I put too much water in a plaster of paris mix - can I cook the water out by reducing without danger?

Answer 1

No. If you really over did it and are talking about soup consistancy. If you are talking about the final cast being very soft and you want to dry it faster, then put it in an oven at about 200F and let it sit over night. It may be too weak to use for what you had in mind. Don't go to a higher temp, it may generate steam inside and explode.