glass splits while draping

[Cheryl]

OK, this doesn't happen all the time but when it does, it's inevitably on an expensive (big) piece.

20x20 4 mm glass, edge bordered with pre-fused gems and frit.
ss floral former

extremely conservative schedule (which I have learned the hard way):
50 dph to 500, :20
50 dph to 1000 :20
75 dph to 1190 :20
75 dph to 960, 2:00
75 dph to 720, 2:00
100 dph to room temp

The glass splits on the way down (no rounded edges). I can attempt pix although I'm not the best at that to be sure.

Thoughts?

[Morganica]

Pics? What's the footprint size on the floral former? Possibilities:

First, you're not really going to get a lot of edge rounding with your schedule, so sharp edges can still mean the glass split on the way up.

Second, if your floral former is, say, 4 inches in diameter, you've got "wings" of 8 inches on a side, with additional decoration at the edges. That's a fair amount of weight needing support while the glass softens. It's possible that even with a slow schedule the weight of the edges are dropping those edges before the more insulated glass in the center is soft enough to move. The way you fix that is to set up some kiln furniture at the very edges of the glass. It'll hold those edges up until the rest of the glass is ready to drop.

Third, if you've got tack-fused decoration on the edges, make sure you've annealed the piece long enough on the flat fused. It could be that the glass has some residual stress and is thermal shocking.

Fourth, how close are you to the top elements? If you're using an 8-10 inch floral former on a shelf in a standard fusing kiln (usually 12-13 inches deep), you may be getting too close.

[Cheryl]

I would have to measure on how close to elements. 4-6 inches I would think. I keep getting farther from the elements but of course I am limited...

Tell me more about the kiln furniture idea.

these are well annealed. 120 mins. @ 960, 120 @ 720. Floral former is about 5 inches at the top. Lots of unsupported glass. I know this is the problem, just not how to fix.

[Stephen Richard]

........

these are well annealed. 120 mins. @ 960, 120 @ 720. Floral former is about 5 inches at the top. Lots of unsupported glass. I know this is the problem, just not how to fix.

I doubt they are well annealed - soaked for long times, yes. But this is not the same as well annealed in my view.
I think you need to review your annealing strategy. The soak at annealing point is to equalise the temperature. The long slow cool is to keep the (reducing) temperature of the piece very close to the same temperature throughout the piece. A soak at 382C will do no useful work except give the electricity company more money.

I think a review of you heat up schedule is in order too. I don't see any reason for a heat soak if you are maintaining the same temperature in the next segment. Relatively even increases in temperature are less stressful on the glass.
When supporting glass with a lot of free hanging glass, I go slowly to about 100ºC and soak there for up to 30mins, then 1.5 times that to around 300ºC with another 30min soak, then twice the initial heat up rate to above 540 - sometimes up to the draping temperature.
A look at the Bullseye tech note on "heat and glass" will give you a lot of information about what is happening to your glass at various temperatures. Study of Stone's book will help too.

I hope this does not sound patronising. I am trying to be helpful.

[Cheryl]

I should know better than to try to recall my firing schedule without checking it which is what I did. 80% of the time, this schedule is perfect. I have made 100s of these vases. (no joke there). The reason for the pause with the schedule the same is just to let the glass catch up - with the uneven masses (at the edges) I would rather take the extra time. This is a 30-hour firing schedule (plus the cool from 400 to room temp).

I will re-post when I have the schedule in hand.

In the meantime here is a representative pic: http://cherylsattler.com/artwork/3052214.html

[Kevin Midgley]

So I had some ideas I pm'd you. Now I am stumped with the additional info provided. Only suggestion is the nature of the edges with their variability is a causative factor and there is some relationship between the glass and the proximity to the elements. It is a tough one to cure. Slow down some more???? Any relationship to the colours being used???

[Laurie Spray]

One thought... Would you really get a "rounded edge"
at 1190?

[Stephen Richard]

I should know better than to try to recall my firing schedule without checking it which is what I did. 80% of the time, this schedule is perfect. I have made 100s of these vases. (no joke there). The reason for the pause with the schedule the same is just to let the glass catch up - with the uneven masses (at the edges) I would rather take the extra time. This is a 30-hour firing schedule (plus the cool from 400 to room temp).

I will re-post when I have the schedule in hand.

In the meantime here is a representative pic: http://cherylsattler.com/artwork/3052214.html

In my view, the glass should not have to catch up, nor the mould catch up with the glass.
The rise in temperature should be slow enough for the rise to be steady and warm both the mould and the glass at the same rate - in general. If you have a rate of advance of 200/hr with an hour soak, it is the same as 100/hr with no soak, and has less risk of heat shocking the glass during the rise in the first segment.
I do have exceptions to that when draping with small contact areas on the mould and lots of free hanging glass. I do soak then. However, after that pause/soak, I increase the rate of advance in the next segment.

I did some tests to help me determine where to put those soaks - the destruction kind. Heat the glass at a rate that broke the glass, reduce the rate on subsequent firings until the glass stopped breaking. I chose that successful firing as my fastest rate of advance. I have found that in drapes a soak at 100ºC is required. I cannot go at the rate of 100ºC/hr without breaking the glass if I don't do the soak (in my kiln).

All my breaks occurred at the heat up phase, not on the annealing and cool phases.

[Kevin Midgley]

The answer is to slow down the firing rate some more to 25 degrees an hour or less
If your kiln is typical you will find that it is capable of heating much faster than a Digitry controller can control if you just turn it on and when it gets near 100F you press the hold key.
Try this experiment. Turn on your kiln and unless you have proportional heating control in addition to the computer, if you try and do a temperature hold at 100 F you'll find there is an overshoot perhaps to 200F. Most kilns are totally overpowered at the low temperature range where heat loss through the kiln walls is not a factor. At the beginning of a firing all the thermocouple is reading is heat radiating directly from the elements, not stored heat from the glass and kiln walls as well.
I remember all too well the clunk sound of disaster always less than 5 minutes into a firing as the glass I was using broke. Spectrum baroque made beautiful vases but had different thicknesses and heat absorption rates because of the colour and clear. I found all edges had to be perfectly sanded smooth if I wanted success as well as a nice fire polished edge in a single firing. Then the pita was having to polish the bottoms so the vase wouldn't wobble.

My vases would break before 200F and usually at the 150-170F or between 800 to 950. A Digitry controller and a fast heating kiln can't always stop it from happening. Your best bet given the nature of the pieces you are firing and note my edges commentary well is to do the initial heating to the 1000 at 25 per hour. Yes it will add time but not much electrical cost to the firing. It is better than having only an 80% success rate.

One final suggestion would be to have the kiln slightly warm when you load it. If the kiln temperature reads 70-100, you can shut it off and then load your blank. That initial heat I found helped reduce the thermal shock to the glass and it was a way to slow the firing down. The thermocouple will read the kiln wall material heat instead of just the elements radiant heating. caveat if the kiln is too warm you'll know quickly

My elements at that time were within....... 2 inches of the glass. Regularly got 3 kiln firings a day from that kiln but never got to a 100% every time success rate.

Before everyone rushes out to try this, there's also the folds-touching-tack-fusing-issue you have to solve to prevent breakages.

[Cheryl]

What I really want is a camera inside the kiln. I want to take pix every 10 mins and pinpoint the problem. I hate lifting the lid that much and the break is sometimes on the back side so I wouldn't likely see it anyway.

The problem with any schedule is that it's fine...until it isn't. Unless the first one breaks.

[Bert Weiss]

The problem with any schedule is that it's fine...until it isn't. Unless the first one breaks.

Cheryl

You must know from reading this board that the above statement is made here all the time. I commented on another thread about the same problem that stresses are cumulative. They can also counteract one another. Start with a level of compatibility stress, add some annealing stress, then add thermal shock, and bango you have a crack. Of these, once the glass has been annealed, the one you have control over is thermal shock. Sometimes you have to slow down beyond the range common sense tells you. Remember that "compatible glass" has stresses in it, just not enough to spontaneously destruct at room temperature.

Last year at this time, I was working on a bowl making project. I cast circles using broken tempered float glass. Then I set them on bowl molds for slumping. My big kiln holds 10 bowl molds. I would do a firing, and some would work, but several would crack on the heatup. Eventually I slowed it to a creep, and they all survived. This was way slower than my common sense ramp speed.