Upper/lower temp/range for thermal shock?

[Cheryl]

I've been working under the (apparently mistaken) assumption that glass was no longer susceptible to thermal shock above about 1,000 degrees. I just re-fired a piece (about 1/2 inch thick, 1.5 x 1 foot, multiple colors, with tack fused elements) with the following, 88-hour schedule, only to find a TS crack (on the way up I'm fairly sure) running across the middle:

35 dph to 1000 1:30
100 to 1225 1:00
200 to 960 2:30
50 to 830 1:30
50 to 500 :01
25 to 100 :01

I think the culprit is either the 100 to 1225 or the cool right after (200 to 960) but would appreciate thoughts. And the kiln goddess does NOT like deadlines.

[Lisa Allen]

I've been working under the (apparently mistaken) assumption that glass was no longer susceptible to thermal shock above about 1,000 degrees. I just re-fired a piece (about 1/2 inch thick, 1.5 x 1 foot, multiple colors, with tack fused elements) with the following, 88-hour schedule, only to find a TS crack (on the way up I'm fairly sure) running across the middle:

35 dph to 1000 1:30
100 to 1225 1:00
200 to 960 2:30
50 to 830 1:30
50 to 500 :01
25 to 100 :01

I think the culprit is either the 100 to 1225 or the cool right after (200 to 960) but would appreciate thoughts. And the kiln goddess does NOT like deadlines.

I had a similar problem with a 1/2" thick piece that had multiple colors, both trans and opaque. But, my crack seemed to have come on the way down, it was sharp and visible but not feelable, the piece did not separate. My solution came with annealing much more conservatively, starting at 1000 and holding for an hour, then creeping down to 960 at 40dph and holding for 2hrs, then creeping again at 40dph to 750, and finally 80dph to 300. I was trying to accomodate all of the varying colors and the differences in annealing points between the trans and opaques. It has worked well for me, no more cracks.

It might be worth taking it back up to full fuse, healing the crack, and taking it down much more slowly cause that is alot of glass to lose. Also, remember to go really slowly back up since the piece is already stressed. I did 100dph to 1000 in my healing fusings.

Hope you can fix it and meet that dasterdly deadline.

Lisa

[Bob]

Hi Cheryl,

Your firing schedule is cautious enough that it should not have caused thermal shock. Did you have large areas of white and black glass together? Were the breaks related to edges between colours?

Fire your kiln goddess...

Cheers,

Bob

[Cheryl]

The cracks in both pieces occurred in large expanses of clear with various colored inclusions. In one case, the glass separated entirely. In the other, the crack (which is quite jagged) is there but the glass is together.

[charlie holden]

You are not mistaken -- glass will not thermal shock above 1000 F. That is not to say that all of your glass is above 1000 F just because your thermocouple says that's the temperature of the air in your kiln. If you are getting thermal shock in a 1/2 inch thick piece after going up at 35 dph then something else is going on. Either there is something keeping part of your glass cool or something heating part of your glass up so that temps in your kiln are radically uneven.

You don't say whether the glass is in or on a mold or not, whether there are many differences in thickness of mold or glass, or whether the edges are near any elements. I'm assuming the second firing was a slump since you only went to 1225 F. What was the set up? Should I guess that it was a drape over a mold? If so then it's not thermal shock.

ch

[Judy Schnabel]

I recently had a problem with a pattern bar piece. I had successfully fused the blank but when I slumped it I ran into problems. My first one cracked on the way up.

I had the nerve to try the same thing twice. On the second slump I took the advice from the board but this time the disaster was on the way down.

Don't know if this will help but I've copied Jim Gregory's advice to me. Jim is the fusing guru at Spectrum Glass.

Take it for what it's worth. Here's Jim's quote:

"My first guess as to why your pattern bar platter cracked, has to do with thermal shock. You probably could not have picked out a more difficult color combination. The thick, dark pattern pieces will absorb and hold heat much faster than the thinner white platter base. As the thick pattern pieces expanded with heat, the cooler, white perimeter stayed contracted, until the dark pieces forced the white to expand rather dramatically; crack. You were wise to use a very slow rate of rise, but it needs to be slower. I'd suggest steps. Go to 200 degrees and hold for 10 minutes; then to 300, hold another 10 min.; then 400, hold for 10, then 500 and hold for 10 min. From there, I'd ramp up at 250 or 300 dph until 1050. Then you can do almost what ever you want.

It is also difficult to fully anneal a thick/thin and dark/light piece after both your first and second fuse. It is most critical to even out all the temperature differences throughout the glass, the kiln shelf and kiln atmosphere at around 960 degrees. If the thickest part of your piece is near 1/2 inch, I'd hold for almost an hour, then very slowly creep down to 800 degrees at about 100 dph or slower. After that, 300 dph should be safe.

There may be a compatibility issue with the white glass. If you have some polarized film, you could confirm the expansion fit by doing a simple compatibility test. I'll do that when I receive your samples."


Judy

[Cheryl]

The purpose of the fuse was to tack elements onto these large pieces, hence the low temp. It was just a straight fuse, no slump. (Turns out I didn't really go hot enough to get the pieces to stick anyway.)

The elements would make some rather radical differences in thickness - but the elements are not where the glass cracked.

might try to post a picture - not how.