Roman structures stand because they were well built of five materials. From the innermost working outward:

• Rubble. Roman walls were an Eskimo-pie sandwich of two exterior surfaces with a chunky filling in between. Rubble, no matter how irregular, provided bulk and counterweight.
• Tufa. Yellow stone cubes -- parallelopipedons, to be precise -- about three inches on a side, stacked diamond-fashion. For surfaces unlikely to take extensive wear, such as walls away from corners, tufa served as a reliable foundation material.
• Brick. The bricks we use today are fundamentally no different from Roan bricks. And the Romans discovered that, properly shaped and properly configured (over a removable wooden barrel frame), bricks could be shaped into arches, and arches into vaults, and vaults into roofs.
• Cement. Stronger than tufa, weaker than brick, cement nevertheless cured into a rigid, extremely strong (if brittle) joining material.
• Marble. Beautiful but easily worn, marble was seldom a structural material (even temple columns were usually only marbled-faced, brick inside giving them strength), merely a glossy finish more attractive and durable than plaster (the common substitute). And from the time of Augustus, the great quarries of Carrara furnished Rome and its possessions with a virtually limitless supply of smooth regular flat rectangular sheets of the highest quality marble.

Today, of course, much of the marble is gone -- worn into irregularity, looted from abandoned sites, blocks rebuilt from temples into churches, from churches into palaces. But the brick bones remain, as we saw at Hadrian's Villa and later in the Roman Forum.

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ã Copyright 2002 David Alexander Smith