You now have a practial demonstration of the need for care in transportation or annealing or compatibility concerns.
You can take the piece up to about 580C for a few minutes and then reduce to annealing temperature. However neither you nor I know what the glass is so the annealing is a bit of a crap shoot. Assuming the piece is 6mm thick, soak at 530C for 30mins and then reduce the temperature at 30C/hr to about 480C, then 60C/hr to 370C. If it is thicker you need more time at the soak and slower temperature falls. However this will not make the visual appearance of the crack disappear.
If the surface texture is not if interest to you, you can tak it up to around 816C for 10mins then reduce afap to the supposed annealing point and follow the above schedule.
If it is formed already, don’t bother. The shape will change, you don’t have the mould to re-shape it either. So in this case you can use it to find out how long it takes to self destruct.
It would also be of interest to you to find out how much stress there is in this piece. Get some polarising filters and place the glass between. Rotate the filters until they are black. Any stress shows up as areas of light. The more intense or widely spread, the more stress there is in the piece.