Uneven slumped bowl

[Lori]

Yesterday I slumped a 10" bowl in my Skutt kiln on the top shelf using a stainless steel mold.

I used the Skutt preprogramming for slumping with slight changes:

300 DPH to 750 (holding 25 minutes after each 250)
600 degrees to 1300; hold 5 minutes at 1150; 10 minutes at 1300
AFAP to 1000 and hold two hours
30 DPH to 970 hold 2 hours
60 DPH to 750 and then room temp.

There are 3 corners of the bowl that did not slump down against the mold. It looks like an old pirates hat. This is the second time this happened. The first time I used the Skutt maximum temp of 1220. The second time i raised the maximum temp to 1300.

Do I need to hold at 1300 longer than 10 minutes to fully slump or do I need to raise the temp? Or a combination? I don't like having to slump the same piece twice.

Also, dumb question but how do you visually look at a piece being slumped? I can't see anything through the peep holes. This leaves raising the lid which changes the internal temp I'm sure.

[Tony Serviente]

Don't despair Lori-You'll get it, and learn so much on the way. You're not hot enough, or holding long enough. Remember, the glass wants a product of time and temperature to have it do our bidding. You can often get the same result by going to a high temp for a short time, or a lower temp longer. I'd try adding 20 degrees to your target temp. Other factor you have to work with is the depth of your form. If the walls are steep, you may have trouble getting an even slump, even if the time and temp are correct. It that's the case you'd have to do progressive slumps in less steeply sided forms, and end up with the deep one. Whenever the glass doesn't do what I want it to, I just think, it's trying to teach me something. I just have to figure out what it is. You're right about lifting the lid to peek, it will invalidate your learning the correct time/temp curves, that is unless you peek after it's on the way back down, then you risk shocking the piece. I just don't reccomend opening it. Even if you have a peep hole the infra red glow makes it almost impossible to perceive depth, and there are no shadows to reference-So just take lots of notes and keep smiling.

[Jerry]

Lori,
I agree with Tony. In addition, a small square shape is a tough one to get all the way down. I'd increase to 1350 to see if that works. You might need a soak of 30 minutes but I'd try the 1350 first. Also, you didn't tell us what kind of project this was, or how many layers. So, for a single layer piece, I think you are beating yourself up with that schedule. I'd go 350 an hour all the way to 1350 and say a prayer to the kiln gods.

Good luck,
Jerry

[Lori]

The project is 2 layers - 1/8" each. It's a bowl and it's not a deep bowl.

I think I'll try raising the heat and the soak time both. This can't hurt can it?

[Cynthia]

Here'a another view point.

Once you are above 1000F it's safe to peek. You won't be holding the lid open long enough or wide enough for too much heat to be released from the glass, so you won't be altering your schedule results by any great shakes.

Don't peek at temps below 1000F

In my universe, I peek always, unless I'm firing a tried and true schedule with the same glasses in the same configurations.

I don't know why you are holding after each 250, unless it's the only way you can increase temp without thermal shocking. If this isn't necessary, then I'd say ramp up at 250 dph to 1300 and hold until it's draped to where you want it to be. You know what rates and schedule you can safely fire at in your kiln...so this is simply a suggestion that may or may not be usable in your kiln.

I think you need to peek so you can see what is happening in order to learn from what you observe. Peek at 1250 or 1300 and every 5 - 10 minutes until you see that you have what you want. Adjust by adding temp or time or both to get the glass moving how you want it to move.

When you drape glass, two opposing sides of a circle (if fully balanced) will drape first, the sides that remain up will be slower to drape down because of physics. There is an increased resistance to drape because of the arc. A square will drape down all four corners first. The heaviest edges will drape first. Watch and you will see how these dynamics play out and repeat the same patterns each time. IF the drape is steep you may get folds.

[Lori]

Is there such as thing as holding at the peak temp for too long? What can happen?

You asked why i hold after each 250 degrees - this seems to be the way Skutt has preprogrammed their rampmaster. I think I'll send them this question. It certainly adds a lot of time to the process - the cycle took over 14 hours yesterday to bring the temp back down to 500.

[Cynthia]

Is there such as thing as holding at the peak temp for too long? What can happen?

You asked why i hold after each 250 degrees - this seems to be the way Skutt has preprogrammed their rampmaster. I think I'll send them this question. It certainly adds a lot of time to the process - the cycle took over 14 hours yesterday to bring the temp back down to 500.

The problem with an overly long hold at your processing temp is that the glass is flowing/bending/streching/moving toward the ground. It will stretch and pull down with gravity and eventually you could end up past the point of no return with a pool of taffy glass on your shelf. That being said...you could concievably hold at your processing temp for over an hour before you achieve the desired result. This is where you will want to record your results at differing times and temperatures so you can learn what to expect from the glass.

I can't see any reason, if your kiln is a glass kiln and fires evenly, why you would need these holds, any hold, every 250 degrees, especially when you are ramping up at such a conservative rate. A standard firing schedule can easly eat up 12 hours time considering how slowly a brick kiln can cool to room temperature. I'll be curious to hear what their answer to your question is though.

[Barbara Muth]

Lori, I would recommend trying to do the slump at a lower temperature for a longer time. You didn't say whether you used BE or System 96 glass, did you?

I slump BE lower than most folks, at about 1100 or 1150 depending on the glass with a long soak of say 30-45 minutes. I often do a shorter soak at 1000 or 1050 degrees to sort of "prime" the glass by ensuring an evenivity of heat throughout the piece. Some folks slump at even lower temperatures than I do.

I find I like the pieces better slumped low and slow.

Spectrum says system 96 slumps at 1275, so taking it to 1300 for a slump isn't a bad idea. I wouldn't go much higher temp-wise because you then risk deforming the piece. I would probably soak at 1150 (as you did) or 1200 for 15 to 20 minutes and then go slowly up to 1300 and soak for half an hour to 45 minutes, then anneal.

Since I am guessing you are using Spectrum 96, I recommend taking a look at their annealing schedules
http://www.system96.com/Pages/FiringGuideF.html
It seems to me that your anneal should look more like:
AFAP to 1000 degrees, hold 10 minutes;
300dph to 955 degrees, hold 20 minutes;
200 dph to 800 degrees, no hold;
then cool
400 dph to 120 degrees.

good luck!
Barbara

[steve_hiskey]

I agree with Barbara on the "lower and longer" philosophy for slumping. I tend to do all my slumping at no more than 1200 or 1250... even on the thick stuff.

so for two sheets, I would do something like
100dph to 1000, hold 10
100dph to 1200, hold 10
drop

for thicker:
100dph to 1000, hold 10
100 dhp to 1250, hold 20
drop

as the piece slowly comes up to 1200, you find that it is already done before even reaching peak temp.