Low temperature breaks in flat pieces

The usual advice in looking at the reasons for breaks in your pieces must be considered in relation to the process being used.  Breaks during low temperature processes need to be considered differently to those occurring during fusing.  

The advice for diagnosing breaks normally, is that if the edges are sharp, the break occurred on the way down in temperature. Therefore, the glass must have an annealing fracture or a compatibility break.  It continues to say if the edges are rounded it occurred on the heat up, as it broke while brittle and then rounded with the additional heat.

This is true, but only on rounded tack and fused pieces.

I exclude low temperature tack fuses from the general description of when breaks occur in flat pieces as it is not applicable at low temperatures.  

Low temperature flat work includes sintering, laminating, sharp profile tack fusing, etc.  There are lots of other names used for this "fuse to stick" work.  In all these cases, the finished glass edge will be barely different than when placed in the kiln.  It stands to reason therefore that you cannot know when the break occurred, as the edge will be sharp whether it broke on the way up or the way down.  

Periodic observation during the firing is the only way to be sure when the break occurred. These observations should coincide with the move from the brittle to the plastic stage of the glass.  Therefore, about 540C.  It can be at a bit lower temperature, but not a lot.  If the glass was not broken by that time, you can be fairly certain it broke on the way down.

Further information is available in the ebook Low Temperature Kiln Forming.