Interchangeable see-through and opaque inserts for muffle furnace

Claim 1

A muffle furnace device comprising in combination: a door means for providing access to and for closing off furnace interior heating space, the door means defining an insertable structure insertable sealable within a furnace access opening with the door structure having inner and outer faces with a passage extending substantially lineally therebetween such that an interior of a furnace space is viewable through the passage from an exterior thereof when the door means is inserted within a furnace access opening; first and second insert structures each having a configuration for sealably inserting within the door structure defining said passage, one of the first and second insert structures being substantially opaque and composed substantially of insulating composition, and the other of the first and second insert structures having a substantially lineally extending channel extending between and through inner and outer faces thereof such that when inserted within the passage, interior space of a furnace is viewable from an exterior thereof, the other of the first and second insert structures including a transparent barrier structure mounted within the channel substantially transversely to a longitudinally elongated axis of the channel providing thereby insulation between interior compartment space and exterior space, whereby alternately intermittently the first insert structure may be removed and replaced by the second insert structure thus providing for intermittent observation interior space of a furnace without excessive heat loss.