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Well, I had a massive explosion apparently. I checked and rechecked the schedule and it was fine. Same schedule that I have used a bunch of times for the same type pieces. So, this is my question.......I noticed a pocket of air trapped in the clear base glass when I was assembling the piece in the kiln for the fuse firing. matter of fact i thought to myself....I should drill this from the bottom as it was a pretty big trapped air pocket, like the size of a lima bean. But, of course I didn't......and, this crazy pocket was located in the upper left quadrant of the piece (where the splitting looks to have occurred). Could this have caused the piece to explode to the degree that it has?? I am hoping that the answer is yes, because it is my only idea. And, I sure don't want to be repeating this little adventure.

Thanks for any ideas!

Lisa,

It's obvious that the glass blew up on the way up... but I see a piece of what looks like black irid blown to pieces and not a piece of clear. Is there another piece of glass there? What was the thickness of the base glass? Was the base larger than the pattern bar piece? If it was, that may have been the cause since you would have uneven heating of the base glass due to the large mass of the patternbar piece... which I assume was prefused.

Tony

Tony Smith wrote:Lisa,

It's obvious that the glass blew up on the way up... but I see a piece of what looks like black irid blown to pieces and not a piece of clear. Is there another piece of glass there? What was the thickness of the base glass? Was the base larger than the pattern bar piece? If it was, that may have been the cause since you would have uneven heating of the base glass due to the large mass of the patternbar piece... which I assume was prefused.

Tony

Shoulda been clearer, clear 16"x16"on bottom, black 16"x16" on top of that and bars in the middle on top of all.

But, what about the lima bean pocket of air trapped inside the clear layer.....could that have initiated the explosion? My ramp up by the way was 300dph to 1000º. That's plenty conservative, isn't it?

Lisa

Lisa,

Sorry! Bummer.

I agree with Tony on this one. It looks like a classic case of thermal shock on the way up. The center portion of the base glass is heavily insulated by the thick pattern bars while the outer edges are exposed to the heat from both top and side elements.

Two simple remedies (I’d do both together):

1. baffle the edges from the side elements.

2. slow you firing schedule waaaay down. There’s no formula for exactly how fast to fire, but I’d guess that anything over 100F/hour is risky.

Incidentally, “I’ve used this schedule lots of times before and never had a problemâ€

I had a similar occurrence last year ... an exploding sunflower, 6 layers near the center and only 2 layers at the edges. I finally fused it successfully with a ramp of 250/hr, and then slumped it with a ramp of 150/hr. It sounds as though I may have been lucky at that -- I'd vote with Lani's suggestion of 100/hr.

(I still keep it in the windowsill of my studio as a reminder ...)

- Marilyn

Lisa Allen wrote:
Shoulda been clearer, clear 16"x16"on bottom, black 16"x16" on top of that and bars in the middle on top of all.

But, what about the lima bean pocket of air trapped inside the clear layer.....could that have initiated the explosion? My ramp up by the way was 300dph to 1000º. That's plenty conservative, isn't it?

Lisa

Assuming your pattern bar piece was about 1/4" thick, you had about 1/2" of glass there. But because of the uneven heating due to the large mass on top, the outside of the black glass started to expand at a much faster rate than the middle of the glass and blew it self apart. Lani's right, slow way down. 200°/hr might work, but you might still be on the "risky edge of things". Is a few more hours in the kiln worth all of the time and effort you put into the pattern bars? If you do decide to use a ramp rate higher than 100°/hr, peek into the kiln at around 1000° to check it. If it has blown up, at least you can still salvage your glass.

Tony

[quote="Lani McGregor"]Lisa,

Sorry! Bummer.

I agree with Tony on this one. It looks like a classic case of thermal shock on the way up. The center portion of the base glass is heavily insulated by the thick pattern bars while the outer edges are exposed to the heat from both top and side elements.

Two simple remedies (I’d do both together):

1. baffle the edges from the side elements.

2. slow you firing schedule waaaay down. There’s no formula for exactly how fast to fire, but I’d guess that anything over 100F/hour is risky.

Incidentally, “I’ve used this schedule lots of times before and never had a problemâ€

Assuming your pattern bar piece was about 1/4" thick, you had about 1/2" of glass there. But because of the uneven heating due to the large mass on top, the outside of the black glass started to expand at a much faster rate than the middle of the glass and blew it self apart. Lani's right, slow way down. 200°/hr might work, but you might still be on the "risky edge of things". Is a few more hours in the kiln worth all of the time and effort you put into the pattern bars? If you do decide to use a ramp rate higher than 100°/hr, peek into the kiln at around 1000° to check it. If it has blown up, at least you can still salvage your glass.

Tony

Thanks Tony. I usually just mindlessly close the kiln up at 1000º without looking (I am almost always venting thinfire) But, tonight I will surely check first! i am going to try 200dph until it proves unworthy......a little risk is a good thing for me these days.

Lisa Allen wrote:Well, I had a massive explosion apparently. I checked and rechecked the schedule and it was fine. Same schedule that I have used a bunch of times for the same type pieces. So, this is my question.......I noticed a pocket of air trapped in the clear base glass when I was assembling the piece in the kiln for the fuse firing. matter of fact i thought to myself....I should drill this from the bottom as it was a pretty big trapped air pocket, like the size of a lima bean. But, of course I didn't......and, this crazy pocket was located in the upper left quadrant of the piece (where the splitting looks to have occurred). Could this have caused the piece to explode to the degree that it has?? I am hoping that the answer is yes, because it is my only idea. And, I sure don't want to be repeating this little adventure.

Thanks for any ideas!

Lisa

When you have an air bubble, the glass on either side is thinner than the solid glass around it. This poses an annealing challenge. With my float glass, i have had glass crack on reheatup over the bubbles, but not badly. It heals up to temp, but not all the way so it is still flawed. Of course the solution was to reheat slower.

With multicolor glasses there is a lot more pushing and pulling going on so the chance for a more pronounced effect is there. I agree with Lani that it can be avoided with good annealing and slow heatup.

Sorry to see so many folks have learned about this the same way I did. I would add that the problem is exacerbated with black glass below because, being dark, it absorbs radiation (heat) faster than white (for example), thereby increasing the heating differential between the 'shadowed' center area and the more rapidly heating exposed glass. I have blown black apart with just a thin layer of white on top like this...so it need not be thick to 'shield' the heat, just lighter...thick and lighter is worse yet. I have also broken clear glass the same way, but by top-weighting the outside edges with steel flat-bar so they would lay flat and not pull-through a dropout mold, and then firing too fast. The outside edges were not expanding as fast as the interior and..... bam! I have learned to ramp-up at 50 degrees per hour when I need to. No problem....I mean, that's WHY I have a CONTROLLER....right? I Go slow when I need to, because it's faster!

regards, Jacques

When I make something like this I usually build a frame the same thickness as the central portion so the piece has a uniform thickness. Maybe this isn't the effect you want, but I've not had any thermal shock problems with it.

Steve

Steve Immerman wrote:When I make something like this I usually build a frame the same thickness as the central portion so the piece has a uniform thickness. Maybe this isn't the effect you want, but I've not had any thermal shock problems with it.

Steve

Well, I am looking for a continuous border with no seams and stacking is the only way I know to get it. I guess Ihave been really lucky, because I have done it successfully probably 15 times before the explosion.

I have slowed my schedule down to 200dph ramping up and hope that that solves future problem and still gives me a schedule that isn't a million hours long. Between this slow ramp and Lynne C's bubble reduction slow down and my overly conservative annealling......we are close to talking days and not hours just to get one of these guys fused

To avoid seams I build a dam for the border as steve suggests. Then I pile on the black around my piece. Sometimes I use a piece of thin white as a base just to get some opacity behind my elements. if the black is high enough so that I fill the dams I usually don't get distortion of the center elements. Sometimes I have to fire a second time to fill in seam indentations, but not usually.

Barbara

Lisa Allen wrote:Well, I am looking for a continuous border with no seams and stacking is the only way I know to get it. I guess Ihave been really lucky, because I have done it successfully probably 15 times before the explosion.

I have slowed my schedule down to 200dph ramping up and hope that that solves future problem and still gives me a schedule that isn't a million hours long. Between this slow ramp and Lynne C's bubble reduction slow down and my overly conservative annealling......we are close to talking days and not hours just to get one of these guys fused

emphasize the seams. cut 3/8" (thickness of back under the pieces + thickness of pieces) strips and use them on edge. put a pattern bar in the corners to make it really unusual. doing it this way you may not even need a back to the pattern pieces, and then they'll show on both sides of piece when done.

charlie wrote: emphasize the seams. cut 3/8" (thickness of back under the pieces + thickness of pieces) strips and use them on edge. put a pattern bar in the corners to make it really unusual. doing it this way you may not even need a back to the pattern pieces, and then they'll show on both sides of piece when done.

I have thought about using strips of black instead of a base sheet and if BE sold precut 3/8th" black strips I would do it in a minute

Kidding aside, that is probably what I will do in fixing my exploded piece.
Since the center pattern bar portion is fine, I am going to cut it out on the tile saw and will probably try the strip method for a border, as the center is now far too thick to add glass under it.

Might have never tried the strip border if it had not been for the explosion....so, good things do come in nutty packages. Seems all I do is cut strips all day long anyhow, what's 100 or so more