Not for the faint of heart

[mnglass]

Here is the problem:


Ouch.

Here is what I did:
Flip and fire. These pieces (3/8 inch thick) had been annealed at
9999 960 60
75 900 90
100 800 0
200 700 0

When I went to slump, they were ramped at 200 dph to 1100. They broke on the way up as they tack fused in the molds (I broke them apart so you could see where the break was. The piece on the left was 5" from the side elements at the top of the photo and the bottom was 6 inches from the side elements at the bottom of the photo. Tow other pieces that were on the left side of the kiln came out fine.

Note that I've done dozens of similar (although not with pattern bar) pieces and almost never had a problem - and certainly two in one firing is indicative of something amiss.

I find it hard to believe that they were poorly annealed (although I won't discount it completely) because the schedule seemed pretty conservative to me. On similar problem, Lani suggested that this was uneven heating with the top being closer to the side elements. So the question is what should I do. Obviously slowing down to a 150 dph ramp (or even 125? 100?) ramp is one answer. But has anyone used a baffle (a tall piece of kiln shelf turned on its end) to shield pieces from the evil side elements?

I'm open to suggestions.

[Twin Vision Glass]

I am sooooo sorry ! I am truly thinking if you looked at the beginning of your schedule and slowed it down to no more than 100 deg. per hour you may have more success. As soon as I started doing this, I have been really pleased with the results. Also try baffles to help disperse the heat and to protect those edges close to the side elements. ( I also like to raise the pieces above the side elements so the heat will circle around the pieces. ) This is just what I have done in my kilns, so perhaps give these ideas a try and see where it takes you. Better to slow down than to loose beautiful work like this.
Leslie

[mnglass]

( I also like to raise the pieces above the side elements so the heat will circle around the pieces. ) Leslie

Leslie,

In the past five years I've made dozens and dozens of pieces (16 and 22 inch platters, ovals, squares etc) and rarely had a problem but in the past month a cluster of breaks. Every time you think you got it, you don't, I guess. My question is that if you raise the pieces above the side elements don't you get scary close to the top element? At least I would in my Evenheat 2541.

Next is 100 dph and baffles. But what is the downside (other than time)?

[kgkennedy]

Look at the way you set up your pieces in relation to the elements. Your pieces heated first from the areas closest to the elements. As the glass got wider it was colder and farther from the elements, especially the long piece on the left, and the uneven heat in the piece cracked it. The ends closest to the bottom of the photo were furthest away from the elements, and stayed colder. Maybe if you turned these pieces 90 degrees in the kiln so the length ran parallel to the elements on the side you could avoid this. Slowing down the heat is a good suggestion. Also do you have top elements on your kiln, as glass heats best when it is heated evenly from the top, the side elements usually cause problems with uneven heat, or uneven sides due to glass heating and sliding too fast in one spot. If you have top elements, can you turn off your side elements? The suggestion to elevate the pieces above the elements is good, but it is most important to have the piece heating evenly across the surface, and top fired kilns do this the best.

[kgkennedy]

Also, you say they broke on the way up. Did you peek below 1000 degrees (which is a no no), or are you saying this because the cracked edges were rounded? If the cracked edges are relatively sharp and fresh then it is an annealing problem, or you vented the kiln to cool it and that cracked the pieces. If the cracked edges are rounded then it is a heating problem.

[steenafullmer]

I have had this happen when the molds/kiln were not perfectly level. Perhaps your kiln is slightly off level a bit?

[Marty]

Why would an unlevel kiln or shelf have anything to do with the problem?

[mnglass]

Why would an unlevel kiln or shelf have anything to do with the problem?

I don't quite see the logic either. I doubt that they are off by more than 2 degrees from level.
no I didn't peek. It is roomtemp to room temp. yes they broke on the way up.

I've subsequently had the same problem on pieces that were baffled with large blocks of kiln shelves so I'm doubting that it was uneven heating from the side elements.

They were fired at 125 dph

Could my thermocouple (which is original) be that far off that I am way out of range for the annealing temperature?

[Twin Vision Glass]

Hi again, these are just my thoughts as I am not in your studio but ! I still feel if you slow down to 100 Deg. Per Hour (or if a full kiln, even less if thick pieces) you will not find it breaks on the way up.(unless the flat fire was not annealed correctly ) (and the outer sides really dump heat)I have had really good success with this slow down approach. They are such wonderful pieces (and also strip cut with dark and light glass side by side) and when I make pieces that have taken so much care like these, I really want them to work. Slowing down does add a few hours BUT at the end , you still have all the glass and the beautiful pieces intact.(especially when side fire is engaged .) You had mentioned it did not happen before. Perhaps you where fortunate. (still try alittle more central in the kiln perhaps too.) Just fire 4 plates instead of 5 at once (or 3 instead of 4)
I purchased a hand held Pyrometer that was suggested in Tech notes 7 . I am sooo pleased I did as I now know my kiln is firing almost perfect to the GB-1 . This gives me great comfort to know my kiln is not the culprit as far as temp. goes. Now I just have to make sure that all other variables are considered when something else goes wrong and not worry it was the temp.(the kiln is just another tool and we have to understand all it's ups and downs ) I strongly suggest this tool to everyone. Puts one more piece of the puzzle together. (also when you are firing on the lower end of the kiln , where is your thermocouple . ) perhaps it is reading the center of the kiln and you are on the bottom. Just a thought as well.
P.S. I also notice you have a kiln shelf right beside the long broken vessels, I wonder if you are changing heat flow with that large shelf and perhaps cool air comes out from underneath the kiln shelf and hits the side of the vessel. Hmmm! raise it above this by an inch . It still gives you lots of room to the elements. This might help the flow of air better.
Leslie