Straight Lines on Thicker Pieces

When you use a partial layer on two or three layers, you will get wavy lines as the thicker glass spreads more than the thinner. E.g., if you have 3 layers, especially near the edges, the fully fused glass will spread out, while the two layer areas will keep their volume. If you have straight lines at the edge, they will no longer be straight at fusing temperatures.

To keep the lines straight on thick pieces, build the design upside down, so that the final top design is down to the shelf. The shelf side is the coolest part of the glass being fired, so it will distort less. In addition, the weight of the glass above will assist in keeping the design elements in place.

This is often referred to as “flip and fire”