This question has been bugging me for a while so I did some ed-u-ma-cation this morning. The link is to a nice little article that describes the difference between ceramic, earthenware, stoneware.
I know some people say they use pottery outdoors in freezing areas with no problems. I now wonder if they have been using stoneware, which is a step up in durability and has less porosity than regular pottery. Just thought it was interesting.

Here is a link that might be useful: Types of ceramic