float glass disaster

[Annah James]

I am working on a project using 3/16" float glass. I am kilncarving over a lot of fiber paper. Yesterday, looking through the posts, I found a firing schedule for float glass that I thought I'd try, since I didn't want to have that cloudy look that I have sometimes gotten - and I THINK it's from firing too slowly...or having it at some temperature too long.

Anyway, the firing shcedule was:
250 / hr to 100 no hold
600 / hr to 1510 hold 20 min
afap / hr to 950 hold 1 hr
200 / hr to 700 hold 1 hr
end

I had three pieces in my kiln - which is top and side fired. (elements only on the long sides, not the short sides) Each piece was 24" x 31".
The two pieces on the ends BOTH cracked - and it looks like it was early in the firing, since the edges are all softened. The middle piece looks fine.

Any suggestions? I have pics but I don't know how to post them on this fourm.

Many thanks!

Annah

[Brock]

Don't need pics, it's thermal shock. You went up too fast. Try:

250 DPH 1510
Hold 20 min.
AFAP 1040
Hold 60 min.
100 DPH 740
Hold 30 min.

Something like that, and there are many other programs, will work. Brock

[rosanna gusler]

i would go slow to 1100f or so with a 20 min hold for evinivity. then afap to top temp.(i think that the devit zone is between 1300 and 1450f approx) then like he said. rosanna

[Annah James]

Ok, I'll reprogram and try it slower. I just didn't want that devit...easy enough to try again.

Thanks a bunch!!
annah

[Lynn Bishop]

Rosanna, I totally agree with you. Go slowly below 1100 or 1200, the larger the piece, the slower you should go. Then, go AFAP to your top temp, which for me is 1545, and again AFAP down to annealing temps. I fire with float almost exclusively, and never get devit. Lynn

[Annah James]

So, a further question - is 'slow' 250 dph or less? What is 'slow' to you? I know what it is to me, but not anyone else....

smiles, and thanks,
annah

[Brock]

So, a further question - is 'slow' 250 dph or less? What is 'slow' to you? I know what it is to me, but not anyone else....

smiles, and thanks,
annah

250 is slow to me, in fact it's glacial. I was being tres conservative.

I'm taking 2 layers of Bullseye, up to 19" square, at 500 DPH.

Brock Craig Breedlove

[Bert Weiss]

I won't comment on the heatup schedule as everybodies kiln is different.

I would only go as high as 1365 and hold for 1 hour. I don't care much for 3/16" float. It is thin and devits. If you get devit with that schedule, you need to go to 1400 for 40 or 50 minutes with a borax overcoat.

Make sure that the tin side is down.

3/8 boy

[Joanne Owsley]

Listen to Bert, he knows things.

[Bert Weiss]

So, a further question - is 'slow' 250 dph or less? What is 'slow' to you? I know what it is to me, but not anyone else....

smiles, and thanks,
annah

250 is slow to me, in fact it's glacial. I was being tres conservative.

I'm taking 2 layers of Bullseye, up to 19" square, at 500 DPH.

Brock Craig Breedlove

My kiln will heat up a 40" x 84" piece of 3/8" float at 600º per hour, 100 to 1000. I do 8" x 8" pieces at 900º per hour to 1000.

That is why I'm a big fan of fiber kilns with roof only elements. And why I love my 3 zone setup.

[Brock]

So, a further question - is 'slow' 250 dph or less? What is 'slow' to you? I know what it is to me, but not anyone else....

smiles, and thanks,
annah

250 is slow to me, in fact it's glacial. I was being tres conservative.

I'm taking 2 layers of Bullseye, up to 19" square, at 500 DPH.

Brock Craig Breedlove

My kiln will heat up a 40" x 84" piece of 3/8" float at 600º per hour, 100 to 1000. I do 8" x 8" pieces at 900º per hour to 1000.

That is why I'm a big fan of fiber kilns with roof only elements. And why I love my 3 zone setup.

Apples and oranges, Bert. I'm not firing a uniform thickness of float, I'm firing 2 layers of Bullseye, plus beads, plus foils. My kiln is fibre, with only ceiling elements, and If I want it can get to 1000 from ambient in 20 minutes. Good for magnets, not much else, at that speed. Brock

[Bert Weiss]

Apples and oranges, Bert. I'm not firing a uniform thickness of float, I'm firing 2 layers of Bullseye, plus beads, plus foils. My kiln is fibre, with only ceiling elements, and If I want it can get to 1000 from ambient in 20 minutes. Good for magnets, not much else, at that speed. Brock

I know, which is why I didn't initially comment on heatup schedules. They are more attuned to many variables than the rest of the numbers.

[Brock]

Apples and oranges, Bert. I'm not firing a uniform thickness of float, I'm firing 2 layers of Bullseye, plus beads, plus foils. My kiln is fibre, with only ceiling elements, and If I want it can get to 1000 from ambient in 20 minutes. Good for magnets, not much else, at that speed. Brock

I know, which is why I didn't initially comment on heatup schedules. They are more attuned to many variables than the rest of the numbers.

Totally agreed, but this problem certainly sounds like thermal shock. so whatever program is used, it's gotta be slower up to 1000. Brock

[Annah James]

Well, MY kiln is bricks on the bottom and fiber above with elements on the top and the long sides. It's 48 x 72.

I have been firing for years in an old ceramic kiln, so have had VERY conservative firing schedules so that the centers of the pieces got hot, too. Since I am not used to top firing, I feel like I am on a brand-new learning curve. I have continued to be 'slow' - since it's what I know, but I have a feeling that I could speed things up in this kiln. I have NEVER taken a firing of BE glass at 500 dph. eek! I clearly have lots to learn. I hope I don't pester everyone too much...Is there a good source for top-fire firing schedules? Are the samples from Bullseye appropriate?

The pieces did not devit, by the way.

onward...
Annah

[Brock]

I hesitate to tell you what to do with your kiln, but if you could turn off thr side elements until 1000, you would have better evenifity TM. You could turn them back on after that with no ill effects. Brock

[Bert Weiss]

I hesitate to tell you what to do with your kiln, but if you could turn off thr side elements until 1000, you would have better evenifity TM. You could turn them back on after that with no ill effects. Brock

I totally agree. The side elements could be helpful for a drop slump or other functions above 1000ºF, but below 1000 they sound like trouble to me. I have heard of kiln setups that can't control both elements seperately. A good techie might be able to set up multizones, if the controller is a Bartlett RTC 1000 style.

[Annah James]

When I had it built, I had it set up for multi-zone and I do have a Bartlett RTZ controller, so I could have my kiln guy show me how to turn the side elements off unless I am slumping. This is a new kiln - only ten firings so far - exciting!

Thanx again for all the advice!

annah

[Lauri Levanto]

Annah,

my kiln is quite similar but smaller.
I have no fancy controller with zones,
but I have a second set of contactors.
Manual switches allow me to cut off side,top or both.

The controller does not know what is going on,
but if I change the settings, the controller follows
in a couple of minutes.

I like to use top elements only for annealing,
but the idea that any sheet glass is warmed up
with top elements only makes sense to me.

-lauri

[charlie holden]

You know the old simple stand by method, that everybody used before they had these fancy-schmantzy ceiling fired fiber kilns, that we now seem to have completely forgotten, is to baffle the wall elements. Just put a little wall of something -- fiber board, kiln shelf, fiber paper -- between the glass and the wall elements. That reduces the amount of direct exposure to nearby infrared radiation. No need to worry about turning those elements off and on at certain times.

ch

[Annah James]

Bert - You said that you didn't like 3/16 float glass - too thin and it devits...so, when I unloaded the kiln this morning - (Yes, I admit, I didn't do it over the weekend, I was fighting the flu) the one 'good' piece was curved up in the center - not flat. Now, my kiln is FLAT. With the schedule that I followed, did it just not flatten down? And if I follow Brock's schedule, will it likely be ok? Oy vay. On top of my bricks I have a layer of fiber paper and then I used thin fire with fiber designs under the glass.

I am gritting my teeth - 12 panels due by end of month - I just want this to all work...

thanks-
annah