I'm making a waterfall with rocks and silicone, but want to use polystyrene as internal support for the structure.

All the research I've done says that polystyrene is safe for frogs, but what about burnt polystyrene?

I know that during the burning process, polystyrene releases toxic gas's through black smoke, but I'm wondering if what's left of the polystyrene would still be safe?

I ask this because I've seen photos online (My 50cm paludarium-like tank mini journal) of someone making a 3D background with polystyrene and burning the surface to shape the landscape. I'm thinking of using this process to stop the polystyrene from breaking off into little pieces my frogs could swallow - as you can see in the link, it leaves a nice smooth surface.

Anyone know?

Thanks