The Collaborative International Dictionary
Pottern ore
Pottern \Pot"tern\, a. Of or pertaining to potters.
Pottern ore, a species of ore which, from its aptness to
vitrify like the glazing of potter's wares, the miners
call by this name.
--Boyle.
Wiktionary
pottern ore
n. (context mining dated English) A variety of ore that tends to vitrify like the glaze on pottery.