Which is why I'm trying to figure out some sort of flat lapping
rig that they could have used, given the technology at hand.
The use of something along the lines of a rotating horizontal lap
didn’t really pop up until fairly late (14th-15th century), prior to
that they did their flat face grinding using abrasives on a flat
plate made of lead, copper alloy, or something like porphyry, as
described in Theophilus’ “On Divers Arts” (circa 1122). Remember,
they were flat grinding stones on the sides of the huge sandstone
wheels in Idar Oberstein, for generations, starting in the 1400s.
There may have been someone who got the idea to use a potters
wheel as a lapidary tool, but so far, there has been no evidence of
such innovation from as early as the migration era.