That sounds good Matthew, and another interesting part of that is that it heats more evenly throughout. Rather than the heat having to penetrate through from the outside.
I remember from one of Steve Chastain's advanced casting books that plaster of Paris (is it okay if I just say poP from now on?) is used for pouring basin or sprue insulating liners in casting work. poP is I think the best insulator found for this.
So if it is such a good insulator, I imagine that would slow down regular convection heating even more, compared to microwave heating. That could account for a good chunk of the time required.