I’ve been involved with exposing and repairing allot of bricks. This one happens to have been in my office. So often where you want to expose the bricks happens to be where some sort of fireplace used to be. In this case the bricks over the “fireplace” opening where in pretty bad shape and needed to be rebuilt. As luck would have it, I inadvertently ended up with two nice pictures to illustrate how to properly close up an old flue.
You don’t close up a flue (which is great passage way into you home for vermin) with cardboard, plywood, plastic, plaster, or pretty much anything else – other than metal and cement. Usually you can chisel out the mortar between two levels of bricks, and slide your metal into place.