I'm wondering, as I roll a substantial silver ingot into sheet, if
I constantly anneal the one side of the silver when it needs it
after a few rolls, will firestain develop on the underneath side,
as well as the top side?
it’s not the flame that does it. it’s oxygen in the air, which the
flame does not exclude. If the silver is protected from the air with
a suitable flux (not all fluxes do this as well as one might wish),
then it’s protected from developing fire scale and fire stain.
Annealing without protection of some sort will give you both fire
scale and fire stain. Your annealing surface does not exclude air
from the bottom surface. Fluxes which work are those which don’t burn
away or get depleted during heating. The white paste fluxes for
example are very active, and great for promoting solder flow, but
don’t always last long enough to prevent some oxygen from reaching
the surface. Fluxes like Prips flux (which you can mix up yourself)
or Firescoff (commercial, pricey, but quite effective), and others
that last during the process, will protect your metal.