Thank you for your help!
By Crafty_Witch from IL
You can combine plaster of paris with plain dirt. Just make sure the dirt is pure. Remove any twigs, leaves, etc. The dirt will will help the plaster to withstand temperatures well over 1,000 degrees Fahrenheit. The downside is that you will need to manually dry the clay. Try using a blowtorch or plain fire.
You can also use fire clay. It's much cheaper than plaster of Paris but has a very high finishing temperature. I don't know that much about it, but hope this helps a little.