In the heat of the kiln, glaze would fuse with the shelf that the object stands on, so glaze is carefully wiped from the base of the object before it is fired.
In lower temperature firings, like cone 06–04, you can use a stilt, which is a small piece of ceramic material with pointed wires sticking out of it. You fire the pot sitting on the pointed wires. This leaves small marks in the glaze which sometimes have to be cleaned up a bit. In higher temperature firings such as cone 10, it’s not practical to glaze the bottom of a vase. The norm is to have what is called a dry foot where no glaze is applied to the bottom. A stain or a colored slip can be applied to the foot before firing if there’s too much contrast between the color of the bare ceramic and glazed areas. It’s also possible to construct a foot ring on the bottom of the vase in such a way that the glaze can at least be applied to the edge of the bottom without sticking the pot to the shelf. That does add a level of difficulty, though, since glaze can move during the firing, and you need to leave a little distance between the shelf and the glaze in case it does move. On my pots, I don’t attempt to do that, but accept the look of the dry foot.
The pot has to stand on something (or hang from something, with its own problems) during firing. It cannot float. During firing, the glaze melts. If you have glaze on the bottom of the pot, when the glaze cools it sets solid and fixes the pot to the kiln shelf or floor. Even if you can get the pot off the shelf, probably taking bits of shelf with it, you will have to grind down the rough bits. More likely the pot will break during cooling as the pot contracts more than the shelf, with (very sharp, beware) bits left stuck to the shelf and the pot ruined (also the shelf). If you can find a way to make pots float, then you can glaze the bottom.
You can hang them, but you need a suitable hanging point, and you may not want to have a hanging hole in your pot, and if you do, that part will have to be unglazed, so there will always be an unglazed part. And some plates appear to have completely glazed bases with no unglazed bits, but if you look carefully, you find breaks in the glaze between base and rim, where the plate has been supported during firing.
There has to be a break in the glaze where the ware is supported, it doesn’t have to be on the bottom, but if it isn’t it will be somewhere else, probably more visible.
Many items have an indented bottom which is glazed, leaving just a thin ring unglazed. Some potters like to do this as to them it looks more professional; others don’t, preferring the handmade look of a bare base. Many factories glaze the base this way, but there is still always an unglazed ring. It has to stand on something during firing!
Picture Credit : Google