split glass

[Thomas Decker]

My screen melt split about 10 minutes into the slump. I am assuming that there was not enough heat in the glass , and therefore the split. I was wondering though weather there could be some other factors that could have caused it? my schedule was as follows 200/1000 30 50/ 1150/ 30 this was a 3/8 thick bullseye mostly browns with a little white . melted on to a clear base
the slumping mold was a 12" ball bowl mold that I have used several times with just as thick or thicker glass. any thoughts ? the following link should show a photo

https://onedrive.live.com/redir?resid=A ... hoto%2cJPG

[Brad Walker]

Thermal shock. Lots of heat on top, not as much on the bottom.

Usually you're too close to the lid elements when this happens. It's most likely with a round mold like the ball surface once, which kind of insulates the bottom of the piece from the heat.

[The Hobbyist]

Two other possible culprits:

1) Compatibility shift during melt

2) Poorly annealed

Jim "The Hobbyist"

[Thomas Decker]

thanks that makes sense.

[Thomas Decker]

ok. since it did not break into separate pieces I was able to flatten and heal the piece. so I guess a longer hold at the 1000 point should take care of the problem when I re slump it?

[The Hobbyist]

Not if it was #1 above. Sometimes taking glass to such high temperatures, as in melts, can cause the chemistry to change and one or more of the glass(es) used becomes incompatible. Reds and oranges are the most problematic but I'm sure it is possible with others. I've had this happen twice. The first time the melt came out intact. I cut out portions and assembled the pieces into a very nice composition but the second firing caused the piece to break. The more recent attempt at a melt incorporating some reds broke when they came out of the kiln.

It should not recur if the remelt (fusing the fissure) is properly annealed. But thick pieces require longer anneals and if that was the fault the first time it will happen again unless you change your program.

Jim "The Hobbyist"

[Thomas Decker]

I really don't think it was the anneal . my schedules are very concretive and have worked very well until now . I am prone to believe that it was not enough heat because for the first part of the slump everything was fine but as it started to drop down into the lower part of the mold it started to separate. My only real concern would be the shifting of the coe.

[Brad Walker]

As I said earlier, it is almost certainly thermal shock from being too close to the top elements when slumping on top of a concave mold where a heat difference builds up between the top and the bottom of the mold.

If the crack occurred while you were firing up or while you were holding during the slump, it's almost certainly not compatibility or annealing. The crack is a fairly typical thermal shock crack for this kind of slump.

Fire slower, lower, and with a longer hold.

[Thomas Decker]

I refired the piece today and it looks good. thank you Brad you were right.