The hell is that? I'm having trouble figuring out how the hell this design would work because you got two entirely different combination of elements and they both have different heating capacities. if they are touching each other, cooling could cause cracking . They heat different ways and right now I'm questioning whether or not that glass is just a cap? Just give an explanation maybe another picture or two.
Ok it's a little hard to see what this really is but I'm going take a swing and say just heat up the bowl as a whole but slowly.
Then you concentrate on the nail. That way you're allowing for heat expansion and time for cool down.