To avoid needling in box cast or dammed work you need to provide a space for the glass to flow into.
This is done by using 3mm thick fibre paper to line the damming materials. The fibre paper is cut to 3mm less than the finished height of the fired piece.
Fire the glass with a long bubble soak. This allows the glass to almost achieve its final height before it becomes less viscous. It will still be higher than the fibre paper and as the glass continues to be more “soft” it will round as it reaches full fusing temperature. There is not enough glass above the fibre – only 3mm – for the glass to run over the fibre, as the surface tension holds it in until 6 or 7mm above the fibre. The top edge of the glass does not touch the fibre or dam, so there are no needles.
Another way to avoid needles in this kind of work is to make the dams larger than the glass being contained. That is, place the dams a short distance away from the glass. The glass will then flow out to meet the dams. Since the glass is not contracting it will not have needles. This is a good solution when the thickness of the glass is not critical. You control the area of the piece by the placing of the dams.