casting program

[Pam Morgan]

Hi all!
I am a newcomer to hot glass. I've been a stained glass artist for years and have done glass painting, and some minor fusing and slumping. I bought a switchplate cover mold and decided to run it with the factory set casting program on my Hot Start Pro (since I don't really know anything about casting). well, it took 17 hours and didn't work! I used scraps of Fremont glass I had lying around, and basically it fused, but not even a full fuse-more like a tack. The program went like this:

150F/hr to 250f hold 15 mins.
250F/hr to 1500F hold 5mins.
1000F/hr to 950F hold 2.5 hrs.
30F/hr to 750F hold 20 mins.

I want to run a test with either float or some system 96. I'm sure I don't need to hold so long for such a small one layered piece. I'd love some program suggestions. The mold is ceramic and measures 5"x 3/5". Thanks for any advice. Pam

[Morganica]

Hi all!
I am a newcomer to hot glass. I've been a stained glass artist for years and have done glass painting, and some minor fusing and slumping. I bought a switchplate cover mold and decided to run it with the factory set casting program on my Hot Start Pro (since I don't really know anything about casting). well, it took 17 hours and didn't work! I used scraps of Fremont glass I had lying around, and basically it fused, but not even a full fuse-more like a tack. The program went like this:

150F/hr to 250f hold 15 mins.
250F/hr to 1500F hold 5mins.
1000F/hr to 950F hold 2.5 hrs.
30F/hr to 750F hold 20 mins.

I want to run a test with either float or some system 96. I'm sure I don't need to hold so long for such a small one layered piece. I'd love some program suggestions. The mold is ceramic and measures 5"x 3/5". Thanks for any advice. Pam

First of all, unless those Fremont scraps are from the same sheet, you can't count on them being compatible with each other, especially if they're very different colors. It may also have considerably more tendency to devit, which in extreme form can look like crystallized sugar.

Second, I don't know the firing characteristics of Fremont, but if it's like float, you'll need to get closer to 1600F, and hold it there for awhile. Any less and you won't get full sintering into the form you want.

Third, I'm not sure why you've got a hold at 250F--that would be the temperature (or a bit less) to vaporize water in the refractory mold, but if you've got a ceramic mold it's not necessary. And if your mold had been plaster/silica, 15 minutes wouldn't have been nearly enough.

Fourth, is your switch plate mold really 3/5 inch (15mm) thick? Seems a bit thick but if that's the final size of the glass, your annealing schedule's a little odd but not that far off. If it's thinner (I'm getting confused by the 3/5 vs. 1-layer), then yes, it's long for what you're doing.

If you're going to do this in system 96, I'd head over to the Spectrum website and follow their recommended schedule.

[Pam Morgan]

Hi thanks for your reply. The Fremont glass was from the same sheet-a beautiful multi color with some gold pink in it. It also did devitrify a bit. As for the hold at 250F I can't explain it-it was the preprogrammed casting program that came with my kiln-the mold is ceramic. I also meant 3.5" wide! Sorry-I'm a lousy typist! The mold is only 3/4" deep max, and I don't think the final product needs to be that thick. If the final product is about 1/4" thick I think that's ok. Do the glass scraps all have to be really small and the same size? I will check out the spectrum website for casting schedule suggestions for system 96. And thank you for letting me know float needs to get so hot-I thought float was softer! Pam

[Bert Weiss]

I would be shocked if Fremont behaved more like float than like System 96. Many glass blowers use a batch comparable to 96.

I haven't enough fingers, toes, teeth, or even bones, to count the number of times somebody posted here saying, I used the preprogrammed schedule and it didn't work. For reasons beyond our comprehension, kiln manufacturers pre-programmed controllers with totally bogus programs. Or at least often bogus programs.

First of all, every kiln behaves differently. Even the same kiln with a different load in it will behave differently. A schedule is relatively simple. Step one is to heat the glass above the strain point at a rate that does not cause it to crack. Once above the strain point, you can go as fast or slow as you need to. The goal here is to accomplish a precise level of heatwork. You want the glass to slump, tack fuse, full flat fuse - whatever is your goal. Heatwork is a combination of time and temperature and is significantly effected by the kiln design and construction, and how it is loaded with glass. For instance, with a brick walled kiln, during the heatup, the bricks are soaking up heat, and during the cooldown, they are letting go of that heat. Fiber walled kilns do not soak heat. So, the schedule I use in my kiln might not achieve the same heatwork or anneal as what might happen in another kiln. This alone is reason not to rely on a pre-programmed schedule. For some reason, they seem to have programmed in useless, waste of time holds, at strange places in the programs.

A schedule has these goals:
1: heat the glass above the strain point without cracking it
2: continue to heat the glass enough to accomplish the desired heatwork, hopefully without causing devitrification. When fusing, adding in a bubble squeeze is often useful. This happens when the glass is soft enough to move, but not so soft that is seals up and holds air bubbles trapped.
3: anneal the glass, which is requires a 15 second soak when the entire mass of the glass is within 5ºC inside the annealing range (in a thick piece this can take several hours)
4: cool the glass to room temperature without cracking it.

I highly recommend reading Graham Stone's "Firing Schedules for Glass". I think Brad sells it in his Warm Glass studio/gallery