Fired to cone 10R. The porcelain contains bentonite and a plastic kaolin, both are contributing iron-stained solubles that come to the surface during drying. They tend to concentrate on this foot ring. The solution is to employ a little barium carbonate in the porcelain recipe to precipitate the salts. These could also be sponged or sanded off in the dry state.
In ceramics, certain compound in clays and glazes can dissolve into the water, then on drying these are left on the surface.
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.