Problem with Slump

[jim burchett]

cracks on bottom side of bowl

I have a bowl made of Be red with some pieces having a light overcoat of White. these were cut into strips, fused, sandblasted, then fired again to a slump. When i took the bowl out it looked great...until i turned it over. It has multiple cracks...see pic

Wondering if there's any ideas of cause? This was taken to a top temp of 1150 in the slump and was in the kiln with several 1/2" thick pieces so was very slow up and down. Is it the red? Thnks

[Bert Weiss]

Those cracks are caused by slight thermal shocks during the heatup. Your fire polish and anneal make them stable now. Because of the stretch during the slump, I don't think you can make those marks go away. If the glass is a candidate for a pot or wire melt, reuse it...

[jim burchett]

Thanks for all the input..I'm thinking that perhaps the white had a problem with multiple firings and became unstable...dont plan to re use this glass..will keep it as a lesson.

[Morganica]

Those cracks are caused by slight thermal shocks during the heatup. Your fire polish and anneal make them stable now. Because of the stretch during the slump, I don't think you can make those marks go away. If the glass is a candidate for a pot or wire melt, reuse it...

Take a closer look at the pattern those cracks are making, especially in the second photo. Thermal shock doesn't repeatedly outline a specific color of glass ; it's more likely to make a couple of big lazy-S cracks than a lot of tiny striations.

I think you've got two possibilities, Jim:

1) The white is a lot stiffer than the red and there's not much of it, so the red's soft and moving down while the white is still brittle. The white's cracking just enough to keep up with the red. I've seen similar cracks when I've gone into the kiln and pushed down on a slump before it was ready to go. The softer glasses were fine, the harder glasses cracked in small longitudinal lines across the bottom of the piece, with the biggest directly below where I pushed.

2) There's a compatibility issue, possibly the glass shifting in multiple firings (although I'd think the red more likely than the white).

Without knowing more I'd probably bet my money on #2, but either's possible.

[Bert Weiss]

I don't disagree with Cynthia about the causes. They are still thermal shocks that happened during heatup (not compatablilty stresses that happened afterward). Compatibility issues between the glasses exacerbated the stress, so that they happened at specific places. The stresses were not great enough to make the lesions longer.

[Warren Weiss]

Jim,
What was your fireing schedule?

Warren

[Bert Weiss]

Compatibility is about 2 glasses being able to survive side by side in perpetuity. Reheating is a different playing field. Sometimes you just have to go slower.

[Brock]

This is so obviously incompatibility nothing else need be said. Speed in a firing CAN be a factor, but Jim said, "was very slow up and down" so that is unlikely. Even if speed had been a factor, the underlying cause is incompatibility. Look at the cracks, they are all on the white axis. In fact, some of the white is completely encircled by a crack. Classic incompatibility . . .

[Morganica]

I don't disagree with Cynthia about the causes. They are still thermal shocks that happened during heatup (not compatablilty stresses that happened afterward). Compatibility issues between the glasses exacerbated the stress, so that they happened at specific places. The stresses were not great enough to make the lesions longer.

I suppose it depends on your definition of "thermal shock--" that would be a pretty broad definition. I would say exactly the opposite: Heat exacerbated or sped things up, as Brock said, but probably wasn't the proximate cause of the cracking.

Rather, the glasses just didn't fit together. There could be a permanent change in their physical characteristics, i.e., the white and red became incompatible (and multifired reds are notorious for this). Or the misfit was temporary; the heatwork was enough to soften the red glass but not the white, and so stress fractures opened up in the white as it was flexed down by the red.

I generally call it thermal shock if the primary cause of the break was rapid temperature change, especially if the break follows the classic "lazy S" crack, whether or not it actually opens up and breaks. A small series of parallel cracks in one particular glass, especially if those cracks outline the interface between that glass and another, doesn't fit that profile.

[jim burchett]

I appreciate the input..needless to say that I will not be pairing these glasses in this way again.Did a similar bowl with orange and fuschia and it worked fine, so I'll assume that the red and the white just dont get along.

[Terry Ow-Wing]

which red and which white are these colors? I had some trouble with French Vanilla and Deep Red Opal on pieces that were 19 x 14 but no problems on small items less than 8x8.

[Bert Weiss]

Stress is stress. Incompatibility creates stress. Thermal inequality creates stress. When you have some of each the glass is more prone to crack. These short lesions are created by a relatively small amount of stress. We know that 2 glasses can be under a certain amount of stress and not break. However when you add a thermal stress to the situation, it can push it to crack. Because the total stress is not tremendous, the crack is short. The placement across the white areas certainly suggests incompatibility is at work. If the glass cracks as it sits around, then certainly the glasses could be incompatible enough to be an irreconcilable difference. 2 glasses can be compatible but still under stress, just not enough to cause a crack. As I said, add some more stress to the mix and you get a crack. I believe a close examination of the glass will reveal that all these cracks happened during the heatup and not after the anneal. This is why I think thermal shock caused the already stressed glass to crack.

[Brock]

As long as you understand that incompatibility is the reason for this problem . . .

[Bert Weiss]

As long as you understand that incompatibility is the reason for this problem . . .

My point is that the level of incompatibility may not be enough to cause the room temperature glass to crack, but when you add a temperature inequality, it rises to the level where it cracks.

My understanding of one example is a difference in COE between 1 and 5 points will put the glass under stress, when it gets to 6 the stress will be strong enough to crack the glass and relieve itself. COE is just one of the factors that can create stress. Add another and it gets worse. Stress is stress, regardless of the cause. you can bend a piece of clear glass between polarizers and see the stress appear as it is bent and disappear as the force that is bending it is removed.

We know that incompatibility alone, and heat shock alone, can cause cracks that look different. A combination of 2 sources of stress can also be in play.

[Brock]

Let me try one more time. There would be no problem if the glasses were not incompatible. Got it?

[Gail L]

Jim, I would be curious if it was 0013 Opaque white. I've used 0013 in some pattern bars, it is SO stiff that when it is fired the other glass around it flattens. It creates a little bit of a dam effect. Not sure this makes sense, but it is so stiff that its really hard to get it to move.

[jim burchett]

Gail, yes it was 0013 Opaque White.