We tried to make a mold of a prop gun and used Crayola clay, having read it was non-sulfur. Being cold in our apartment, it took a while to solidify but the top eventually did overnight (Smooth-On Mold Star 15 Slow). However, when removing the clay, it seems that the under part of the mold is still not solid. I assume it is from the clay. I am wondering if we can use a razor to cut the mold where it wasn't in contact with the clay and use it for another attempt. This mold took probably half a gallon of silicone, so it's pretty costly to screw it up. We had a hard time finding clay in any quantity that wasn't stupid expensive, but I ordered some Van Aken Plastalina so we can try again.