Diagnosis of breaks during slumping processes is often difficult because the temperature is not high enough to be able to apply the usual rule.
In looking for the reasons for a break in fusing processes, sharp edges imply the break occurred on the way down in temperature, but rounded edges indicate the break happened on the way up to the top heat.
This not a universally applicable diagnosis.
At low slump temperatures, the edges will be sharp in both the case of a break on heating up, and in the case of breaking on the way down in temperature.
The best test to determine when the break occurs is to observe periodically during the heat up. You will be able to see if the piece breaks before the top temperature. If it is whole at top temperature, the break occurred on the way down.
If you have been unable to observe the progress of the firing, you will need to diagnose when the break occurred. The test here is not whether the edges are rounded or sharp, because at normal slumping temperatures, the break will be sharp in both cases.
If the break occurred before the top temperature, the pieces will shape separately. Therefore, If the pieces no longer fit together, the break was on the rise. If they do, the break was on the way down. Place the pieces very carefully together to see if they form part of a continuous curve. If they do, the break was on the cool down. If they almost match, or do not match at all, then the break was on the rise in temperature.
In general, when the break is on the cool down, there is an overhang of the glass on the mould which causes the break. But the most common break of a slumping piece is caused by a too quick rise in temperature.
For a flat 6mm piece, the slump temperature rise should be less than 2/3 as quick as the rise for the fusing. If you have a tack fused piece to be slumped you should reduce the rate of advance to at least half of that for a smooth, flat piece of 6mm. Thicker glass with tack fused elements will need to be even slower.