The soluble salts have formed the brown coloration on the bare clay foot ring. While the actual salts layer is very thin, it is glassy and enough to glue parts of the base to the kiln shelf (the latter did not have adequate kiln wash or sand). The glaze line is close to the foot and this complicates the problem. There are a couple of solutions. Sand the foot ring at the dry stage to remove the soluble salt layer. Use a more refractory kiln wash that offers a powdery, non-stick surface.
A firing issue in ceramics where the foot rings of vitreous ware stick to the kiln shelf. Removing them leaves sharp fragments glued to the shelf.
A common problem with dry and fired ceramic. It is evident by the presence of a light or dark colored scum on the dry or fired surface.
In ceramics, certain compounds in clays and glazes can dissolve into the water, then on drying these are left on the surface.