What physical processes are in operation when an unglazed earthenware salt pig is used to dry salt? I would have thought that the porosity of the unglazed earthenware is an essential part of this process, but I have been surprised to find that glazed earthenware pigs are just as common in the shops as unglazed ones.
I'm afraid you're not very clear with your description. As I understand it, salt pigs require an unglazed internal surface to work properly. The external surface is often glazed to resist chipping etc.
When you say that glazed earthenware pigs are just as common in the shops as unglazed ones, are you talking about the glaze being on internal or external surface?
Perhaps the glazed one are made to be fashionable rather than functional by people who have no idea how a salt pig is supposed to work.
Ones that are glazed on the inside are absolutely useless for sucking the moisture out of the salt.