What you see is the skin
Stand in front of a Roman wall at Ostia and you will see neat rows of thin red bricks. It looks like a brick wall in the modern sense. It isn’t. Behind the face is a core of Roman concrete, lime mortar packed with rubble, and the bricks are its facing. The trick is in the shape. Many facing “bricks” were triangular tiles, laid with the point pushed into the concrete and the long side on the outside. Every few rows, the builders laid a course of large square tiles, the bipedales (from the Latin for “two feet”), to bind face and core together.
This brick- and tile-faced concrete is what scholars call opus testaceum. Under the empire it was by far the most common way to build a wall, and it spread across the Roman world.
Why brick won
Before brick, Roman builders faced their concrete with small stones. The oldest method, opus incertum, used irregular pieces set at random. Then came opus reticulatum, small square-based blocks set diagonally like the mesh of a net. The architect Vitruvius, writing under Augustus, compared the two: the net pattern “is the most beautiful, but is very liable to split”, because it bonds badly, while the older method is uglier but stronger. Brick facing eventually replaced both.
It was cheaper, for one thing. Bricks and tiles could be mass-produced in kilns and laid by workers with less training than a stonemason needs, so opus testaceum cost less than walls of cut stone. Bricks were also regular, so a wall could rise straight and true.
Mud or fired?
Read Vitruvius on bricks and you may be surprised. His chapter on bricks never mentions a kiln, and the bricks it describes are dried in the air: they should be made of good clay in spring or autumn, so that they dry evenly, and left for two years before use. Fired brick appears in his advice on walls. In Rome, he says, the law did not allow walls on a public street to be more than a foot and a half thick, so to fit the crowds the Romans built upwards, with stone piers and walls of burnt brick, “multiplying the number of stories in their houses”. Augustus then had to limit the height of new buildings on public streets to seventy feet, about 21 metres, according to the geographer Strabo.
Where brick did the work
The apartment blocks of Ostia, Rome’s port, are the best place to see it on a domestic scale. The House of Diana, from the second century CE, rose four storeys in brick-faced concrete, with shops on the street and flats above whose value dropped the higher they were. In Rome, the drum of the Pantheon is a cylinder of concrete faced with brick, with brick arches buried inside its six-metre-thick walls to channel the weight of the dome. Large buildings, though, were rarely made of one material. The Colosseum has a travertine frame and façade, tufa for the secondary walls and concrete for the vaults.
Brick facing, then, was a habit of Roman architecture at its most practical: concrete gave the mass, brick gave the shape and the finish, and the two together could be built fast, high and cheaply.
Where to see it
Ostia Antica, Rome’s ancient port, has whole streets of brick-faced apartment blocks, the House of Diana among them. In Rome, look at the Pantheon’s drum from Piazza della Rotonda.
Sources
- Encyclopaedia Britannica, “Opus testaceum” (topic page and Western architecture: triangular tiles, bipedales, cost, opus reticulatum and incertum); “Pantheon”; “Colosseum”.
- Vitruvius, On Architecture 2.3.1-2 and 2.8.1, 2.8.17 (trans. Gwilt, LacusCurtius).
- Strabo, Geography 5.3.7 (trans. Jones, LacusCurtius).
- Turismo Roma (Roma Capitale), “House of Diana, Insula of Jupiter and Ganymede and Insula of the Yellow Walls”.
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