float glass disaster

Bert Weiss · Post by **Bert Weiss** » Mon Jan 12, 2004 6:15 pm

Annah James wrote: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

Annah

I would not fire above 1435 to prevent the big bubble. The glass is too stiff to blow the bubble at that temp. Take long enough time there to get your heat work done. Soak 20 to 40 minutes or longer.

A bubble means that either the shelf is not flat or somehow gasses can build up and expand under the glass. If it was simply warped, it means that cold air got in during the cool down and made it warp. The warp is fixable with another firing. A bubble is not because the glass stretched to make it happen and it will never "unstretch".

Annah James · Post by **Annah James** » Mon Jan 12, 2004 9:24 pm

Bert:

So when Brock says to go to 1510 degrees, I shouldn't? I could take it to a lower temp and hold it longer...will it devit? (The piece is warped, not bubbled, by the way - about 1/2" tall in the center...)

I am holding off the next firing until I feel like I have SOME understanding of what I am dealing with here...oh my.

Also, one of you talked about putting the tin side up. How do you tell??

sheepishly - I 'thought' I knew how to fuse glass....

Annah

Annah James · Post by **Annah James** » Mon Jan 12, 2004 9:25 pm

Bert:

So when Brock says to go to 1510 degrees, I shouldn't? I could take it to a lower temp and hold it longer...will it devit? (The piece is warped, not bubbled, by the way - about 1/2" tall in the center...)

I am holding off the next firing until I feel like I have SOME understanding of what I am dealing with here...oh my.

Also, one of you talked about putting the tin side up. How do you tell??

sheepishly - I 'thought' I knew how to fuse glass....

Annah

Brock · Post by **Brock** » Mon Jan 12, 2004 9:32 pm

Annah, thr 1510 was from your schedule. Here:

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 wouldn't go that hot myself, but I was dealing with your question about why the piece broke, and it is thermal shock. I gave you a better rate to heat up and just used the top temp. you had posted, assuming, (I know, I know) that 1510 was required in YOUR kiln. Brock

Annah James · Post by **Annah James** » Mon Jan 12, 2004 11:18 pm

Hmmm. I got the 1510 from an old thread I found on the board as I was looking for firing schedules for float. I doubt that I would have to go that hot - I don't generally.

All the variables that we deal with make algebra look easy!

So: What if I do this:
250 dph to 1450 hold 1 hour
afap dph to 1040 hold 1 hour
100 dph to 740 hold 30 min
end

Does this seem reasonable? Do you think the pieces will be flat and not warped, devitted, and perfect???! Oh dear, I am asking for WAY too much, aren't I?

You guys have been great. I appreciate all the feedback...

annah

Brock · Post by **Brock** » Mon Jan 12, 2004 11:32 pm

Why not try Bert's suggested top temp. of 1435?

Annah James · Post by **Annah James** » Mon Jan 12, 2004 11:54 pm

Brock:
I want to get a really crisp line with fiber paper below. Do you think I will get that at 1435? I'm game, I just want a good, deep impression.

annah

Bert Weiss · Post by **Bert Weiss** » Tue Jan 13, 2004 1:01 am

If your piece is warped, I suspect a big cold air leak somewhere. This would cause thermal shock and warpage. Plugging up the hole should solve both problems.

I like to kiln carve at 1365 for 1 hour soak. Fuse at 1420 with 40 minute soak. For fusing I also soak for about 45 minutes to an hour at 1220 or so to squeeze out as much air as I can.

rosanna gusler · Post by **rosanna gusler** » Tue Jan 13, 2004 8:28 am

So: What if I do this:
250 dph to 1450 hold 1 hour
afap dph to 1040 hold 1 hour
100 dph to 740 hold 30 min
end

like bert says do a soak around the 1100f-1200f range. then there is nothing to be gained by going slow up to process temp except devit. rosanna

charlie holden · Post by **charlie holden** » Tue Jan 13, 2004 11:04 am

Annah James wrote:Bert:

So when Brock says to go to 1510 degrees, I shouldn't? I could take it to a lower temp and hold it longer...will it devit? (The piece is warped, not bubbled, by the way - about 1/2" tall in the center...)

I am holding off the next firing until I feel like I have SOME understanding of what I am dealing with here...oh my.

Also, one of you talked about putting the tin side up. How do you tell??

sheepishly - I 'thought' I knew how to fuse glass....

Annah

You find the tin side with a UV light. Search the archives; there's tons in there on this. And the tin side should be down, not up.

I would like to see the element layout of your kiln. How many elements are in the wall compared to the ceiling? This isn't a big, coffin Olympic glass kiln is it? My guess is that the warp is partly a function, again, of the edges of your glass being hotter than the center. The middle of the three panels had more even heat so it didn't thermal shock, but your ceiling element isn't hot enough to get the glass up to the same temperature as the edges. You could both baffle the wall elements and raise your shelf so it is closer to the ceiling elements. Also, slow down a little at the top temp.

An aspect of firing that you should understand is that glass "sees" the IR, (Infrared Radiation); it is effected more by IR than other forms of heat, such as the temperature of the air in your kiln. Any glass that is openly exposed to a glowing element is going to get hotter and softer faster than any glass that can't see the element. Glass that is closer to an element that it can see, gets hot much, much faster than glass further away, just like a piece of paper is illuminated much more as it is brought closer to a light bulb. (That's because the IR is spreading out into three dimensional space as it radiates away from the element.) Ceramics, on the other hand, can't "see" the IR. That's why you can load a ceramics kiln with pots inside larger pots and many shelves of pots stacked above and below each other, and all the pots will bisque the same. If you baffle the wall elements of your kiln, you will hide the glass from their IR and even out the temperature of your glass.

Another way to simplify this whole process is to settle on a set soak at whatever top temperature you are fusing or casting at. Is an hour hold at 1465 the same as 20 minutes at 1520? Who knows? I can say for sure that an hour at 1465 is less than an hour at 1520. In a class I took with Ted Sawyer, head of research and education at Bullseye, he soaked everything the same amount of time.

ch

Bert Weiss · Post by **Bert Weiss** » Tue Jan 13, 2004 12:04 pm

charlie holden wrote:
Annah James wrote:Bert:

So when Brock says to go to 1510 degrees, I shouldn't? I could take it to a lower temp and hold it longer...will it devit? (The piece is warped, not bubbled, by the way - about 1/2" tall in the center...)

I am holding off the next firing until I feel like I have SOME understanding of what I am dealing with here...oh my.

Also, one of you talked about putting the tin side up. How do you tell??

sheepishly - I 'thought' I knew how to fuse glass....

Annah
You find the tin side with a UV light. Search the archives; there's tons in there on this. And the tin side should be down, not up.

I would like to see the element layout of your kiln. How many elements are in the wall compared to the ceiling? This isn't a big, coffin Olympic glass kiln is it? My guess is that the warp is partly a function, again, of the edges of your glass being hotter than the center. The middle of the three panels had more even heat so it didn't thermal shock, but your ceiling element isn't hot enough to get the glass up to the same temperature as the edges. You could both baffle the wall elements and raise your shelf so it is closer to the ceiling elements. Also, slow down a little at the top temp.

An aspect of firing that you should understand is that glass "sees" the IR, (Infrared Radiation); it is effected more by IR than other forms of heat, such as the temperature of the air in your kiln. Any glass that is openly exposed to a glowing element is going to get hotter and softer faster than any glass that can't see the element. Glass that is closer to an element that it can see, gets hot much, much faster than glass further away, just like a piece of paper is illuminated much more as it is brought closer to a light bulb. (That's because the IR is spreading out into three dimensional space as it radiates away from the element.) Ceramics, on the other hand, can't "see" the IR. That's why you can load a ceramics kiln with pots inside larger pots and many shelves of pots stacked above and below each other, and all the pots will bisque the same. If you baffle the wall elements of your kiln, you will hide the glass from their IR and even out the temperature of your glass.

Another way to simplify this whole process is to settle on a set soak at whatever top temperature you are fusing or casting at. Is an hour hold at 1465 the same as 20 minutes at 1520? Who knows? I can say for sure that an hour at 1465 is less than an hour at 1520. In a class I took with Ted Sawyer, head of research and education at Bullseye, he soaked everything the same amount of time.

ch

Charlie's point makes sense to me. Uneven heat would cause a warp the opposite way as a cold spot. If that is the case "Baffle and Go"

And tin side is down! The tin oxide layer protects the glass from sticking and tin bloom (cousin of devit) happens more readiy on the up side if it is tin.

Annah James · Post by **Annah James** » Tue Jan 13, 2004 4:42 pm

Ok, you all...I will check the archives about the tin coating and research where to get a uv light.

Re the firing schedule: this is what I'm thinking of doing, based on ALL the excellent feedback I have been getting:

250 dph to 1450 hold 1 hr
afap to 1200 hold 20 min
afap to 1040 hold 1 hr
100 dph to 740 hold 30 min
end

Does this seem real?

I don't think I have a air leak - there is a channel that the bell lid drops into and it's filled with sand to form a positive air lock.

I'm going to launch this today unless I hear screams of disbelief from the gallery...
many thanks!!
Annah

rosanna gusler · Post by **rosanna gusler** » Tue Jan 13, 2004 8:29 pm

[quote="Annah James"]Ok, you all...I will check the archives about the tin coating and research where to get a uv light.

Re the firing schedule: this is what I'm thinking of doing, based on ALL the excellent feedback I have been getting:

250 dph to 1450 hold 1 hr
afap to 1200 hold 20 min
afap to 1040 hold 1 hr
100 dph to 740 hold 30 min
end

hi annah, did you mean 250dph to 1200 hold 20 min.....then afap to 1450..............seems like you might have transposed some numbers. rosanna ]

Annah James · Post by **Annah James** » Tue Jan 13, 2004 9:22 pm

Rosanna:

somehow I thought that the soak at 1200 had to be on the way down. Is that wrong? I can do it the other way...

250 dph to 1200 hold 20 min
afap to 1450 hold 1 hr
afap to 1040 hold 1 hr
100 dph to 740 hold 30 min
end

like that??

annah

rosanna gusler · Post by **rosanna gusler** » Tue Jan 13, 2004 9:36 pm

the soak at around 1100f-1200f on the way up is to let everything even out in the kiln and to squeeze out trapped air if you have multiple layers. then go afap to process temp. no chance of shocking glass or molds then. the hold at 1030 or so on the way down is for annealing. rosanna

Annah James · Post by **Annah James** » Tue Jan 13, 2004 9:45 pm

OK, I'll go program the controller and get it going. Whew! I really want to take a workshop on firing schedules for top-fire kilns. Anyone out there going to teach one??

annah

Bert Weiss · Post by **Bert Weiss** » Wed Jan 14, 2004 1:44 pm

Annah James wrote:OK, I'll go program the controller and get it going. Whew! I really want to take a workshop on firing schedules for top-fire kilns. Anyone out there going to teach one??

annah

Annah

I'm teaching painting on float and 90/96 glasses in a couple of weeks in Hillsboro Oregon. I will be covering firing schedules. Info in the classifieds or Email me or Harry or John Groth.

Annah James · Post by **Annah James** » Wed Jan 14, 2004 6:29 pm

OK. I ran the firing last night. The pieces look good, with nice crisp lines, and they are NOT FLAT. They are bowed in the middle by about 3/8".

My theories are:
1. Since the edges that are down, are the ones nearest the side elements, there is some uneven heating happening in the kiln.
2. My thermocouples are not set to properly adjust the top elements, being set too low in the chamber, and too close to the side elements.

Any other ideas?? I have no air flow - the chamber is tight, so it's not that.

I don't want to move the suckers until I can re-fire them - maybe get them flat next time...

hmmm.
Annah

charlie holden · Post by **charlie holden** » Thu Jan 15, 2004 11:24 am

Annah James wrote:OK. I ran the firing last night. The pieces look good, with nice crisp lines, and they are NOT FLAT. They are bowed in the middle by about 3/8".

My theories are:
1. Since the edges that are down, are the ones nearest the side elements, there is some uneven heating happening in the kiln.
2. My thermocouples are not set to properly adjust the top elements, being set too low in the chamber, and too close to the side elements.

Any other ideas?? I have no air flow - the chamber is tight, so it's not that.

I don't want to move the suckers until I can re-fire them - maybe get them flat next time...

hmmm.
Annah

Did you change anything other than the schedule? How far are the edges of the glass from the wall elements? How far is the surface of the glass from the ceiling elements? How many wall elements are there? How many ceiling elements? Can you raise your shelf so that it is above the wall elements without being too close to the ceiling elements?

My advice stands -- baffle the glass so it is not directly exposed to the wall elements. This will even out the heating and mean that all of the glass gets about the same heat treatment as the glass in the center got on your last firing. So you will have to lengthen your soak or raise your top temp to slump the center completely. Two hot firings is getting risky with float and you may get devit.

Does your kiln have zone control? If it does, are the zones broken into left-right-center, or top-middle-bottom, or what? Only in a top to bottom zone system will the thermocouples likely have any control over your ceiling elements, seperate from the wall elements. If this were the case, you would likely have one thermocouple up high in your kiln, and another down low.

If you just have thermocouples down low then the ceiling elements are probably on the same controller as the wall elements. If you can push your thermocouple further into the center away from the wall elements, it would read more accurately. But it wouldn't solve your problem since it would be reading cooler temps than it does now, and therefore turning your kiln on more often. Your problem is too much heat from one direction and uneven heating in general.

ch

Annah James · Post by **Annah James** » Thu Jan 15, 2004 11:22 pm

Did you change anything other than the schedule? How far are the edges of the glass from the wall elements? How far is the surface of the glass from the ceiling elements? How many wall elements are there? How many ceiling elements? Can you raise your shelf so that it is above the wall elements without being too close to the ceiling elements?

My advice stands -- baffle the glass so it is not directly exposed to the wall elements. This will even out the heating and mean that all of the glass gets about the same heat treatment as the glass in the center got on your last firing. So you will have to lengthen your soak or raise your top temp to slump the center completely. Two hot firings is getting risky with float and you may get devit.

Does your kiln have zone control? If it does, are the zones broken into left-right-center, or top-middle-bottom, or what? Only in a top to bottom zone system will the thermocouples likely have any control over your ceiling elements, seperate from the wall elements. If this were the case, you would likely have one thermocouple up high in your kiln, and another down low.

If you just have thermocouples down low then the ceiling elements are probably on the same controller as the wall elements. If you can push your thermocouple further into the center away from the wall elements, it would read more accurately. But it wouldn't solve your problem since it would be reading cooler temps than it does now, and therefore turning your kiln on more often. Your problem is too much heat from one direction and uneven heating in general.

ch

OK. I did change the firing schedule. As I was loading the kiln this time I thought that the edges of the glass were too close to the sides, so I only did two pieces instead of three and I centered them in the space. The top elements are 18" above. Since the sides that are "down" are the sides that have the elements closest to them, I think that the side elements in the kiln either need to be baffled, or I need to set up the thermocouples so that one is reading more in the center of the space - and program the controller so that the top elements are firing more than the side elements. I think that your take is right on - that it's uneven heating.

Do you think I could flatten these pieces out by doing a slump firing where I hold it longer at a lower temp, and baffle the side elements? Charlie, I REALLY appreciate you taking the time with this...it's always hard to learn something new when I'm working on a commission piece....oy

annah