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Every now and again - like this morning - I open the kiln only to find that a piece is in multiple pieces - sharp edges, like you'd get if you dropped it on a nice hard concrete floor. Generally the glass is in about a zillion of these pieces - the whole thing is criss-crossed with cracks.

This used to happen to me sometimes when I did melts and Steve K. thought it was because a corner of the glass was touching something - a kiln brick for example - not properly kiln washed and the tug was sufficient to basically throw off the integrity of the whole piece.

Today's piece wasn't a melt, just a nice rectangular platter, but I did use dams that, quite frankly, probably could stand to be scraped down and re-kiln washed. But I'm wondering whether that's really the problem?

I was using a schedule that I've used many times before, roughly: 200 to 1000, hold for 1 hour, AFAP to 1450, hold for 1.5 hours, AFAP to 960, hold 2, so on to 830 and a very slow cool.

Is the glass:

1) Stuck to part of your dams?

2) Stuck to the shelf?

If not, it sounds like thermal shock. Your schedule would seem to preclude that, but when you have empirical evidence . . .

Brock

What does "so on to 830" mean? Since you have sharp edges on your breaks, this is the temperature region where you are having problems and need to really be in control of your descent.

How thick is the glass and what do you mean by "very slow cool"?

Tony

No real sticking - altho there might have been a tiny tug when I moved one dam. Definitely no sticking to the shelf. And the other piece in there - slightly thinner - was fine.

Thermal shock doesn't make sense to me altho it's true, it's hard to argue with the evidence. The bits are definitely the kind of sharp-edged evidence TS leaves behind.

I can't remember the exact schedule, but it's something like (after 960): 50 dph to 830, hold 1.5h, 75 dph to 700, hold 1 min., 83 dph to 200, hold 1 min., 100 dph to 75, off

This piece, by the way, is only about 3/8 to 1/4 inch thick. It's a terribly conservative schedule that I use for much thicker/bigger pieces, but was too lazy to change when I popped the current load into the kiln (figured overkill was okay). That's why I'm so surprised with the result.

very conservative indeed... surprising that you had thermal shock... makes me wonder how accurate your thermocouple is. For example, if your thermocouple was reading high, you could be starting your anneal at 900° rather than 960°... no way of really knowing that without comparing it to another thermocouple or maybe cones... have you tried the BE "knowing your kiln" tipsheet? That should give you some assurance that you're in the right temperature range.

Any chance someone opened the kiln when you weren't looking?

Tony

If the TC was really off, I'd expect this to happen much more frequently, which is why the dam sticking comes up as a possibility. But what else could it be?

And the cat (the only being in the household stupid enough to open one of MY kilns mid-cycle) can't lift the lid. Lucky for him - he's on life 8.9 of 9 already!

Cheryl,

that "tiny tug" is the reason your work thermal shocked. clean your damn dams and re-kilnwash them.

Sara

Cheryl wrote:No real sticking - altho there might have been a tiny tug when I moved one dam. Definitely no sticking to the shelf. And the other piece in there - slightly thinner - was fine.

Thermal shock doesn't make sense to me altho it's true, it's hard to argue with the evidence. The bits are definitely the kind of sharp-edged evidence TS leaves behind.

Hi Cheryl.

I agree with Sara on that one. But, in case you don't believe that that's the problem, I refer you back to earlier WG discussions where it was suggested that you can introduce stress in a piece in an overlong annealing if the kiln's evenivity is off. Run a check your kiln (as per BE) to rule that out.

Another thing. If I were to do a one and a half hour soak at full fuse, I would probably put some fiber between the dam and the glass. In fact I always do that, even though I kilnwash the dams.

Finally, though, if the piece is that thin, you may not have needed dams in the first place.

good luck next time around!

Barbara

dams were to keep strips from tumbling over. Alas! If only that were the problem!

Okay, okay, I'm kiln-washing tonite. Next up: Saturday baths, whether I need them or not.

IMHO- it's not thermal shock, the schedule is fine (if a little eccentric) for that thickness. Thermal shock doesn't shatter in a gazillion sharp pieces; sticking to a mold or shelf does it, and it happens on the way down from the slump, below 1000F.

Close examination should show more than just one place where it stuck.

I have had a piece break on the ramp up, and found that the thermal couple getting accidentally pulled out caused it. The piece was broken into about 100 wedge shaped pieces and the cause was thermal shock. The kiln controller was trying to heat the kiln for the initial ramp for a slump but could not get the temperature to read above 80°F. The end result was a kiln trying to heat up ASAP. I have learned to make sure the thermocouple is in place before I fire.
I have also experienced pieces that stick to shelves. In the sticking case the cracks were almost always parallel to the point of contact and the broken pieces were in the shape of strips or radiating rings from the point of contact. In this case the cause is not thermal shock, but stress cracking.
Take a look at you fragments and look for signs of rounding out of sharp edges. The rounding out of the edge can only happen on the ramp up. If the edges are very sharp it happened on the ramp down.

Tim

I think that most of the cracks I have been confused about in my glass (i.e. the ones where I can't find an obvious, I-can't-believe-I-did-that reason) seem to come from a sticking problem.

In my case, I am sure it is my kiln goddess's way of chastising me for laziness - which I definitely have a problem with. But sometimes I think she is just ornery and likes to keep me on my toes. Thinks OTHER folks seem to get away with (like reusing fiber paper or only kiln washing every x number of times) she doesn't care about. I find it quite irritating

Do try the fiber paper barrier if you don't mind working with fiber paper. It seems to help.

Good luck!

Linda