My husband and I are fairly new to glass (a little over a year) and this weekend, we had our first major fail.
Because we are learning, we have been bouncing around to all sorts of techniques. I made a 12" mosaic style bowl. I cut very small 3mm shapes to make my mosaic. I worked inside a steel round form. Pieces were tight in some areas, but space was visible in most places. I capped it with a 3 mm clear. First firing was a high tack fuse. Slumped in a 12" round mold, somewhat shallow. Followed Bullseye recommendations for deep form 3-step process, but after the first firing, it looked great, so I stopped there.
The first bowl was a hit, sold immediately, but I saw issues I wanted to remedy. I made a second piece, 10" this time. I wanted more definition between pieces so this time I spaced the glass a little further apart. I also worked on top of the clear blank this time. I did another tack fuse but held it for less time, and I was very pleased with the results.
Even though it was a smaller piece (10" instead of 12") I slumped in the same mold, the same schedule. When I removed it, a piece broke apart in my hand. The rest of the bowl had cracks forming all over it. Cracks did not necessarily fall between mosaic pieces. Cracks were sharp. Because I still make rookie mistakes, I realized I had left a peep hole plug out. I figured this was likely my problem-- that the kiln cooled too quickly.
This is the slump schedule we used:
250 to 1200 hold 05
Afap to 900 hold 1.00
100 to 700 hold 00
Afap to 70
We knew it would not be a sellable piece, but I was quite attached, so we put it back in the mold and fired this schedule:
250 to 1225 hold 30
600 to 1420 hold 05
Afap to 900 hold 1.30
75 to 700 hold 30
100 to 500 hold 15
Afap to 70
When I removed the piece this morning, there was a brand new crack traveling the length of the entire piece.
So, what are we doing wrong? I have watched bullseye video on breaking, and figured it was due to uneven glass thickness, but we felt we compensated for that in the 2nd firing?
We are using a skutt 818 with elements around the side only. I am having trouble uploading an image but will keep trying. (The image is available through the link below. It is the only image I have and is pre-firing. It does illustrate the distance between the glass pieces, if this is important)
https://drive.google.com/file/d/1EHzoS8 ... p=drivesdk