Casting window glass frit

[DonnaG]

I have this friend with a problem...really I do have a friend.

He is a high school art teacher and is wanting to introduce his students to the world of fusing. Yah him! I have given him supplies on occasion and advice but now it's a bit out of my realm and I'm hoping someone on the bulletin board might be able to help.

They are using a ceramic kiln, skutt KM-1227 and casting snowflakes and made the ceramic molds themselves, about 3" wide and maybe 1/4" deep. The kiln will hold 5 shelves, which they are using since the classes are full (about 150 snowflakes at a time). Someone gave them a bunch of window glass, so being industrious, they made their own frit. However, the problem comes when firing. Some of the snowflakes are firing fine while others are barely tacking together. I know kilns have warm and cool spots, but I'm not sure how to advise a firing schedule since I've never used a ceramic kiln, or for that matter, window glass.
They started with:
400-1100-10
250-1245-30
300-1540-20
999-1015-120
80-800-0
They have tried a few batches and keep increasing the top temp -- last I heard, they were up to 1800 for 30 minutes.
Thanks in advance to everyone.

[Larry Lunsford]

I actually have the same kiln and have had a very similar problem. I simply had way too much stuff in the kiln for a single firing. My shelves were too close together and I just didn't get good heat penetration and distribution throughout all the stuff.

Ask your friend to make sure to have 2 inches of open air above each piece.

Also, with a kiln stuffed to the max, slow down the ramp rates (maybe 250 dph max).

If the glass for the snowflakes is spread too thin, the pieces will not merge. The finished thickness should be at least 1/4inch to get it to all fuse together and flow into the mold. 3/8 or more would be better. The frit needs to be 1.5 to 2 times the finished height.

Were the molds pre-fired? If not, that's another problem.

Larry

[Morganica]

Heat distribution is one problem--ganging shelves 5 deep in a ceramics kiln is usually going to require a longer, slower firing or some way to distribute air between shelves and ensure that heat gets to the pieces in the center. If you're casting into molds, that adds to the shelving insulation and increases the amount of heatwork you'll need. IOW, he's probably creating a heckuva hotspot/coolspot situation and slowing down will help.

Glass formulations, probably another problem. I've worked with float that softens and flows at one schedule, and float that is much harder and doesn't flow at all with the same schedule. If your crew crushed a bunch of random window glass, and tossed it all into containers, they've probably created layers of different types of glass. They could simply be encountering harder and softer glasses that flow at different heat levels. (And since nobody guarantees compatibility from float mfg to float mfg, that's something else to think about--I always confine my projects to as much frit as I can make from a single sheet.)

Schedule is a third issue. Float glass simply isn't formulated for fusing, so it takes some extra work to come up with a good schedule for it. Float also can have bad devit tendencies, so you're trading off between preventing devit with a too-slow ramp and actually getting the stuff to melt.

Ask them if the glass is turning white at the edges. If it is, the glass is devitrifying, i.e., crystallizing. Devit has a MUCH higher melting temp, and if it's on those edges they'll stay solid and sharp well past 1800F.

I wouldn't worry much about the initial ramp up to 1100F, but with five shelves in the kiln I'd hold at least an hour at each segment starting there, to make sure you've got equal temps throughout the kiln. I'd also raise the top temp to about 1570F and try process holds between 30 and 60 minutes. It'll probably devit, but once he's established that the whole kiln is now fusing the glass into the molds, he can start shortening hold times until he gets a good fusing schedule that doesn't devit so much.

[jim simmons]

If they are making snowflakes is devit really going to be a problem?
Looks like frost.
Jim :>)