weird crack

[lauren]

i forgot to take a picture before i put it back in the kiln to try and fix it, but this piece i was working on a few days ago had the weirdest crack in it! it came out of the kiln fine, today when i went to go move it off my work table it had split in two. it was 5 layers of BE thin, the four bottom layers cracked apart and the top (clear) layer did too, but it was a different crack line! running in roughly the same line, but definitely separate cracks.

i dont remember the exact schedule i used, i do recall i looked up a firing schedule for it since it was so thick...i'm prepared to admit i didn't anneal long enough even tho i thought it'd be ok, i'm just intrigued by the two cracks thing. is that just another kind of stress crack?

danke

peace
- l

[Cynthia]

i forgot to take a picture before i put it back in the kiln to try and fix it, but this piece i was working on a few days ago had the weirdest crack in it! it came out of the kiln fine, today when i went to go move it off my work table it had split in two. it was 5 layers of BE thin, the four bottom layers cracked apart and the top (clear) layer did too, but it was a different crack line! running in roughly the same line, but definitely separate cracks.

i dont remember the exact schedule i used, i do recall i looked up a firing schedule for it since it was so thick...i'm prepared to admit i didn't anneal long enough even tho i thought it'd be ok, i'm just intrigued by the two cracks thing. is that just another kind of stress crack?

danke

peace
- l

Are you sure that the clear top was the same coe as the four lower glasses?

Five layers of BE thin would be only be 7.5 mm right? which isn't that much thicker than 6mm, or 1/4", or would only be 1/16". or 1.5mm thicker than standard, so your anneal schedule shouldn't be excessively longer.

I am wondering if your anneal is off, but also without more info and particularly the schedule, it's hard to make a call.

What shape was the crack? Were the edges of the glass on the crack sharp or rounded? Are you sure the crack happened on the way down? If it was thermal shock, it could have happened on the way up and the different glasses would have broken apart in different places.

Give us all the dirty details please as best as you can recall?

[lauren]

i don't use anything but BE, the clear glass i used was new stuff i just got in so i'm open to the possibility that a mistake happened but i don't think so...

the crack happened sometime after i took it out of the kiln, the piece had been sitting out for about 2 days before i went to move it and noticed it was cracked.

the crack was on the bottom four layers kinda reminded me of a stairway, sharp angles and steps, while the crack on the top layer followed the same general path but more of a straight line.

dwelling on it, i must have done the math wrong, i did a firing schedule more appropriate to a piece 3/8" thick ....i remember i made it a li'l faster since it was a pretty small piece, but i referenced the BE schedule 375 dph to 1000, afap to 1500, hold for 10 minutes, afap to 960 held for 45 minutes. i'm pretty sure i sped up the heating time so that it took a little less than two hours, and i only let it anneal for half an hour, maybe less. i then let the kiln cool naturally to 400 when i opened it up and let it air cool from there on cuz i was kinda impatient.

all those temperatures are approximate, without a digital controller i do my best to keep it at those temperatures, but especially with annealing temps it's an 'approximate' at best.

maybe it was opening the kiln when it was still so hot? i usually don't do that, that's about all i can think that i did differently...

thanks for theories, i'm kinda bummed the piece might not make it but this crack is definitely an interesting one

peace
- l

[Cynthia]

...dwelling on it, i must have done the math wrong, i did a firing schedule more appropriate to a piece 3/8" thick ....i remember i made it a li'l faster since it was a pretty small piece, but i referenced the BE schedule 375 dph to 1000, afap to 1500, hold for 10 minutes, afap to 960 held for 45 minutes. i'm pretty sure i sped up the heating time so that it took a little less than two hours, and i only let it anneal for half an hour, maybe less. i then let the kiln cool naturally to 400 when i opened it up and let it air cool from there on cuz i was kinda impatient.

maybe it was opening the kiln when it was still so hot? i usually don't do that, that's about all i can think that i did differently...
peace
- l

Bingo Lauren. Knowing you opened up your kiln at 400F clinches it. The outer skin cooled very rapidly and the interior remained hot creating stress in the glass...Thermal shock.

Wait at least until 200F to open the kiln, and then just prop it an inch at most.

[Jerry Barnett]

Thermal stress arising from temperature differences in the glass as it cools from its strain point to room temperature, in combination with residual annealing and compatibility stress, can be enough to break the glass. The hotter the kiln is when opened, the easier it is to have sufficient temperature difference develop. Like Cynthia, I wait to about 200.

But this thermal stress is temporary, diminishing as the temperature difference diminishes, and is gone when the glass reaches room temperature. If the piece made it to room temperature without cracking, it was not opening the kiln at 400 that caused the break.

Jerry