We all at times take short cuts or economies which lead to less than desirable outcomes. One of these is to piece together fibre paper. Often the marks of the join – which are always there – are just too obvious to leave. The question becomes whether the fibre paper join marks can be eliminated.
Yes, there are at least two ways to remove these marks.
One is to cold work the bottom with a flat lap or wet belt sander.
The other is to use the kiln to re-fire the piece.
One method would be to put fresh fibre paper or kiln wash on the shelf and fire. This will require temperatures near the full fuse to achieve enough heat at the bottom of the glass to effect a significant change in the markings.
My suggestion for removing fibre paper marks - while it is still flat - is to fire upside down to fire polish to get rid of the marks without much changing the desired final texture of what will be the top. This is because the underside of the glass will not have the same heat effects as the top side. This also has the advantage that you can observe when the marks are eliminated.
When fired, flip over, clean the piece well to remove any fibre or kiln wash, and take to a quick fire polish to remove any marks - if necessary - caused by the upside-down firing. This quick fire will be a slow rise to ca. 600°C, and then quickly to the 740°C to 770°C range. This will cause the minimum change in the surface of the piece.
You will need to observe when both the evidence of the line disappears, and in a subsequent firing, when the final top surface of the piece is fire polished.