The Sumerians used mud bricks to decorative ends, not only with the quintessential niched-and-buttressed façade (established in the 4th millennium BC) but also with arches and domes.
By the 2nd half of the third millennium, vaulting (radial, pitched-brick and corbeled) had been established. Delougas (1967) mentions the implantation of an arch, made by stacking flat-laid bricks to the spring of the arch, then placing bricks with their lengths parallel to the thickness of the wall.
Sometimes the first bricks after the spring were made with voussoirs (specially-made wedged-shaped bricks) while toward the center invariably normal bricks were used.
Arches were often asymmetrical, an indication that wooden centering was not used to support their construction.
However, their broad arc fitted well over the outer curve of a man's slightly bent arm -- perhaps the mason built up part of the curve over one arm, holding them in place with his arm, then bringing the arch further. The spring line on the side of the broader curve was usually lower than that one the other side.
The quoin itself on the broader spring line was flat-stacked to a lower point, indicating the irregularity was intentional and part of the building process. Across the the top of the arch was a course or two of flat bricks, followed by the traditional lay thereafter.
Pitched-brick vaulting was often enacted by local masons since it did not require wooden centering, as described by Oates (1990).
Successive rings were laid with their edges cross the long axis of the wall, each ring included at a slight angle to its predecessor. In this way, the rings support each other and timber is unnecessary.
When the rings meet in the middle, the gap in the crown of the vault is filled with ring segments of diminishing size until a small hole remains that can be filled with brick fragments.
There were different types of wall surface decoration.
The cheapest and easiest was to impress patterns into a play, a technique not attested archaeologically but supported by ethnographic evidence. More costly was to implement frescoing, which has rarely survived to leave archaeological evidence, but which is well attested at the Uruk era tripartite temple at Tell Uqair.
Clay cone decoration was the most prestigious. It was a distinctly Sumerian technique found in the Uruk era and typified by the Mosaic Temple in the Eanna precinct: the use of colored cones, arranged in geometric patterns and stuck into the thick plaster of a façade. Other techniques were to inlay displays of wealth into plaster, such as the intricate gold leaf and semiprecious stones at the Royal Tombs at Ur.
From a house at the citadel in هه ولير Hawler in كوردستان Kurdistan العراق Iraq, an example of an arch and also impression decoration in plaster. Note alley drain in background. Image © L M Clancy, 2010.