I really liked this! It is often difficult in a short story to develop tension well along with a good plot, but you had me gripped from the start. The ending was a twist, but somewhat predictable, although it was not what I had envsioned, so perhaps that makes it a good twist?
You had the obsessive nature of the artist completely right; his endless reproducing of the person. However, I would have liked a little more detail about thecopies; what mediums had he used, or were they all sculptures? I was also a little confused by his sculpting in the clay then at the end mixing a palette, which indicates paint?
I had the oddest feeling that Jon had encased her in the clay so was making her coffin! Perhaps I just have a totally bizarre sense of mystery. The ending was my least favourite part of the story, but I can see how you wanted the continuum of the obsession past the story end.
I love this short story mystery genre so will be looking for more from you with interest.