Roman Keystones

Perhaps is it time to let the cat out of the bag.  Some years ago, I was flown down to St. Charles, MO to build an oven for a very large project.  It appeared that the masons doing to bulk of the stonework would be there for a few years.  I was there for a much shorter time to build a 4'x 5' oven to be incorporated into the entertainment kitchen along with a charcuterie.   

St.Charles oven complete and Charcuterie space (to left)

One of the perks of going far afield to build ovens is meeting regional masons and learning/exchanging skill sets.  A remarkable addition to my knowledge came about when we were ready to close the crown of the dome.  Rick said it needed a 'Roman Keystone".

All arches and all domes need a keystone, the brick or stone that locks the entire structure into place.

A Roman keystone does this too, except it is cut from a rectangular brick.  For arches, the keystone locks the side bricks in place and allows the removal of the arch form.

I began to adapt the idea to my round domes.  It eliminates the last ring of slivered brick and allows for a measured finale.  There.  That's how it's done.