Cracked/broken glass during Firing

kmoorehead · Post by **kmoorehead** » Sun Sep 28, 2003 2:47 am

I am fairly new to glass fusing/slumping. I have had a few successful firnings until today. The pieces on the lower shelf were found to be broken when I opened the kiln. Both were Bullseye on fibre board and beween 2/3 layere thick. One piece had been tack fused previously and the other had been fused and slumped previously. (I was attempting to slump this piece in another form)

When I opened the kiln the pieces were almost 1 cm apart, had flowed over the side of the mold and the crack edges rounded.

Does anyone know if the heat up is to rapid?? The program I am using is:
250C per hour to 500C,
700C in 1.5 hours
730C in .5 hour
Down to 530C in 2 hours
430C in 4hoursjavascript:emoticon(':?')

then cools naturally in about 8 hours to 100C

Jerry Barnett · Post by **Jerry Barnett** » Sun Sep 28, 2003 9:51 am

Since it appears to be thermal shock on the way up, the initial heating rate is too fast for the conditions.

What kind of conditions? I understand this to have been a slump firing, and suspect from the multiple shelves that the kiln is side elements.

Bullseye's web site (http://www.bullseye-glass.com/techinfo/heat.html) suggest that the initial heating rate for the initial fuse of 300mm diameter, two 3mm layers be 250C. (That rate assumes top elements.) It then says: "For a side-fired slumping of a pre-fused piece, increase the initial heating (time) by at least 2.5 times." That would mean no more than 100C for the subsequent firing.

I usually cannot bring myself to slow down that much on the subsequent firings, but in this case 100C may still be too fast. Because of their varying thickness, tacked fused pieces are more susceptible to thermal shock than full fused pieces. (They also require greater caution in annealing.) Also, the tendency of side-fired kilns to heat glass unevenly would be increased by the shielding of the upper shelves.

Jerry

Barbara Muth · Post by **Barbara Muth** » Sun Sep 28, 2003 10:54 am

I agree with Jerry on this. A while back I was heating my work up at 200F per hour and it was still too fast for that work. Everything was breaking in the kiln. Depending on the arrangement of the elements and the shelves and the pyrometer, it is possible that you could also be heating up a lot faster than you think on the lower shelf.

Barbara

gone · Post by **gone** » Sun Sep 28, 2003 12:35 pm

The lower shelf gets heat from the edges so you have a high temperature differential between the edges and the middle of the piece. It helps to space you shelves further apart and you have to go a lot slower with multiple shelves because of the evenivity thing.

Els