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Roman Building Materials

It was Augustus who made the boast (according to Suetonius) that he found Rome of brick but left it of marble. This cannot well refer to private houses but to public edifices, to the buildings of the State, which he constructed of stone. His successors for the most part imitated him in this respect. The Empire was to build of stone, and was to employ the Arch not merely underground in sewers, where it could not be seen. The Arch, however, could be made of brick also, so that it was possible anywhere, without stone or marble, which nature has to furnish. Brick, on the other hand, is made by man from the common earth which is found generally and not alone in Paros or Pentelicus. We may regard brick as somewhat plebeian, marble is certainly aristocratic or imperial. An indulgence in costly and exclusive marble is one of the characteristics of the Empire. The royal Arch of the Cloaca Maxima, made of large blocks of travertine, has lasted; the later plebeian Arches of brick have quite vanished. Brick is indeed perishable, having a tendency to go back to the soil whence it was taken; but stone, if of a good quality, persists with Nature's own endurance. When Augustus transformed Rome from brick to marble, the change was from a republican to an imperial material, corresponding to the change in government.

Rome, having universal sway in the Empire, is accordingly to have an imperial universal Architecture. The question rises: What building materials has she for such a purpose? These must not be too far away, and otherwise easily accessible and workable. Transportation of large masses was very difficult in those days, nor was there any steam to drive colossal implements. Nature placed in the vicinity of Rome several kinds of stone which furnished the original found ation of her architectural character, and rendered possible her constructive greatness. Without these bountiful presents handed to her almost outright, she never could have built the mag-* nificent homes of her Institutions which have furnished the ideal pattern for the future.

First to be mentioned is the stone called travertine, taken from the hills in the territory of Tivoli. It was hard, compact, quite uniform in grain, homogeneous; it could resist crushing and disintegration by the weather; we may say that it had a decided individuality which would assert itself against any outside assault. Yet it was not difficult to quarry and to shape with good tools. It was a limestone, deposited by water in slow and regular formation, not by volcanic action. When burnt, it made an excellent lime, for which Rome was famous,' and in a way unique. Home used lime, mortar, cement with their cohesive power for binding together very diverse materials, which fact curiously reflects the city's character and function in history. Greece laid stone on stone in her public structures, without any tie of mortar or cement, the whole being united chiefly by nature's gravity.

Next to be noted is the stone called peperino, composed largely of pebbles and volcanic ashes solidified by heat. Its name comes from the seeming peppercorns which lie imbedded in its composition, making it a mixed salt-and-pepper reck. Evidently this stone is heterogeneous, in contrast with the homogeneous travertine, and is the product of sudden volcanic eruption rather than that of gradual sedimentary deposition. It comes from the Alban Hills which rise up a few miles from Rome across the compagna, and from which Rome herself, or at least her Latin element, originated. Here again we must hear the stone speaking out its Roman nature. For surely that was supremely the character of Rome, especially on its Latin side: it fused in the volcanic fire of conquest many foreign and recalcitrant peoples, and thus held them together in the unity of its spirit, yet also preserved in each a kind of national individuality.

The Roman Empire itself was a sort of political peperino, out of which its vast institutional structure- was built, in a peculiar subtle harmony with the material of many of its architectural edifices. Let us say then, putting together our two kinds of Roman stone thus far described, that Rome had primarily its original elementary homogeneous character, the slow deposit of the stream of time in the evolution of untold ages (as the River Teverone deposited during countless geologic ages its beds of travertine); and on the other hand that this unitary homogeneous Rome had the power of fusing and making one with itself the scattered heterogeneous elements of the folkraces strewn over the globe.

But the most important and characteristic Roman buildiug material goes by the name of pozzolana, supposed to be derived from Pozzuoli near Naples, where it is also found. It may be called an earth composed of fine sandy particles, which, being mixed with small stones and water, takes a semi-liquid consistency like mortar; this mortar can be made to assume quite any shape by being put into a mold usually of wood. Then it hardens or " sets; " the mold is removed and the object comes forth as solid as an iron casting. Here we see a kind of artificial stone or concrete, which has the peculiarity of being made by man who seizes it in its process of formation, which shows three stages: the loose substance, the semi-liquid mass, and the solidified object. Such is the significant fact: in the previous sorts of stone, nature made the material and man chiseled it into shape from the outside; but now man makes his material, and causes nature to produce the shape from within, through her process.

The transition from the nature-made stone to the man-made stone is sugtive and hints a great step taken by Rome. For this special use of pozzolana belongs to the imperial stage of the city, though republican Rome knew of it as a base for mortar. The Dome of the Pantheon, belonging to Hadrian's time, is made of pozzolana, and is as solid as an inverted iron kettle resting upon its circular wall. It seems to have taken centuries for Rome to develop the possibilities of pozzolana which is the very soil lying around it and under it everywhere. Note then that the common earth upon which Rome stands, is capable of making a cement which binds together the most diverse materials. We saw a similar mixture of different substances in peperino, but that was the work of nature, not of man. It has been observed in many Roman ruins all over the world that the stone often cracks and disintegrates while the Roman cement alongside of the stone remains perfect. Middleton's striking expression is that Rome is the eternal city through its pozzolana. Certainly Roman Architecture gets its most original characteristic from the use of this native substance, which not only unifies diversity of materials, but overarches the greatest structures of Rome.

It was this pozzolana that rendered possible the bold constructive forms of the imperial period. The wide span of the vaulted ceilings in Baths, Basilicas, and Temples are built of this material; so are the ribbed cross-vaults and finally the domes. These forms are not in the present case made of voussoirs of stone, or even of brick; they are, so to speak, poured. Moreover another peculiarity is that they have no thrust sidewards, as has the stone arch; the vault above is a solid piece of rock and holds itself together. Hence there "is no need of such buttresses as we find in Gothic cathedrals to counteract the outward push of arch, vault and dome built of separate layers of stone. In the present case aroh, vault, and dome are in principle a single stone hollowed out and set down inverted on a wall, which must indeed be strong enough to receive the total vertical pressure from above. The Dome of the Pantheon may be regarded as a huge porcelain cup turned upside down with a hole in the bottom. Such a structure will correct its own lateral thrust.