After the damper has been set in place at the top of the throat and most of the channel has been formed with firebricks, stones are laid over the opening.
They are mortared to form a structural 'jack arch', even though there is also an angle iron supporting this span of stones. ( The angle iron leaning against the stones in this photo isn't it . That's just there temporarily to hold the key stone in place until the other stones across the opening are set )
Once all the arch stones are mortared together, the top of the throat is bricked up. I braced a stiff slab of styrofoam at the correct angle to temporarily support this corbeled brickwork until everything set.
Then comes the messy job of getting up into the firebox and filling all the joints and parging the entire throat up to the smoke chamber.
Actually Mark said he enjoyed this part. My heart just isn't in it. In fact, I hate it. Chunks of mortar always fall in your face.
I'd much rather be hearting dry stone walls.