I tried making a dam for the first time and the result was kind of strange. I had a 3 layer design for a desktop 77.5" x 28.75". 2 layers of 10mm float with a layer of coarse homemade frit in the middle. The front and back were not dammed only the two ends. I made the dams with firebricks covered with 970 1/32" paper. The dams were about 1/4" from the glass edge(I cut the glass 1/2" short). The bricks were wedged in to the ends of the kiln. The glass spread and the ends curled up then the whole thing shrunk about 3/4" which as I look at it is about 1%. I ended up with a short slab with an upturned edge. I can cut off the sticking up glass edge with a diamond saw and those edges were always planned to be covered up.
Wouldn't I like one of those Fellissatti's about now.
A few more details. The frit stopped 1" from the edges and the front edge, a wavy line, was rolled (top piece bigger than the bottom piece by the glass thickness). The back edge which was stacked. Front and back came out perfect.
Did you use glass repelling bricks for the dams instead of regular bricks by any chance? I would have though that it might pull in toward where the glass was thickest but it sounds like it was of even thickness, although at that size do things act differently?? I have no idea so I didn't write with advice, just wanted to say, it sounds cool, can we see a picture??
-Amy
Sounds to me like the glass expanded enough in the long direction during ramp up to jam against the dams before it got soft. Then it softened, fused and settled. That's when the ends turned up, since they were in contact with the fiber paper, maybe even tightly jammed against it, while the frit was being compressed.
Then it contracted again on cool down. Given that it contracted 3/4 of an inch while cooling, from a definite limit set by the dams jammed against the kiln walls, I think you could anticipate that it originally wanted to expand that much as well. Next time try giving yourself 7/8 of an inch open space and see if it ends up 1/8 inch long.
If you know the COE of the glass, there may be a way to calculate how much it expands between room and fusing temp. There is some good dam info in Bullseye's "Working Deep" article, though they aren't talking about anything nearly this long.
I'm no expert on dams, but I suspect the strange results were because only the ends were dammed. It may have "contracted" because it was spreading out along the front and back edges and pulling the ends in as it spread.
charlie holden wrote:Sounds to me like the glass expanded enough in the long direction during ramp up to jam against the dams before it got soft. Then it softened, fused and settled. That's when the ends turned up, since they were in contact with the fiber paper, maybe even tightly jammed against it, while the frit was being compressed.
Then it contracted again on cool down. Given that it contracted 3/4 of an inch while cooling, from a definite limit set by the dams jammed against the kiln walls, I think you could anticipate that it originally wanted to expand that much as well. Next time try giving yourself 7/8 of an inch open space and see if it ends up 1/8 inch long.
If you know the COE of the glass, there may be a way to calculate how much it expands between room and fusing temp. There is some good dam info in Bullseye's "Working Deep" article, though they aren't talking about anything nearly this long.
Charlie
I think that the solution is probably to estimate the amount of spread, cut the glass that much smaller and then set up the dam at the outer limits. By only being 1/4" smaller than the dam, the glass bumped them good and curled up. If the contact was gentle it probably would have worked as anticipated.
I have been able to trim off the curl with a 4" diamond saw. That area will have to be covered. I planned for that originally anyway.
I just did my first test with BE black and dichro frit covered with clear frit. I cut 1/4" strips of black irid and placed them in a 1/4" LD board with 2" square cutouts. Then I made a square to fit inside the border. Next I loaded in dichro frit covered wit medium clear BE and fired fast to 1000 and slowly to 1375 and held for an hour. This time the dams worked fine. I got a lumpy surface which is what I was shooting for.
I think that the solution is probably to estimate the amount of spread, cut the glass that much smaller and then set up the dam at the outer limits. By only being 1/4" smaller than the dam, the glass bumped them good and curled up. If the contact was gentle it probably would have worked as anticipated.
Well, I would phrase it like this -- cut the glass to the size of the countertop, estimate the amount of expansion and set the dams at that outer limit. It seems that if you cut the glass substantially shorter than the final length that you want, it will end up too short. In other words, it starts out short, expands out to touch the dams at top temp, then contracts back to its original size on cool down. Of course that assumes that the glass isn't flowing out any, just expanding and contracting. If the glass is flowing, it seems that you would need dams all the way around the piece and the dams determine the final dimensions of the slab. In that case you have to try to calculate the contraction from fusing temp, regardless of how much spread there has been. Trial and error would be my method.
Again, I recommend the Bulseye "Working Deep" article. You can download it in pdf format. They have really worked out some interesting methods of building dams and calculating volumes.
I would have though that it might pull in toward where the glass was thickest but it sounds like it was of even thickness, although at that size do things act differently?? I have no idea so I didn't write with advice, just wanted to say, it sounds cool, can we see a picture??