System 96 casting plates- getting rid of bubbles

[Peter Zelle]

hi there,

i am casting large open face molds with frit on the bottom and 3 layers of system 96 casting plates on top. the plates are 6"x12"x1/2" thick. i am getting mostly clean and clear results but in a few spots there will be a 1" bubble trying to breach the surface. this is not great.

my firing cycle is 10 hours to 1250F and a 3 hour soak at 1250F. then taking 2 hours to 1500F. the surface is smooth within 20 minutes at this point and i shut the kiln off and pull the plugs until about 1200F.

to get rid of these bigger bubbles i am thinking of trying a few things-

1- soak longer at 1250F or longer at a 1200F

2- break up the plates into smaller pieces on the top layer

i am guessing the bubbles are being trap in the center where the plates are stacked and the air has a longer way to travel to get to the edge. just a theory.

any ideas or suggestions would be greatly appreciated.

thanks, peter

[Morganica]

Or 3- Stop using frit.

Seriously, frit is usually the biggest bubble generator in a casting, followed by cone-shaped gaps between billets. If you really need to use frit (you're mixing colors or something), try prefiring your casting plates with the frit on top. That'll allow the air to escape and you'll have a much easier time with bubbles.

Other ways I've reduced bubbles in my sculptures:
--Take the mold to 1525F instead of 1500F, and hold it there for at least four hours. That'll help more bubbles to rise to the top and burst. If that doesn't work, increase the time.
--Change the way you pack the mold--keep the glass stacked in the center so that there's at least an inch or two between the mold walls and the glass. That encourages the glass to spread into the mold from the bottom center and move out and up the mold sides, which is more likely to push the air out. If you need to have more than one stack in the mold, keep at least an inch of space between them.
--Don't stack broken billet/plates so that the gap between them is wider at the bottom than at the top, or you have gaps surrounded by glass with no way for air to easily flow out. You're more likely to have the glass soften, come down and seal the gap, trapping air.
--Construct billet molds and prefire your billets so they're already close to the final mold shape. If the mold has a lot of twists and turns that might trap glass, or I've done a pate de verre cover (for a face, for example) that needs a clear cast back, I'll make a custom shape, cast it with the glass I intend to use, and then clean up the mold, rough-grind the surfaces and use that as my billet. It is a real pain to do, so I hardly ever.

[Peter Zelle]

anyother suggestions out there?

thanks, peter

[Peter Angel]

I would agree with Cynthia. It's got something to do with the way you are stacking the glass.

Any chance you could take a photo of your stacked glass and upload it here before your next fire?

Pete