Mica could enter the pot as temper (with quartz sand) as micaceous
sandstones are not uncommon at least in Europe.
I came across an interesting example recently from a kiln in Swansea, S
Wales. it produced pots which appear t be late C15/early c16 in date
paralleling similar vessels from the Cotswolds (Minety area) suggesting
a source for the potter. The Cotswold pots are tempered with oolitic
limestone but the Swansea pots appear to have been tempered with an
oolitic sand probably from a beach along the nearby Gower peninsula. The
pots re unknown from consumer deposits so it looks as if the kiln was
short-lived- possibly not much pot being used in this period which seems
to be dominated by English and Continental imports.
paul