Large bowl woes continue!!!

Anea · Post by **Anea** » Fri May 30, 2003 8:27 am

It is about 5:00 am over here in California. Does everybody remember that lovely20" round tile bowl that bubbled on the slump in the ball surface mold from bullseye? Well I woke up at this very early hour to check on that piece when it would be at its peak temp for slumping 1130, and it has two cracks in it I am bit upset and discouraged right now. (this is the second large piece that has gone bad for me in two weeks and I normally do larger items without a problem)

Anyway, the cracks are in the exact same spots on both side. They effectively cut off the sides of the round circle making it sort of like a rectangle with round edges on the shorter sidse. I am pretty sure that it is from the extra holes we drilled. I may have put too many. We probably added aboout 6-8 holes (can't remember exactly and I can't look), but I took them around around the one hole provided, 2 there I think and then made a square of holes around those, all about 2-3 inches above the other, heading up to the sides and to edges of the mold. I am thinking that there are holes (1 on each side possibly) right along the horizontal line that the glass cracked in

It looks like it cracked the sides completely off. And I am thinking that now my $100 mold is also useless, unless there is a way to fill in the holes.

My schedule was beyond conservative so I don't think it was that. 50 dph set 1130 hold 30 minutes, and this was the fourth firing at this rate. This piece has been in my kiln since Saturday!!!

So please learn from me, do not put your holes within the last 2-3 inches of a mold, near the edges and top sides of the mold.

Aimee
aka
Anea

Jackie Beckman · Post by **Jackie Beckman** » Fri May 30, 2003 9:01 am

I really don't think it was the holes -

charlie · Post by **charlie** » Fri May 30, 2003 11:52 am

i doubt that it was the holes either. you can fill them with just making a paste of kiln wash and dabbing a little in the hole, since of course you didn't make them too large.

are the edges of the crack soft or sharp? that'll tell you when they were made (ramp up/down) and you can change the schedule to fix it. of course, the more often you heat it up and the longer you anneal, the larger your chances that the coe changes to cause cracks. eventually you'll have to just abandon the glass and slice it up for something else, probably smaller.

Cynthia · Post by **Cynthia** » Fri May 30, 2003 1:42 pm

I agree with Those AZ folks. It wasn't the holes. Your mold is probably fine, especially if you treat it with care...

I have questions to add to charlie's. What colors are the glasses you are using? How many times has this piece been fired? Any hifgh fired or component glass pieces in there that were fired prior to incorporating into the piece? I'm beginning to suspect the possibility of incompatibility.

Sometimes if you bend a large piece too rapidly it will split apart, but you went slow as a tortoise...

and so I think there is a compatibility issue or an annealing issue that has caused the glass to crack once you stressed it again with heat.

One idea anyway...

Anea · Post by **Anea** » Fri May 30, 2003 1:55 pm

This piece waws being fired its fourth time. It cracked on the way up as I looked at it when it was at its highest temp during the slump and it had not begun to come down yet.

The annealing schedule was the same for each firing and I went up at only 50 dph to 1100 and then if I was full fusing 100 dph to 1450:
From process temp AFAP to 960 hold 2 hours
60 dph set 700 hold 30 minutes
Cool to room temp without peeking

The colors varied, but the base was clear, the only opaque glass was white and gray, oh and a tiny bit of yellow, and then there were transparent pieces of blue/gray and other blues. It was a tile plate so the thickness varied from 6 layers down to two depening on where the tiles were, which is why I have been so careful with it.

If it wasn't the holes then I guess it just didn't like being fired fourth time. I believe it was properly annealed and the COE must have changed then? There were no high fire pieces, it never went over 1450. My kiln is a jen ken so there are side and top elements, I can fit a 23" shelf into it. The base glass was BE and the elements were all UB. Hope that is all the anwers to the questions. This is just a bit baffling

Aimee
aka
anea

Jackie Beckman · Post by **Jackie Beckman** » Fri May 30, 2003 3:15 pm

Since you happened to mention "baffling" I'll suggest thats just what you do if you try to repair this again. Here's what I think the problem could be - You say the piece has been fired 4 times, which makes it a little touchy to begin with, but shouldn't really be a problem. Its been breaking since the second firing, right? But given that it IS the fourth firing and your shelf is only 23" and your piece is 20", that means you are pretty close to the side elements. If that kiln has the option of turning off your side elements, do it, if not, baffle with fiber board, cut up shelves, or whatever you have handy. I think that even though you are ramping up slow enough the direct heat from the side elements are causing the edges to heat up quicker than the center and you're thermal shocking the piece.

Jackie Beckman · Post by **Jackie Beckman** » Fri May 30, 2003 3:32 pm

Oh, and another thing to consider - I just re-read your post and realized that the base glass was BE and all the elements are Uroboros. That could very easily be your problem. I use some Uro mixed in occasionally and although I've never personally had a problem, I know others have once in a while. Yes, they both may have a COE of 90 but they won't fit together every time in every situation. Is the crack coming from the same spot each time? If so, compatibility may be your problem.

Rob Morey · Post by **Rob Morey** » Fri May 30, 2003 3:36 pm

Aimee,
I think that Jackie has a point there. Also, try holding at 700 for about 20 minutes. That is a stress point and it may help to do a little soak there. Hey, do you remember that big blue piece that blew apart when I tried to slump it. That was the third time in broke. So I smashed it up, took out all the high fire elements and used it the rest in another piece this week. Guess what, it broke again, on the way up. It refused and I'm going to try to slump it again, but I won't be very surprised if it breaks again because I think that the glass is not compatible any more, probably never was because of the high fire stuff in there. Add to that I fired it at least four times. Though you don't have anything weird in the piece, it may be a compatibility issue in which case, that horse may be dead.

Later,

Morey

Anea · Post by **Anea** » Sat May 31, 2003 1:06 am

Here is the final verdict everyone. It finally cooled enough for me to open the kiln. The holes were further away then I thought. But the way the glass cracked was directly on line with where the tiny bit of deep yellow was. Apparently it must have been the part that changed COE. On one side it cracked right along the line of the yellow saquare on top to exclude the yellow (i.e. the yellow square and everything past it is the edge that cracked and broke completely off) and on the other side, everything on the other side of the yellow edge broke off, leaving a direct line of glass left on the larger part where the yellow was included. So the yellow must have been the evil one. I also tend to agree with the sides possibly heating up more and thermal shocking, but I just wonder why it didn't happen until the fourth firing.

thanks for all the help and advice, I think this piece is a goner, off to the mosaic table land cemetary

aimee
aka
anea