Large bubble during slump

Please help! I have never had trouble with this before. But the last two times I have fired a slump, using two different molds, I have ended up with a large bubble in the middle. Both molds have holes in them, and I always check, and they are not clogged.  I'm using the pre-programmed slump, which has never been a problem before. Does anyone have any thoughts on why this would suddenly be happening to me. And do you think it's possible to refire, and fix the bubble, or do you think my project is doomed? :( Any input would be appreciated. Thanks!

Are the air holes in the

Are the air holes in the mold under the bubble?

What is the schedule?

Helios Kiln Glass Studio


"Bubbles" in slumping

It is difficult to be certain, as you do not give the temperatures or rates of advance for your slumping, nor the shapes you are slumping, but would put a small bet on too high a temperature or too long a slump.  If either of these occur, the glass can begin to move within the mould.  As there is no where for the glass at the bottom to go, it rises.  So I call these uprisings.  They are not bubbles in the usual sense.

Using a pre-programmed schedule gives these kind of results.  The programmers have no idea of the mould or glass you are working with, so they put in some middle of the range idea for a slumping programme.   Secondly, even if you are doing your own programming, you  need to observe at intervals for a short time each to see what your glass is doing.  You will learn that thicker and thinner glass in the same mould behaves differently.  You will learn that simple curves are different from complex curves, sharp curves are more difficult than smooth ones, that small diameter moulds work differently than large diameter moulds.  So with those number of variables you cannot expect a pre programmed schedule to work all the time.

An observation:  you would (rather I would) never bake a cake without checking on it near the time I expect it to be done.  Why we expect to walk away from heating glass and still expect to get perfect results still eludes me.

Refiring:  Sometimes it is possible, but you need to make it flat first.  If it won't flatten easily, then yes, your project is doomed to be started again from scratch.  This time peek in the kiln during the 50C prior to the expected slump temperature.  I bet you will find the slump is completed considerably before that part of the ramp is done.  So be sure you know how to stop the ramp and proceed to the next before you start slumping.

Stephen Richard

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Thanks so much for the help

Thanks so much for the help with my problem. I was using system 96 glass in a very gently curved square mold. The preprogrammed rates were 300/hr to 1250 for 15 min. hold, and then hold at 960 for 1 hr. I don't always check in when it gets close to firing, because often I throw things in the kiln after work, and I'm asleep when it reaches the set temp. I know, not a perfect system, but sometimes the only way I can get things done. I also have my kiln in the garage, and it gets pretty cold out there sometimes, so I wonder if that would have anything to do with my kilns preformance. On a happier note, we did put an extra hole in the mold and fired it again. My husband is a tool and die maker. His theory is glass is pretty much the same as steel in many ways, and he thought is was an issue with the gas escaping. you could see where the original slump had covered the holes, and the rest is history. So he drilled another hole right in the middle of the mold.  I didn't fire it flat, I just slumped it again. Amazinly enough, it came out ok. I was really surprised as this plate was a series of strips, so I thought for sure that they would have distorted in the refiring process. So my project was actually saved! I guess I am going to have to start firing on my days off, or weekends so that I can keep a better eye on things. Thanks so much for taking the time to respond. I appreciate the experience. I'm learning, slowly but surely! :)

Once you have established

Once you have established the temperature for the slump of that kind of project through observation, you can go back to the overnight firing.  I think you will find that you can often achieve your slump in this mould at 625 or 630C with a short soak rather than the 675C you did use. This will provide you with a less marked bottom surface.  Best wishes

Stephen Richard

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