To create the expansion gap, 3 courses of bricks have been removed from the rear wall. This was a good opportunity to take a picture of the back of the reducer arch.
I don't think this gasket materiel will completely seal the oven, but the gap should help maintain the integrity of the brickwork and the gasket will keep some of the smoke / gasses and steam from escaping.
Now this is complete, the mortar will have to be left to dry for a few days and then curing fires will be started again to drive out any remaining moisture before the oven can be used again for baking. This should give me time to build to exterior cover that will hold all the insulation.