Ancient Roman Walls
The materials of which ancient Roman walls, or parietes, were made varied according to the era in history, the place, and the costs of transportation. Stone and unburned brick were the earliest materials used in Roman Italy with timber used for temporary structures, as in the addition from which the tablinum, developed.
For private Roman houses in early ancient times and for public buildings in all times, walls of dressed stone (opus quadratum) were laid in regular courses, as they are in the modern era. As the tufa, the volcanic stone first easily available in Latium, was dull and unattractive in color, over the wall was spread, for decorative purposes, a coating of fine marble stucco which gave it an impressive finish of dazzling white.
In houses of the less well off, not for grand public buildings, sun dried bricks (the adobe of southwestern states in America today) were mostly used until the beginning of the first century B.C. These brick walls were covered with stucco, for protection against the weather as for decoration, but even the hard stucco has not permitted such walls to survive into the present time.
In classical Roman times another material came into use, better than brick or stone, cheaper, more lasting, more easily worked and transported, which was employed almost exclusively for private houses, and very generally for public buildings. Walls constructed in this new way (opus caementicium) are either called by scholars of ancient Rome "rubble-work" or "concrete" but neither description is exactly correct. This is because opus caementicium was not laid out in courses, as is the rubblework of more recent times, while on the other hand larger stones were used in it more than in the concrete which is used for walls and buildings today.
Way of Making Concrete Walls
In ancient Rome lime and volcanic ashes were used with pieces of stone as large as a fist. Brickbats occasionally took the place of stone, and sand was sometimes used instead of volcanic ashes; potsherds crushed fine were better than the sand. Harder stones made better concrete and the best concrete was made with pieces of lava, the material that roads were paved with.
The Method of Casting of Concrete Walls.
The method of making the concrete walls was the same as that of now. The method employed by the Romans can be seen in the picture above. First, upright posts, 5 by 6 inches thick, and from 10 to 15 feet in height, were fixed about 3 feet apart along the line of both faces of the projected wall. Outside these were nailed, horizontally, boards 10 or 12 inches wide. Into the middle space semi-fluid concrete was poured, receiving the imprint of posts and boards. When the concrete hardened, the framework was removed and raised and so the work went on until the wall had reached the desired height.
Walls made like this differed in thickness from a seven inch partition wall in an ordinary person's house to the eighteen foot thick walls of the Pantheon of Agrippa. Such concrete walls far more durable than stone walls, which could be taken down stone by stone as easily as they were put together ; concrete walls were a single slab of stone throughout the whole, and large parts of it might be cut away without weakening the wall overall structurally.
Such walls were impervious to the weather and were usually faced with stone or kiln dried bricks. The stone used was normally the soft tufa, not nearly so well adapted to stand the weather as the concrete itself.
Practice in early times was to take bits of stone all with one smooth face but of irregular size or shape and arrange them, with the smooth faces against the framework, as fast as the concrete was poured in; when the framework was removed, the wall presented the appearance shown in A above. This kind of wall was called opus incertum. In later times the tufa was used in small blocks with the smooth face square and of a uniform size. A wall so faced looked as if covered with a net (B) and was therefore called opus reticulatum. A corner section is shown at C.
Bricks for Facing Wall
In either case the exterior face of the wall was usually covered with a fine limestone or marble stucco, which gave a hard, smooth and white finish. The burned bricks were triangular, but their arrangement and appearance can be more easily understood from the picture.