I find it interesting that most places I look list the slumping temperatures for fused glass to be between 1225 degrees F and 1260 degrees F. They also always provide the disclosure that this depends on the kiln as each kiln fires differently and you really need to get to know your kiln and keep track of what works and doesn’t work to develop your own schedules.
I originally was using the schedule that was suggested with my kiln which was 1260 degrees F and at first, it worked great. But as my plate size increased, several large plates started to show a problem where a bubble developed in the bottom. Since I really didn’t have a lot of knowledge yet, I thought this meant that I was either not getting it hot enough or not holding it long enough, so I tried both firing slightly higher and holding it longer, but this only seemed to make matters worse.
Then I enrolled in a cold working class and mentioned this during a break and both the instructor and another student immediately piped up that I was taking it too hot. The heat was pushing the glass down in the mold and hence then pushing the center up into a bubble.
In their opinion, you never need to go over 1180 degrees F for a slump. And I recently slumped a large plate that was only 3mm thick rather than 6 mm thick and it has a slight start to a bubble in the center. So I think for 3mm thick pieces, I could go to an even lower temperature like 1160.
Since I lowered my temperatures on my ceramic plate slumps to 1180 degrees F, I have had no further bubbles. Yeah!
A side benefit is that at such low temperatures, the kiln wash does not wear off as quickly and so I can get several slumps on one application of kiln wash!
Okay, so there is always a caveat. I labeled this as specific to plate molds as these tend to be shallower. I have not tried a large deep dish mold yet (and do want to try one :-)), but if I use a drop out mold, coaster mold, or slump over a stainless steel form, then I need to go to a higher temperature in order for the glass to drop or slump enough into the mold.