The firing of glazed wares is different from unglazed ceramics in a few key ways. First they have to be fired in a kiln (no open firing or pit firing), and secondly the pots have to be loaded into the kiln in such a way that none of them are touching. The vessels cannot touch because during firing the glaze completely melts, which once cool creates a glassy surface over the pot. Any pots that are touching when the glaze melts will be fused together. This means that to make glazed ceramics potters had to also have kiln furniture.
Kiln furniture is anything that is used in a kiln during firing and includes things such as shelves, posts, and stilts (or tripods). In particular I wanted to talk about stilts as it just so happens that we have some from Ur!
Stilts can be used in a few ways: they can be used to allow vessels to have glaze on the interior and exterior (including the bottom), as well as to nest and stack them in the kiln. The glazed vessels from Ur are good examples as they have glaze all over and show evidence of having been stacked and nested. For example take a look at 31-43-603, a glazed bowl from U16314) with glaze over the entire interior and exterior.
This object also has three small marks on the interior and on its base: these marks are from the use of stilts during firing that allowed the bowl to be nested with similarly shaped bowls as is shown in the reconstruction below.
Most of the bottles, including 31-43-646 (shown at the top of this post) only have markings from a single stilt on one side and so were probably not stacked but set sideways on a stilt. However, there are a few that do have two sets of markings such as 31-43-631.
With the marks on both sides it is likely that this object had at least one other object stacked above it as seen in the reconstruction below.
The thing though that I find truly remarkable about the stilts from Ur is how little this form has changed. Potters today who want glaze to cover the entire vessel (many leave the bottom, or foot unglazed instead) still use stilts and the design is virtually identical to the ones from Ur. Below are two examples currently available from ceramic suppliers.