I’m not sure what you mean
I’m not sure what you mean by “sink hole”. At first I thought it was about a hole for a sink! But assuming you mean a bubble has formed and burst leaving you a hole in the blank, I will proceed. It sounds like the bubble has formed under the glass, rather than between – is that correct?
1. What are the possible causes?
As you haven’t published your schedule or information about your kiln, I will give some general observations. Inadequate bubble squeeze – temperature or time; uneven heat distribution in kiln; inadequate release of air from under piece; low spot in kiln shelf. these are some of the possible causes.
2. What can be done to prevent holes forming in the future?
Check on the things above. You can also put what the americans call “chads” round the edge (3 is the minimum on a circular piece). These can be little balls of glass formed previously, or simply small shards of broken glass. They do not have to be big. These support the edge of the glass keeping it from sealing at the edges before the middle. Although if I am correct and the bubble has formed under the glass this will not help. If you kiln is side fired, you need to baffle the edges of the glass from the direct heat of the side elements. Also firing more slowly will help.
3. Is it possible to salvage the piece by filling the hole and re-firing it.
Probably not. Mostly these look repaired.
4. What are the likely out comes if I slump the blank with the hole as is.
Ugly, in my opinion.