Abstract: Prior to the present invention, there has not existed any equivalent to the present furnace door for a porcelain furnace in which door there is provided a lineal passage for alternate insertion of an opaque insulator insert and a viewing structure defining a lineal see-through channel having a transparent heat barrier at an outer end thereof utilizing air space within the channel extending to an interior-furnace end for insulating air, and an outer end of the passage defining female step(s) and the respective inserts each respectively including a male step(s) which mates flushly with the female step when inserted to provide an improved heat seal for the inserts each respectively.