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  in Ancient Rome(pottery used to lighten the vaults of Circo di Massenzio) Introduction: art or engineering? During the last decades of the XVIIIth century and for almost the whole XIXth century the views of J. J. Winckelmann (Reflections on the Painting and Sculpture of the Greeks) were regarded as absolute truth; according to them, Greek art of the period 480-323 BC (Classical period) was the peak reached by the Ancient World: what came after was called Hellenistic and dismissed as decadent. This negative view included the works of art of the Roman Empire which were thought to be just copies of the patterns established during the Classical period. When they departed from those patterns they were labelled as extravagant and lacking harmony. The recognition Ancient Romans did not receive from art historians came from modern engineers who investigated and admired the construction techniques the Romans used to build roads, aqueducts, baths, tribunals, circuses, walls and obviously temples and houses. Arches
The Romans learned from the Etruscans the use of arches to make large openings in a wall; the gates of the Etruscan towns (see for example Arco Etrusco at Perugia) show the first examples of arch. The laws of Physics explaining the conditions required for an arch not to collapse were not fully understood until the XIXth century; yet the Etruscans, and after them the Romans, developed empirical methods for designing arches which still stand more than 2,000 years later.
The key element for building an arch is the solidity of its side walls which have to withstand the pressure discharged by the keystone through the voussoirs to the springers. Centina (centring) is the wood structure upon which the stones of an arch were laid during construction.
An important aspect the Romans paid attention to was the choice of materials: travertine proved to resist stress with limited strain and was widely employed to build arches. Roman architects found also a way to link the arch to the wall which was both effective from a structural viewpoint and decorative from an aesthetic one.
Pontifex (builder of bridges) was the title the Romans gave to their highest priests and later on to the emperors; this because the Romans were the greatest bridge builders of antiquity due to the widespread application of arches; using the same technique they achieved impressive results in the construction of aqueducts. Stadiums, circuses and theatres were other buildings where the Romans showed their awareness of the strength of arch-based structures.
Arches were used even in buildings where their presence is not immediately visible. The Pantheon rests on eight pillars and "discharging" arches ensure the pressure is directed towards them; the sunken panels of its ceiling have no doubt a very decorative effect, but they also reduce the weight of the structure; to this purpose Roman engineers also used vases or hollow bricks (see Tor Pignattara). The structures supporting the Roman market at Smyrna show interesting examples of both barrel and rib vaults.
Walls
The earliest walls built in Europe were constructed placing stones one upon the other without any mortar to bind them together (dry-stone walls). Near Rome examples of such walls can be seen at Alatri, Segni and at other locations south of the city: they are called cyclopean, because archaeologists felt that only the mythical Cyclopes could have moved the enormous boulders which made up these walls. Improvements in the tecnique used for cutting stones led to the construction of walls with stones having the same size (Isodomum - Vitruvius - De Architectura). In order to strengthen the wall, blocks were placed alternately with the longer side (stretchers) or the shorter side (headers) on the face of the wall (opus quadratum). Romans were so fond of the texture effect of opus quadratum that they continued to use this technique even after having developed more effective kinds of masonry. The wall built at Foro di Augusto with the blocks projecting from the surface inspired Renaissance architects in designing the bugnato (rustication) of many Florentine palaces.
The Romans developed a very effective kind of mortar by mixing pozzolana, a volcanic ash of the region around Naples, with lime; they obtained a cement which was resistant to water. In his work De Architectura (a treatise on architecture dedicated to Emperor Augustus) Vitruvius so described pozzolana: There is a species of sand which, naturally, possesses extraordinary qualities. It is found about Baiæ and the territory in the neighbourhood of Mount Vesuvius; if mixed with lime and rubble, it hardens as well under water as in ordinary buildings. This seems to arise from the hotness of the earth under these mountains, and the abundance of springs under their bases, which are heated either with sulphur, bitumen, or alum, and indicate very intense fire. The inward fire and heat of the flame which escapes and burns through the chinks, makes this earth light; the sand-stone (tophus), therefore, which is gathered in the neighbourhood, is dry and free from moisture. Since, then, three circumstances of a similar nature, arising from the intensity of the fire, combine in one mixture, as soon as moisture supervenes, they cohere and quickly harden through dampness; so that neither the waves nor the force of the water can disunite them. (translated by Joseph Gwilt, London: Priestley and Weale, 1826 - digital text by Bill Thayer). Most Roman buildings are made up of opus caementicium, a sort of concrete which was laid into timber structures until it hardened. The resulting walls were very solid, but not nice to see, so very often some sort of facing was applied.
The Romans made use of fired bricks; the manufacturing of bricks was perfected during the first century AD: factories branded their products as they were responsible for their quality; bricks were generally longer and narrower than the bricks we use today. There were also round and triangular bricks which were used to imitate columns and other architectural motifs.
The way the bricks were laid is often associated with a specific period: the texture of course-laid brickwork at the time of Emperor Hadrian impressed Renaissance architects who imitated it in many buildings. Contemporary architects have used "Roman brick" too.
An unusual kind of facing was based on specially-shaped tufa stones: the points of the stones were inserted into opus caementicium,
while their square bases formed a diagonal pattern resembling a fishing net (reticulatum). Today many
streets of Rome are paved with porphyry stones which create a similar diagonal pattern.
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