Great molds for making rock walls. Use Hydrocal plaster mix and pour into the mold. I first lightly spray the inside of the mold with a H2O, dish soap mix to help with absorption and releasing the rock from the mold. Wait till the mold sets about 30 min. Then gently and carefully starting at the outer edges free the finished rock from the mold then set the rocks on a piece of wax paper on a flat surface for a day or two.