Did I read somewhere that if you watch the sims moodlet when they start sculpting, it will determine the outcome of the sculpture, I did some experimenting and got all sorts of different outcomes. I think again it is random. Nicely decorated was a masterpiece, good and great. Decorated was good and brilliant. I guess this is like everything else, ya never know what you are going to get with a certain profession. It has really been pretty neat so far.
Edit: I posted this somewhere else, but I did make sim ice sculptors at level 4, the book says lvl 6 and I got a masterpiece at level 5 and the guide says level 9. He was already maxed on sculpting skill, so not sure if that makes a difference.