What are some good tools to use for carving plaster? I love carving plaster! I have an assortment of tools that I've repurposed for carving plaster, but I'd like to know what else there is.

Are certain tools better suited to carving or detailing plaster? It can break apart more easily than wood, is harder than clay, and is sometimes prone to chipping.
Stone carving tools might be good to try, such as rifflers, files rasps. Made for stone they would easily survive being used on plaster. 

Image below is from http://www.fine-tools.com/liogier-riffler-rasps.html