Then using a string and marking pen build a compass half the diameter of your oven 18 or 21 and mark the circumference of your oven floor.
Tiling a pizza oven dome.
Reading through your replies to various comments it looks like glass or porcelain tiles are best to use and to use a thinset mortar to bond and grout the tiles.
It is to do with the sand and afterward place the bricks onto them.
Given the variety of colors and styles of tiles available at hardware stores the design possibilities are huge.
These ceramic and glass tiles present a very convenient medium to finish the dome of the oven since their flexibility allows them to repeat the shape of the dome.
A brick pizza oven dome is usually constructed as either an igloo or barrel dome out of medium density fire bricks both shapes are constructed in much the same fashion with bricks mortar and usually with a supporting frame.
Construction of an igloo dome does not necessarily require a frame if completed by a competent brick layer.
Tiles of different designs can be interchanged on same oven.
I love in the north east of england in the uk we have an outdoor pizza oven which i am looking to mosaic tile.
Then apply a tile and grout sealer to seal the grout.