I've been meaning to take and post these pictures for quite awhile, now. So, don't fret yourself by thinking that my gorgeous blue and orange ribbon stitched quilted plate has suddenly shattered!No, this plate is the first generation plate; the one made entirely of scrap glass, and because of that, alas, I found some incompatible glass laying in the wrong bins at the studio. However, this "unfortunate" incident lends itself nicely to a fabulous learning experience.
Many beginners to glass fusing ask if they can use old stained glass scrap that they've got lying around. The broad answer to that question is "No" due to reasons of incompatibility. You see, glass that has been specifically formulated for fusing "plays nicely" only with other glasses that have also been specifically formulated in the same way. The label "COE," or "Coefficient of Expansion," is the general term that describes the rate in which glass expands and contracts upon heating and cooling. If you are using two glasses that have different COEs for fusing purposes, then your final result will end up looking similar to my lovely sample plate. (Again, the COE will generally get you through the long explanation, but if you want all the juicy details of compatibility, check out TechNotes 3: Compatibility of Glasses from Bullseye.)
However, you may not have a disaster like this the first time out of the kiln! For reasons best left to the Glass Gods, this particular plate did not crack until after it's second time out of the kiln. Only when I was engraving the bottom of the plate did it "pop" in two cracks diagonally down the middle. Initially, I thought it was my engraving that broke the plate.
So, I put the plate back in the kiln to fuse it back together at a full fuse, once again. Fortunately, the third time out of the kiln was a total crack up, around each and every one of the orange pieces, allaying my guilt at having cracked my plate upon engraving.
So, what happened?!? By looking at the slivers that have formed in circles surrounding almost each and every piece of orange glass, we can be suspicious that this glass is not compatible, which all of the other glass that I was using happens to be. In other words, the rate at which the orange glass contracted upon cooling was slower than the rate at which the rest of the glass contracted. As the glass became less fluid, the only way for it to get rid of all the stress building up inside was to crack!