I've seen quite a few nice setups with the molded/custom rock backgrounds attached to the back of cages.
What is that made of ?
For the hobbyist typically some sort of carved foam covered with polymerized concrete, tile grout, epoxy/sand, acrylic emulsion/sand, sculpting epoxy or modeling paste.
Its also often painted. What paint is used? is it safe for the animals?
I think artist acrylics but I'm not sure. I do know that concrete dye is also mixed in for color so often there is no paint at all. I saw one background that was nothing but carved foam painted with Drylok Masonry Waterproofer that was tinted. DMW is a hydraulic cement paint so it has natural texture. It is also aquarium safe so it should certainly be reptile safe.
I only saw a picture of this background so I can't say for sure how good it looked in person.
There are more complex ways to make fake rock but I won't get into those here.
I've heard of "self-extinguishing" PVC. What is this type called so i can look up some suppliers?
This is expanded pvc which is a type of sign board. Sintra, Komatex and Celtec are common brand names.
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Currently keeping a small collection of various Gonyosoma. Both G. janseni and G. oxycephala.