When durability is more important than fine detail, Insulating Cement is the castable mold material to use. It is great for making slumping and bending molds as well as forms to cast into. Insulating Cement mixes with water and cures overnight. Sold in 5-pound boxes.
Apply lots of release agent (I use Pam) on your model.
It should cure 24 hours, full strength in 48 hours. Then fire at 200 degrees per hour to 1250F. Turn off the kiln with the lid closed and let it cool to room temperature.