Careful microwaving! Start at low power, heat for a few minutes, let cool, repeat at slightly higher power for a few minutes, let cool, increase power a little, etc.
Otherwise you can have an orange hot spot in the MgSO4 that will be hot enough to melt the pyrex; the heating is uneven enough to break it also. You avoid this by using a turntable, and by doing the drying in increments of increasing power.