Hi Susanne, First, you can
First, you can avoid this in the future by cutting your fiber strips (used for dams) 1/8″ shorter than the final thickness of the piece. When you do this, surface tension will give you a nice, rounded edge.
Second, in your firing schedule above, I would replace this:
There is no real benefit to slowing down between the squeeze at 1250 and the full fuse at 1500. In fact, the slower heating in this range may contribute to increased devitrification.
If you are going to grind the spikes and simply fire polish, you can likely do it without dams. If you are going to repeat a full fuse (with a hold at fusing temp) then you will need to re-dam the piece.
The biggest change you will need to make on the re-fire is to reduce your initial ramp to avoid thermal shocking the thicker piece. It is very kiln dependent – but as a rule, I refire 3/8″ pieces at 100F/hr. That may be overly conservative – but I can’t remember the last time I broke a piece
Helios Kiln Glass Studio