lump in bottom
- April 6, 2009 at 6:08 pm #9336dianewParticipant
I just finished slumping a dish and I got a big lump in the center of the bottom. This is not a air bubble, just a lump. Any suggestions?April 8, 2009 at 5:09 pm #11232kcofcaParticipant
We have experienced the same thing in our glass class but not consistently with the same slump mold or glass. The first guess was the hole sizes, maybe make them larger by a small amount. Our best guess is maybe the firing schedule is too close (Jen Ken Kiln) for some of the glass (all 90 COE but different colors and embellishments) on some of the slump molds. Maybe a slower, longer firing schedule?!? If you find an answer we would appreciate it. So far it comes and goes at random. It’s hard to calculate a random variable. Keep your journal.April 9, 2009 at 1:21 pm #11233dianewParticipant
Thank for your infomation. Yes, I have done this same mold many times and this the first time it has happened. What do you mean by “the firing schedules’s are too close?” (and yes, I do use a Jen Ken) When you say slower or longer firings, does that mean the “hold” times? I do keep a journal since the day I started and it has been so helpful. Thanks for the reminder though.April 9, 2009 at 8:37 pm #11234kcofcaParticipant
That was an ambiguous statement. I was making the complaint that the preset firings were not reliable for all the variables we put in the kiln. If you fire a lot you probably already tweak your settings. For our class the firing is preset and it just doesn’t fit all slumps or fuses.May 7, 2009 at 9:39 pm #11235Stephen RichardParticipant
I have found that a firing that is too high/too long will cause the glass to continue sliding down. Having nowhere else to go, the weight causes the bottom to begin rising. This is a consistent experience across several kilns and with multiple users.
So keep the temperature down to the minimum required. To find that out, watch the slumping in stages (do not stare!). Look at the piece for a second or two every five minutes after you reach your desired temp. If it already has slumped adequately, you are firing too high. Reduce your temperature in subsequent firings and watch to find what the required temp and time is. There is absolutly no substitute in slumping but to watch and learn what your mould and glass require.
Stephen RichardMay 8, 2009 at 5:37 am #11236AnonymousInactiveMay 8, 2009 at 6:21 am #11237Stephen RichardParticipant
There are two things that seem relevant to my diagnosis on why raised areas appear on slumping:
The experience I have is with moulds that have the vent hole in the centre at the deepest part and also have vent holes on the side of the mould, making me certain that there is ventilation for the glass to push out the air as it falls. I always do a visual inspection of the vent holes before using, and looking at other’s with the bubble on slumping problem, they also have free holes.
The second is more theoretical. If you are slumping at such a temperature to seal the glass to the mould, you are firing too hot anyway. Or put more positively, a low temperature slump (that is, a slump at the lowest temperature to achieve the desired result over an extended period of your choice) A low temperature slump will allow the glass to conform to the shape of the mould without softening so much that it takes up all the markings of the mould. Therefore, there are spaces for the air to escape from under the glass all the way to the top.
It is high temperature slumping that causes most of the problem of creating a bubble at the central bottom part of the slumped glass. Of course, trapped air can be a problem also. But if you have a bubble with no trapped air possible, because the vents of the mould are open, you have to consider other possibilities.
These are the things that make me certain that not all bubbles are caused by trapped air.
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