Sometimes the edges of pieces can show the places where the glass layers join.
This happens when there is not enough heat to completely fuse the pieces together, as in a tack fuse.
It can happen to only some pieces, due to differences in temperature within the kiln. If this occurs take note of the cool spots, or better yet, run a test for the cool areas of your kiln as in this Bullseye Tech Note. Then avoid those areas for full fused items.
If you run your kiln as fast as it will go, you can be left with seams at the sides if you do not soak for long enough at the top temperature. I usually try to achieve my desired results with a 10 minute soak. Going slower to top temperature can give you fully formed edges at a lower temperature.
You can also get visible seams if you are using iridescent glass, mica, metal or other inclusions that come to or near the edge of the piece. The solutions are to move the inclusions further in from the edges. With iridescent glass, you should remove two or three millimetres of the iridescence to allow the glass to stick together.
If you want to conceal the layers of colour used to build up the piece, you can cut your top piece - assuming it is not clear – 3mm larger all around. This allows the top layer to fold over and enclose the whole piece, concealing the different colour layers.