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what is wrong with my kiln. Actually, I think I know what is wrong, but hopefully someone can tell me how to fix it. This is the 2nd piece I've fired and has been ruined. Anyway, to make a long story short, when I opened my kiln today, this is what I saw. Here are the specifics:
Bullseye glass, Irid. side down, black on top. Top temp 1470, hold 10. I think it's either going higher than the 1470 or holding longer than the 10 min. It drooped down over the sides of the shelf - which was a vermiculite board, kiln washed, and papyrus paper on top. How can I check to see which is the case. I did not check on the progress of the firing as it was 5 degrees here and my kiln is in my unheated shed. My kiln is a Skutt 14". The first piece this happened to was a kiln carved mold (flat) that the two pieces were on. They also drooped over the sides. I did fire it twice to a full fuse in between these two drooped pieces to 1465, hold 10, and they were fine. I'm sorry if I'm rambling but I'm hoping someone can tell me how to check this. It's too much money down the drain.
Thanks for any and all help.
Feeling frustrated and cold in 2 degrees Pittsburgh. Patty

More info please.
Full schedule
Full lay up
Size of blank in relation mould.
Preliminary view indicates volume control problems or thermal shock, firing too high, and you have an annealing break. Lots of problems indicating some basic errors

off the hip....too fast heat up thermal shocked the glass causing it to blow apart before it fused back together. . why? dunno. but too fast. sometimes kiln controllers get cranky at low ambient temps. need more info like he said. r.

How close to the edge of the shelf was your disc? How close to the walls of the kiln was your shelf? Is your kiln shelf level? Does your kiln have any side elements? Firing to 1470 (held for 10 min.) did not cause your glass to break. Glass can be fired up to 1700 or more without causing subsequent breakage. There is probably nothing wrong with your kiln. All the information that others requested is needed.

Warren

Warren,
Your statement "Firing to 1470 (held for 10 min.) did not cause your glass to break. Glass can be fired up to 1700 or more without causing subsequent breakage. "
in this simple form is not right. It is possible to break the glass with too fast an initial rate of advance. I suspect this is why there is an open clear space where the top broke (possibly from temperature shock) and subsequently broke through the bottom layer. Etc.

It is not the top temperature so much as the speed of change of temperature. The rate of change is more likely to cause these fractures than simply the top temperature.

I know: eggs, teaching, sucking in some order is what I am doing

Steve,
Of course you are correct. In my statement I was referring to the top temperature only rather than the rate of rise. That is why the real answer to her problem exists in the information requested but not yet supplied.

Warren

Pat K. wrote:what is wrong with my kiln. Actually, I think I know what is wrong, but hopefully someone can tell me how to fix it.

I don't see anything wrong with your kiln. It looks more like a firing problem.

Bullseye glass, Irid. side down, black on top. Top temp 1470, hold 10.

This isn't enough information. We need to know the complete firing schedule, not just the top temperature and the hold time.

I think it's either going higher than the 1470 or holding longer than the 10 min.

Neither of these caused your problem. The problem was probably caused by going too fast from room temperature to 1100F, but you haven't told us how fast you went so we can't be absolutely certain.

It drooped down over the sides of the shelf - which was a vermiculite board, kiln washed, and papyrus paper on top. How can I check to see which is the case. I did not check on the progress of the firing as it was 5 degrees here and my kiln is in my unheated shed. My kiln is a Skutt 14". The first piece this happened to was a kiln carved mold (flat) that the two pieces were on. They also drooped over the sides. I did fire it twice to a full fuse in between these two drooped pieces to 1465, hold 10, and they were fine.

I can't really understand what you're saying here. Was the piece larger than the mold? If so, how much larger? Generally, if a piece is larger than a mold you will have trouble with the firing.

It's too much money down the drain.

Fusing is a fairly expensive hobby. It's a good idea to do your learning and experimenting use less expensive glass. At any rate, we need a little more information about your firing schedules to give you a definitive answer.

Okay - sorry it's taken me so long to reply to all of you.
Glass - Bullseye - two layers - clear irid facing down on shelf on papyrus paper, black on top
Size - 13" in diameter ; shelf was vermiculite - 14" round sitting on top of the shelf that came with the Skutt Firebox 14 on top of 1" kiln posts

Schedule - preprogrammed Skutt Slow fire to full fuse -
250 300 10
400 1470 10
AFAP 950 60
100 800 15

I've used this schedule for two layers many times and have not had this problem That's why I thought it was the kiln staying at the top temperature too long or going past the 1470. The time on the kiln for how long the process took was 9 hr. 15 min. Usually, this takes 7 hr. and some minutes. Maybe because it was so cold outside and my workshop is an unheated shed although the glass had been in the house, it caused the thermal shock. I just don't know.
Thank all of you that took the time to respond. I appreciate your willingness to help.

Patty

Patty,
It looks like your disc came within 1/2 in. of the walls of the kiln. I don't know if your kiln has side elements (does it?) but that would make it difficult to heat uniformly at that rate of rise in temperature. At 2 layers (1/4 in) your disc should not have expanded and run off the edge. It could happen if the shelf was not level. Check it with a level.
I would slow down the rates of increase. Try:
225 to 1150 no hold
150 to 1250 hold 10 min (This is a bubble squeeze to minimize bubbles.)
450 to 1470 hold 10 min.
AFAP to 900 (the Bullseye recommended annealing temp.) hold 60
90 to 750 then off.

There is no benefit to holding 10 min. at 300.
These preprogrammed schedules are rarely useful and need to be modified in general and specifically for special conditions.
Buy Brads book on fusing and you get a better feel for what is necessary. Glass is too expensive to waste.

Warren

Patty, probably the saddest words I ever hear a fuser say are "but that schedule always worked before..." especially because they usually say it right after it stops working...

So...you've got a 14" Skutt firebrick kiln (The Firebox?) with a 13" black glass disk on a vermiculite shelf. If you've got the Firebox, then your glass is coming within a half-inch of the kiln wall at four points. Skutt says the kiln is rated for a 12" glass disk so the glass is probably too big for the kiln--just because you can squeeze a shelf into the space doesn't mean you should. If the glass is that close to the kiln wall it's much harder to maintain thermal equilibrium across the glass...which is what leads to thermal shock.

It's (brrr) cold in Pittsburg, you're firing in an unheated shed and that isn't a large kiln, so the glass was probably pretty cold going in--that probably accounts for the extra time it took for the kiln to heat up. The flowing off the shelf was probably because of the thermal shock. Black glass is soft and tends to flow quite a bit with not much heat, so when the glass thermal-shocked, it wound up hanging off the edge of your vermiculite shelf.

The shock itself...I think in a cold shed, heat-absorbing black on top, insulated clear on the bottom, 400dph that close to the walls...probably enough to cause it. Thermal shock would not be possible at fusing temperatures, so the kiln going past 1470 or staying there too long couldn't be the source.

When you're just starting out, working with small pieces of glass, there's nothing wrong with the pre-programmed schedules. At some point, though, you start wanting to make bigger things, and preprogrammed schedules can't compensate, so you start getting expensive disasters. That's the point at which you need to stop worrying about X minutes at a top temperature, or Skutt Slow Fire to Full Fuse, and start worrying about developing your own schedules based on what the glass should be doing at each segment.

Not trying to correct Warren, but if you're going to be firing in a very cold climate that close to the kiln walls, I'd go even slower on the initial ramp--probably 200dph, and I'd increase the bubble soak to 30 minutes. I'd also consider putting a space heater out in your shed to warm things up before you start up the kiln, so that you don't have those lid elements doing such an enormous amount of work.

And...if you're gonna spend this kind of money on glass you really should invest in a couple of books and/or some classes on basic kilnforming. Bullseye has an educational video subscription series that's under $40/year and is really good at explaining the basics of schedule development.

Cynthia,

That makes good sense. Probably the thermal shock movement pushed it off of the edge.

Warren

Cynthia and Warren,

Thanks so much for your help. I guess I was pushing the limits, trying to get a big a piece as I could. I guess I'll be saving up for a bigger kiln. I do own at least seven books which I have read including Brad's and Stone's, but probably need to reread on a regular basis. I've learned almost as much from this board as I have from the books:)

Thanks to all who help further the learning process.

Patty