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I have a jen ken oval 13", I just tried the following:
RA 400
F 1450
Hold 15 min.
off

I am using Spectrum 96, my problem is on some pieces the center has thinned alot, and on another I have two extra large bubbles in the piece,
what did I do wrong? Also I checked in the peep hole at 1398 is when the extra large bubble started, yet the rest looked good, meaning the layers were beginnig to meld together with nice rounded edges. Then when I checked at 1425 with 13 min left to hold I had what I described above, so I basically shut the kiln down. I checked and opened kiln at 225 and thats's when I found what I've described. Obviously my kiln has a naughty streak, and I need to correct it asap. If you have any advice, by all means send it my way, I would appreciate the input.

next time I'm going to do:
RA 400
F 1398
Hold 10 min.

Thanks for your time
camaro

From another post:

Over the years I have tried many kiln washes, and have found the coarser ones to be far more problematic. The bubbles that erupt through the glass, that are trapped between the bottom layer of glass and the shelf, can be eliminated by your firing schedule. A rapid rise to top temp. is usually the cause for these bubbles. A weak point is where they will appear. A weak point is where elements of your design meet, ANY gap there can be the locus of these bubbles. I do all my firing on kiln washed shelves, and haven't had a bubble like that in years. Brock