First thing I ever covered with clay was a glass salt shaker I found in an Op shop. I did the pepper shaker too, but do not like how that one turned out, so am planning to see if I can remove the clay and start again.
And recently for one of our Smoosher challenges I covered a small glass jar with clay. I love the idea behind covering things with clay, making beautiful, functional items. I would really like to experiment further and cover some other pieces.