ramp speed for thick cast pieces that need refiring

[barneyzeitz]

I have been frit casting glass centerpieces for 15 years and have had to refire many times to fix flaws. I am initially ramping at 1.0 degree/min to 1000F. This takes 12 hours. The thickness of the centerpiece varies from 3/4 " at bottom to 1 1/2" think in some areas. Do you think I could go faster in this initial ramp to 1000F? Thanks

[Morganica]

How are you refiring and why? Are you encasing the whole piece in refractory and firing? If so, and the refractory's pretty strong, I'd ramp to the limits of the refractory, not the glass. Even if the glass shatters, it's in place inside the mold and will heal. I use that technique to attach separately-cast components, and to mend pieces.

If you're firing in the open kiln, to put a polish on the piece or to attach it to a blank, that's very different. You need to consider whether there were any stresses or fractures from the prior anneal/coldworking/etc., and if there were, especially considering that you're doubling thickness in some places, I'd stay in the 50-75dph range until you're past the strain point.

It also depends on what kind of glass you're using. High-lead crystal is a lot more forgiving than soda-lime, for example.

[Bert Weiss]

Barney

Welcome to this forum. I'm going to put the answer back on to you. You have a pretty good clue already. There are no rules for this. Ramp speed is all about evenivity. This depends on a combo of glass, kiln elements, and mold/shelf. Given that you have a variation of thickness from .75 - 1.5, it is a big challenge.

So I ask what does 12 hours really mean? Start at 9PM and go till 9AM. Is this a loss, or just kiln time while you sleep?

I have no real clue, but is 12 hr too fast? I heat a .75" slab of even thickness float glass at 8 hr from 100 < 1000. That is conservative enough to never have had a failure. Add to that the factor of doubling thickness in some places and I would put the brakes on.

OK, there is a rule. Ramp speed is proportional to the amount of work, cost of glass, and importance of the piece. The more you care, the slower you want to go. Take a day if you are nervous.

On this forum, we often hear, "I always used this firing schedule before with no problems, why did this piece break?" The answer is always, not enough evenivity...

[Morganica]

Barney

Welcome to this forum. I'm going to put the answer back on to you. You have a pretty good clue already. There are no rules for this. Ramp speed is all about evenivity. This depends on a combo of glass, kiln elements, and mold/shelf. Given that you have a variation of thickness from .75 - 1.5, it is a big challenge.

So I ask what does 12 hours really mean? Start at 9PM and go till 9AM. Is this a loss, or just kiln time while you sleep?

I have no real clue, but is 12 hr too fast? I heat a .75" slab of even thickness float glass at 8 hr from 100 < 1000. That is conservative enough to never have had a failure. Add to that the factor of doubling thickness in some places and I would put the brakes on.

OK, there is a rule. Ramp speed is proportional to the amount of work, cost of glass, and importance of the piece. The more you care, the slower you want to go. Take a day if you are nervous.

On this forum, we often hear, "I always used this firing schedule before with no problems, why did this piece break?" The answer is always, not enough evenivity...

Well, no. Sometimes the answer is incompatibility. Sometimes the answer is "I changed colors of glass." Sometimes the answer is "I made the piece a lot thicker in one spot..." etc. Thermal equilibrium is frequently a factor, and usually slowing down solves the problem. But not always.

[barneyzeitz]

thanks for the feedback Sounds like I should stay with what I am doing.

[Bert Weiss]

Barney

Welcome to this forum. I'm going to put the answer back on to you. You have a pretty good clue already. There are no rules for this. Ramp speed is all about evenivity. This depends on a combo of glass, kiln elements, and mold/shelf. Given that you have a variation of thickness from .75 - 1.5, it is a big challenge.

So I ask what does 12 hours really mean? Start at 9PM and go till 9AM. Is this a loss, or just kiln time while you sleep?

I have no real clue, but is 12 hr too fast? I heat a .75" slab of even thickness float glass at 8 hr from 100 < 1000. That is conservative enough to never have had a failure. Add to that the factor of doubling thickness in some places and I would put the brakes on.

OK, there is a rule. Ramp speed is proportional to the amount of work, cost of glass, and importance of the piece. The more you care, the slower you want to go. Take a day if you are nervous.

On this forum, we often hear, "I always used this firing schedule before with no problems, why did this piece break?" The answer is always, not enough evenivity...

Well, no. Sometimes the answer is incompatibility. Sometimes the answer is "I changed colors of glass." Sometimes the answer is "I made the piece a lot thicker in one spot..." etc. Thermal equilibrium is frequently a factor, and usually slowing down solves the problem. But not always.

Cynthia, lol. I originally wrote usually, and then changed it to always, knowing it was a risky statement. When I think about it, a case of near incompatibility can indeed cause the piece to break. The undefined term evenivity can certainly be stretched to cover, the interplay between uneven heating and other stresses present in the glass. Heating a piece of uneven thickness is rarely even, it can go slow enough to avoid breakage though.