A frequently asked question is “how many times can I re-fire my piece?”
This is difficult to answer as it relates to the kind of glass and the firing conditions.
Kind of glass
Float glass is prone to devitrification. This often begins to occur on the second firing. Some times it may be possible to get a second firing. Sandblasting the surface after getting devitrification will enable another firing at least.
Art glass is so variable that each piece needs to be tested.
Fusing glasses are formulated for at least two firings, and may be fired a number of times. The number will depend on the colours and whether they are opalescent. Transparent colours on the cool side of the spectrum seem to accept more firings than the hot colours. Both of these accept more firings than opalescent glasses do.
The higher the temperature pieces are fired at, the fewer re-firings are possible. So if multiple firings are planned, you should do each firing at the lowest possible temperature to get your result. This may mean that you have relatively long soaks for each firing. The final firing can be the one where the temperature is taken to the highest point.
You do have to be careful about the annealing of pieces which have been fired multiple times. A number of people recommend longer annealing soaks. However, I find that the standard anneal soak for the thickness is enough. What is required is a slower cooling rate, maybe 5C/hr less on each subsequent firing at tack or full fusing. The slump firing can be annealed at just less than the standard. Of course, having put all this work and kiln time into the piece, the safest is to use the same cooling rate as the final high temperature firing used.
In general slumping is at a low enough temperature to avoid any creation of additional stress through glass changes at its plastic temperatures. But any time you heat the glass to a temperature above the annealing point, you must anneal again at least as slowly as in the previous firing. Any thing faster puts the piece at risk of inadequate annealing.
Testing for stress after each firing will be necessary to determine if there is an increase in the stress within the piece. In the early stages of multiple firings, you can slow the annealing and if that shows reduced stress, it will determine your previous annealing schedule was inadequate. When reducing the rate of annealing does not reduce the stress, it is time to stop firing this piece at fusing temperatures.