It is to do with the sand and afterward place the bricks onto them.
Tiling a pizza oven dome.
The dome itself a series of self supporting circular brick chains that curve inward until they meet at the keystone at the oven top.
Construction of an igloo dome does not necessarily require a frame if completed by a competent brick layer.
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.
Tiles of different designs can be interchanged on same oven.
You can also frame and decorate the front portion of the dome arch with granite or other materials.
To get a perfect shape dome put the sand in the shape of a dome.
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.
Given the variety of colors and styles of tiles available at hardware stores the design possibilities are huge.
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.