Had some nasty leftover Pizza Hut in the fridge so I gave it a go. I successfully created charcoal briquettes...
http://www.abload.de/img/pizza_burned19tpym.jpg
And this was on a gas stove-top, medium heat, and for like three minutes. I'm presently airing out the smell of burning out the window. Also I melted the styrofoam plate a tad. It's looking like just a minute would've done it.
I need to just remain out of the kitchen.
Their crust is greasy as hell, probably part of the problem. But you walked away I'm sure.