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I recently moved from 5500' to 8000' above sea level. I have not had a problem full fusing glass. However, I have tried to do a slump on four plates and each plate has come up with a hump in the middle, indicating the slump firing did not work. I have asked another group and I feel I have stumped them on this issue and I thought of you. There are holes in the mold. The firing schedule is as follows: 150 ph 300 hold 15, 300 ph 1235 hold 15, Full 950 hold 60, 150 ph 800 hold 10, 300 ph 10 hold 0. There are no indications of any other problem, bubbles, etc. These plates have each been full fused before the slump firing. I made four of these plates last year and did not have a problem with a hump in the middle. Maybe I need to hold at 1235 for longer that 15 minutes. Are there any suggestions?

Just a wild guess, but have you tested your kiln for temperature accuracy after the move? Sometimes the bumping and jostling can unseat the thermocouple or loosen a connection, so that the temperatures aren't quite as advertised...

Hi,
Some questions...were there 4 separate firings or all fired at one time?
Were your molds elevated or were they sitting on the shelf? If sitting on the shelf, then holes in the mold wouldn't make any difference for letting the air be pushed out. If elevated but fresh kiln wash since they were used before, any chance that the holes were plugged with new kilnwash?
For your kiln do you need to heat that high / that long for a slump?
Have you done anything else since you moved that would indicate differences in Your kiln performance? If it's not your kiln, then I think it's some thing else that you've done to change what you did before.
I think the answers to these questions will help us diagnose what's changed and what will return you to fusing/ slumping success.
Ciao,
Linn

Mary Kay from bullseye has indicated that some experience there leads them to observe that the centre of the slump is the last to settle. If this is the case, you could try a longer soak. I normally try to get my slump done in 30 minutes. If it takes only 15, I feel my temperature is too high. Remember that too fast a rise heats the top surface much more than the bottom, making it more difficult for the glass to conform to the mould.

Blame the different wall wiring to the kiln.
If you were to have measured the distance from the power panel to the kiln in both locations,
you might have found that the distance in the new location is greater or the wire used in the walls is
not as thick.
Additionally your new location may be regularly giving you lower electrical voltage than you old one.
Given that a longer firing time makes your electricity bill higher, having the wiring changed to a heavier gauge
in order to reduce line voltage drop may become extremely cost effective.

I don't quite understand the reason for your first hold at 300 deg.

With my slumps, I ramp to 1100, hold 30, and then proceed to slump temp. Often, by the time I actually hit the slump temp, my piece is "done". There's a lot of relaxing of glass that occurs at that 1100 deg F.

Dana W.

Morganica - I don't know how to test my kiln for temperature. I have done several full fuse firings without a problem.
Linn - 4 Separate firings as I have a paragon FS16. The mold is elevated on a kiln shelf. The slump schedule was one for Spectrum system 96 so I do not have to have the temperature that high, I just did because it was a firing schedule that worked well.
Kevin - I am just renting this house and the wiring in the garage is not exposed so rewiring would be okay as long as I got the owners permission. This is a dedicated circuit so I was fairly confident in the results as that is what I had before the move.
I have noticed that firing takes about 3 hours longer than before as I have kept a log, religiously, since I started fused glass. I know that everything takes longer as you increase in altitude. I was amazed at the 3 hours however.
I was told to use the Spectrum System 96 for a firing schedule and that is why I chose this schedule.
I have also made sure the holes are not plugged.