Sometimes people experience sharp, needle-like points on the bottle after it is slumped.
As the bottle expands and softens, it conforms to the surface of the mould. When the cooling begins, some parts of the glass are trapped in the tiny pits of unevenness that always exist in the mould or in the separator. As the glass retreats, the glass is stretched until it breaks off, leaving the sharp needles.
Remedies relate to separators and temperatures. This of course, assumes you already have a good coating of kiln wash or similar separator on the mould.
These additional separators can be fibre paper or powders. Thinfire laid on the bottom of a bottle mould can provide additional separation between the bottle and the glass. This works, because with a slow rate of advance, the Thinfire will have turned to powder before the bottle begins to slump. This powder will not interfere with any designs on the mould. Papyros will work on smooth moulds, but not so well with textured bottle moulds, because of its more fibrous nature.
This use of powered paper indicates that you could use a cheaper solution. Just dust a fine film of kiln wash on the mould. I do this by placing the powdered kiln wash in a sock and shake the sock above the mould. This will allow an almost invisible layer of fine powder to fall onto the mould. This is enough to provide an additional layer of separation between the glass and the mould.
It is quite common for people to slump bottles at tack fusing temperatures to do both the flattening and the slumping at one firing. This is quite hard on the mould and softens the glass enough to promote the needling.
It may be better to use two firings – one to flatten using tack fusing temperatures, and one to form the bottle at slumping temperature. This lower temperature will avoid the needling, as the bottle will not soften enough to form the needles during the slumping. The reason many people avoid this is because the bottles tend to devitrify on second firings. If you do this two-stage slumping, you will need to apply a devitrification solution to the upper surface of the flat bottle to try to prevent it.
You can take a different solution to the two-stage firing. As lower temperatures reduce the possibility of needling, you can simply soak for a longer time at the slumping temperature than a normal one stage tack and slump. You will need to peek in at intervals to determine when the slump is finished, of course. After a few firings though, you will get a good idea of the amount of time required to complete the slump. An additional advantage is that at the lower temperatures, devitrification is less likely.