Hi! Last week I fired a 10" 5mm flat mosaic piece perfectly 400 deg F - 1225 - hold 1:00 600 deg F - 1490 - hold :10 AFAP - 900 deg F - 1:00 150 deg F - 700 - :01 down to 100 deg F before kiln was opened
This week I fired a 11" 6mm flat mosaic piece and got a huge bubble! I thought that my firing schedule reflected the 1 mm and 1" increase in size... 300 deg F - 1225 - hold 1:15 600 deg F - 1490 - hold :10 AFAP 900 - 1:00 100 deg F - 700 - :01 down to 100 deg F before kiln was opened
I'm using a Skutt GS22cs (22" clam shell with top elements). The piece was place right in the center. The original piece was driftwood gray with "scraps" and the second, celadon with "scraps." Both pieces were placed smooth side up on a solid piece of clear Tetka, mosaic on top. Both were fired on a mullite shelf with thin fire. Both were tack fused prior to adding the aventurine blue frit powder to the "cracks/lines." The frit level was well below the 3mm level. The kiln was not opened at all during the firing.
In addition to the big bubble with a dimple on the side of the bubble, there is another slight dimple to the left of the bubble. Is this connected to the bubble problem? At first I thought that my kiln shelf has an indentation, but I checked with a flashlight and couldn't find one. Also, the two pieces were placed in the same centered location on the shelf.
I'm going to try to duplicate as much as possible this piece for firing next weekend. Thought that I'd add a pre bubble soak at 1175 deg F for 30 min and then increase the bubble soak time to 2 hrs at 1250 deg F. What do you think? I'm attaching some photos, the file names should be self explanatory. Thanks so much for any advice. As you can see, I'm just starting out and still learning my kiln!
celadon-slight-dimple-reized.jpg (75.26 KiB) Viewed 6370 times
celadon-before-tack-fuse-resized.jpg (144.01 KiB) Viewed 6370 times
celadon-after-full-fuse-resized.jpg (156.59 KiB) Viewed 6370 times
For starters, your firing schedule does not have a proper bubble squeeze. A bubble squeeze is not just a long hold at 1225. A bubble squeeze requires a slow ramp up to 1225, giving the glass time to drop from the middle outward, and squeezing the air out as it goes.
For system 96, I normally ramp to 1100 deg F at a rate suitable for the project, then ramp 50 deg per hour up to 1225 & hold 60, then continue as usual.
Hi, Looking at the bubble, it seems it would be worth considering using the other side of the shelf once you check it's completely flat. The very large bubble looks like those you get when there is air trapped between the shelf and the glass or air created by humidity (too much glasstac, shelf not completely dried). You don't need to adjust your schedule for 1 mm thickness variation. The 1st ramp up @ 350/H is fine, I would not go faster especially since you've already fired the glass once to tack fuse. It would be a good idea to program your next one in a way that allows you to check and monitor when it gets close to the process temperature of 1490. I Hope this will help. Tks, Geraldine
Hi! Thanks for your input. Last weekend I fused a piece similar in size and layout to the original with the bubble. It fused perfectly! 200 deg F to 1100 deg F. hold :45 100 deg F to 1250 deg F. hold 1:00 300 deg F to 1490 deg F hold :10 AFAP 900 deg F. hold 1:00 100 deg F. to 700 deg F. I think that my first schedules were probably sealing the edges too fast. Thanks again for the input.