Recently I made a 8″ square plate using some red and green glass trying to get a piece ready to sell for the holidays! With Bullseye glass, you end up with what are called soft edges on the sides of the sheets (the natural edge from their production process) and so I wanted to incorporate these soft edges into my design.
Let me describe the layout:
- a 7-3/4″ 2mm thick sheet of clear
- a 7-3/4″ 3mm thick sheet of what I call Christmas glass
- using the soft edges, I wanted them to drape of the sides of the plate so using their width of about 1″, I put about 3/4″ on each edge and then had about 1/4″ hanging over the side
- and for these edges, I then overlapped them at each corner
I loved my design, fired it using a normal full fuse schedule and then opened the kiln when it was done! Argh! It had a crack down the center of the piece of Christmas glass.
I had never had anything like this happen before. So I wasn’t sure it was my firing schedule and I had gone too fast, or something wrong with my layout, or something wrong with the Christmas glass. So I decided to send an email to Bullseye and say, “Help!” They gratiously asked me many questions about kiln, position in kiln, type of kiln. layout of glass, thickness of glass and so forth.
Bottom line is that in the center, it was only 5mm thick and on the corners where I had overlapped things, it was 11mm thick. So as it was heating up, it could not heat evenly across the entire plate and hence it cracked. According to Bullseye, another possible problem could have been a large bubble in the center due to the center having the least thickness.
How not to have this happen? Basically, two options. The first is to make a design where it is even across the piece. However, sometimes this might not allow me the creativity that I want. So next option is to heat the piece up very, very slowly and the same when annealing it on the way back down to room temperature. I do plan to try this again (perhaps with a cheaper piece of glass) and use a very slow firing schedule and see what happens. Experimentation is so much fun!