The process takes advantage of two things - heat and weight. The glass on the shelf side moves less than the top as it is not quite so hot, and the weight of the glass above keeps the lines the way they were cut. The glass on the top of the piece begins to move first and fill the gaps that are left between the pieces.
|This piece has been assembled with the final upper surface on the shelf and the base sheet placed on top.|
The simplest method to achieve straight lines is to fire the piece with the final surface down to the shelf. After fusing, turn over and clean any surface contamination, usually by sandblasting. Wash and polish dry. Then fire the new surface to a fire polish temperature.
|The same piece fire polished after cleaning the fused glass.|
This technique works best on pieces that are of one uniform thickness.
There are other factors at play in obtaining crisp lines.