Hi Carolyn: Not much of a proffessional (lol) here but to me it looks like the glaze like on stoneware (earthenware) items. Eg like the older crocks and things...
Are the letters AH on the bottom?
Earthenware examples:
Click Here and on that page there is pottery with initials of J.A.H. not that it is the same, but thought I would mention that...
Maybe that link can be book-marked here as a resource page too....as interesting BOOK