Reversible jamb blocks and method of using the same

A furnace doorway comprising:

• a. a frame having a first side and a second side opposite the first side with a top spanning therebetween, the first side of the frame having a first set of a plurality of first attachment elements arranged symmetrically about a first vertical axis of symmetry positioned on the first side and a second set of a plurality of first attachment elements arranged symmetrically about a second vertical axis of symmetry positioned on the first side, said first and second vertical axes of symmetry being non-collinear;
• and
• b. a plurality of jamb blocks, each jamb block comprising a first attachment side and a second attachment side opposite the first, both attachment sides configured to adjoin the first side of the frame, both attachment sides having at least two second attachment elements arranged to correspond to two of the first attachment elements of at least one of the sets of first attachment elements on the first side of the frame, each of said corresponding first and second attachment elements configured to releasably attach to each other, such that either of the block attachment sides can be releasably attached to the same set of first attachment elements of the first side of the frame.