Thank you for posting on this frequently asked question!
Seams in kilnforming have something in common with seams in clothing--you can minimize them through careful craftsmanship, but they are almost never completely invisible.
Using dams on the periphery can keep seams minimal, because these reduce the movement of the top surface of the glass.
Also, the glass surface against the kiln shelf has the least amount of movement at process temperature, so firing face down can also minimize seams, if you don't pick up shelf primer or paper.
Having your pieces clean and fitting tightly minimizes seams.
I hope this helps. Let us know if you try any of these variables, and what results you observe.
Mary Kay Nitchie
Bullseye Glass Co.
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