I had an emax monolithic crack, same place 4 times, I tried slow cooling, hold fix material, wow after waxing that sucker 4 times I could do it with my eyes shut, anyway whatever I tried didn't work until I called the dentist and asked if I could fill in the hole in the centre of the prep with a block out wax before I made it again, this worked out perfectly, no more crack. So am pretty sure that now and again you get a prep with weird geometry that stresses the crown when cooling. Is definitely a thick thin scenario, when I held the cracked units up to the light there was a thin area in the shape of a horseshoe with some bulk on the one side. I have been pressing FCC for a year now and only had this happen on this one case. Live and learn?