Cut lines in glass did not heal

[Carrie Loosz]

I tried a technique that I read about in a book, but it did not turn out very well. I took two layers of 90 COE glass to a full fuse holding at the top temp. of 1480 degrees for 10 minutes. To let the air bubbles escape from the bottom, I cut the "Clear" 3mm base layer into sections and topped with a single piece of 3mm "Red Stringers on Clear." Even though the piece fused nice and flat, the cut lines on the base piece did not heal very well, and it looks like the piece is cracked. You can even feel a ridge along the cut lines on the back. I'm not sure what I did wrong, but I'd like to fix it, if possible. Would refusing the 1/4" slab be advisable to heal the cut lines so they blend and become undetectable? If so, would I slowly take it to the the same processing temp. and just hold it longer, or would going hotter be the solution? I'm still somewhat new at glass fusing and would appreciate any advise offered.

[Morganica]

I don't think you did anything wrong, but the glass touching the kilnshelf is more insulated than the rest of the glass, and so it tends to be the last to fully fuse. This is the reason you can "flip-n-fire" fused pieces i.e., fire them upside down so that the lines between pieces stay sharp and defined.

I'm not quite clear on why the bubbles needed to escape from the bottom--were you having trouble with bubbles before? In any case, you can get rid of the glass seams in several ways:

-Fire longer/hotter so that the glass against the shelf fully melts into its fellows (which can be difficult to do without messing up something else)
-Change your stack order, so that there's a single piece of glass against the shelf and the smaller cuts are on the next layer
-Change your design, for example, use the seams as a design element, or put a softer glass between the seams (such as black noodle or stringer), so that it softens first, "glues" the pieces together seamlessly, and then becomes an outline for those pieces
--Change the way you cut your clear components so that the seams are directly under an opaque or very dark component higher up, hiding the seams

[Carrie Loosz]

Thanks so much for the education and solutions, Morganica. The reason I chose to cut the bottom layer into several pieces to let the air escape is because the last time I stacked 2 single larger pieces of glass of together, I got a lot of air bubbles. With that piece, I inserted small chunks of clear glass in between the bottom & top layer close to the outside edges so that the glass would start fusing together in the center first and then work its way to the edges letting the the air bubbles escape in the process. I had read about that technique and thought I'd try it. I still got air bubbles that came close to bursting through the top, and it also resulted in bumpy edges where the small chunks of glass were placed. I didn't like that result at all. So, this time around, I thought I'd try the second technique I'd read about for the same size piece. I was using shelf paper instead of kiln wash, so I thought it would turn out fine. Only a few air bubbles, but the obvious cut lines that didn't blend together were a disappointment. I usually have one piece of glass for the base layer and then create a design with many small cut pieces stacked on top. The results are usually excellent. I will keep all of your comments in mind for future pieces and will try the flip-n-fire technique on this piece and hopefully the seam lines will disappear.