). The shock can be violent enough to push the pieces apart.

Hard to tell what happened, though, without the schedule. What firing program were you using?

for the green and black: 300 To 900 hold 15, 100 to 1200 H 40, Full to 1450 h 30, 1000 to 960 h60, 200 to 725 h 10, 400 to 100 h 0, original firing, the clear piece was originally fused with this
for the clear piece it was the same except high temp was 1475, there were other pieces in both firings that came out okay

Well, it's not uncommon to have some of the work in the kiln do just fine and other pieces thermal-shock. It depends on the composition of the pieces, the colors you're using, the design, etc. And in some cases, the pieces that look fine could still be stressed but just not have cracked (yet).

As a rule of thumb, I begin my anneal at 1000, hold for an hour and step down 50 degrees to 950, hold for an hour, step down 50 degrees - etc. till I hit 850 degrees. Then I cool 300 degrees per hour till I hit 300, then let the kiln naturally cool to room temp. I have a Skutt GM1414, and I pack that bad boy full.

In addition, are you sure all the glass was 96? I ask because I use to buy glass from a shop, and they would sell glass scrap containers that were full of Spectrum glass. I knew for a fact that the Spectrum glass in the container was not all 96, but that didn't stop them.

Judd wrote:Well, sometimes two glasses find out they're no longer compatible. Glass grows old and changes, and suddenly they're no longer interested in the same thing. Some glass wants to be optometric glass, while the other wants to be decoration. It's especially difficult on the glass children.

Huh? Is there a supporting source for this? How old is old? I'm getting old and changing too, but I'm not a stable chemical amalgam.

I do not think COE changes with years.
I have suffered recently of temporary incompatibility.
The piece had a large base of Tekta. On top was fused a 4x7" oval of 1808 light sky blue tint.
The blue absorbed readily the low infrared around 450 C (=ca850 F) while Tekta lets it go trough. The have a compatible COE, rate of expansion but achieved a different level of expansion. BANG.

That is one reason to ramp up slowly so that the lazy Tekta does not lag after.

Disclaimer: This is only my educated guess.

Jerrwel wrote:

Judd wrote:Well, sometimes two glasses find out they're no longer compatible. Glass grows old and changes, and suddenly they're no longer interested in the same thing. Some glass wants to be optometric glass, while the other wants to be decoration. It's especially difficult on the glass children.

Huh? Is there a supporting source for this? How old is old? I'm getting old and changing too, but I'm not a stable chemical amalgam.

poetic license

poetic license[/quote]
The osteopath tells me than I'm suffering from atrophied funny bone.

Judd wrote:Well, sometimes two glasses find out they're no longer compatible. Glass grows old and changes, and suddenly they're no longer interested in the same thing. Some glass wants to be optometric glass, while the other wants to be decoration. It's especially difficult on the glass children.

Satire. Right?