A brief description of the main Roman masonry techniques. 3rd. Part
We continue with the third part of this serie of four articles about the main Roman masonry techniques.
Oppus Testaceum. This constructive technique, invented by the Romans, is the one that uses the baked brick: much harder, more resistant and more durable than the traditional clay/mud brick, without firing, whose constructive technique was named Oppus Latericium. The size of the Roman baked bricks generally followed a module normalized in 3 different measurements -photo 1-- each of them with its own name: bipedal (two feet each long side, 59.2 cms), bessal (the most small, 2/3 of foot each long side, 19,7 cms) and sesquipedal (1,5 feet). This last one could be divided in triangles. In the case of clay/mud bricks, there were also three kinds of brick namely: lydian brick (with dimensions of 1 foot by half foot, 29.6 x 18.8 cm), tetradorum brick (four spans, 1 foot by 1 foot, 29.6 x 29.6 cm) and the pentadron brick (five spans or what is equal 37 x 37 cm). Although they were usually of rectangular geometry, examples of tubular (circular and square section) and circular bricks (semi-cylindrical, cylindrical and quarter-cylindrical), used respectively on roofs and columns - photo 2 -, are not lacking. There are also cases of bricks cut into capricious shapes intended to draw designs with which to adorn the facades of monuments. This particular constructive technique is denominated Oppus Figlinum.
Photo 1.- Set of Roman bricks with different module. Roman city of Regina. Casas de Reina, Badajoz, Spain. 2nd century AD.
Arranged in longitudinal rows joined with lime mortar -photo 3-, the fired brick, hereinafter referred to simply as brick, proved from the beginning (first century BC) to be an optimal material for construction due to its cheapness and good technical characteristics, to the point of rapidly becoming widespread throughout the Empire. In fact, many of the constructions of early imperial period were built with this material and not with stone. This preference for brick will reach its zenith in the last two centuries of the Empire, when the available money for the erection of new buildings was being reduced year after year together with the splendor and the power of Rome.
Photo 2.- Ancient city of Carteia. San Roque, Cádiz, Spain. Quarter-cylindrical bricks used in the construction of columns.
Regarding the modes of use, they were mainly two: as a main structural element, support of the weight and the tensions of the structure in question and as lost formwork to be filled later with the usual Oppus Caementicum more or less rough –rubble agglomerated with lime mortar in many cases--. In the first case, usual in non-critical structures such as small vaults, dividing walls, openings and others, the advantage of the brick lay mainly in its great cheapness and ease of use combined with its suitability to make demanding geometric shapes as, for example, the typical Roman arches. When, on the contrary, the burdens to be borne by the future structure were high enough to discourage the exclusive use of the Oppus Testaceum, the walls were raised in the very robust Oppus Caementicum: more expensive to execute but still economical. Having said this, it is easy to understand the second way of using brick: without a doubt the most used cladding throughout the imperial era, well ahead of the Oppus Spicatum and Reticulatum, which in fact were practically relegated by the Oppus Testaceum to the category of finishing of pavements.
Photo 3.- Roman villa of Carranque, Toledo, Spain. Example of typical Oppus Testaceum. 4th century AD.
It is usually quite simple to date the Roman buildings erected in Oppus Testaceum thanks to the custom of stamping the bricks with the seal of the manufacturer and, from 164 AD, with those of the consuls of the year since it is at that date when all the brick production factories became directly dependent on the emperor, being suspended the custom of stamping the bricks with the name of the producer. This practice would not be resumed until the reign of Diocletian, back in the final years of the third century, once the production was again decentralized by the aforementioned emperor.
Photo 4.- Roman rampart of Nicaea (Iznik, Turkey). Mixed wall of Oppus Incertum and Oppus Testaceum usual on the late Roman buildings.
The process of firing the brick was carried out in basic ovens, made with cheap materials. The fuel used as a source of heat was wood, sometimes also coal, reaching temperatures in the combustion chamber -praefornium- from 450 to 500 ºC. This praefornium was located under the actual firing chamber -laboratorium--: the space where the bricks were placed to be fired (in shelves disposed for that purpose) and whose floor -solea- presented a series of holes intended to communicate one camera with the other and thus allow the passage of heat from below to above. Finally, the oven had an opening in the upper part of the laboratorium through which the fumes were evacuated, as well as a small grid in its front part by which progress of the cooking process could be verified. The duration of this process depended on several factors such as the atmospheric conditions, the dimensions of the oven or the properties of the fuel used as a source of heat. Three days would be an average term in normal conditions.
Oppus Mixtum. Also called Oppus Listatum, this is the name for the constructive technique consisting of the combination of two or more different techniques. It is, therefore, a somewhat ambiguous name that can be used to accommodate a very large number of different factories, although in practice only a few variants of all possible were used. In fact the normal thing is to denominate Oppus Mixtum to the structures formed by alternating bands of Oppus Reticulatum/Oppus Incertum on the one hand and bands of Oppus Testaceum/Oppus Vittatum on the other.
Photo 5.- Roman theater of Mérida, ancient Emerita Augusta (Badajoz, Spain). Oppus Mixtum made with alternating rows of bricks with different modules.
It is a relatively late technique, since it was not used until the reign of Tiberius (14 - 37 AD). It´s tought that the reason for its use is the reduction of the pernicious effects caused by earth movements in the structures of Oppus Reticulatum/Oppus Incertum: too prone to fracture diagonally and completely (from top to bottom). In this way the damages were limited to the area between band and band of Testaceum (photo 4) or Vittatum, with which the deterioration was much smaller.
Photo 6.- Ruins of the Baths of the ancient Cemenelum (Nice, France). Example of mixed technique of Oppus Vittatum and Oppus Testaceum in the best Gallo-Roman architectural tradition.
Another variant, also quite used, was the alternation of rows -or groups of these- of bricks of different module (photo 5). The result was an irregular type of Oppus Testaceum considered also as a form of Oppus Mixtum and whose purpose was again to reduce the damage in the event of earthquake. Finally we can mention a third combination, this one more of decorative character, in which the participant materials are on the one hand the small ashlar masonry --Oppus Vittatum-- and on the other the fired brick, Oppus Testaceum (photo 6).
Oppus Craticium. By this name is known the constructive technique whose bearing structure is made with wooden beams and poles, filling the rest of the space -the walls themselves- with rubble agglomerated with white mortar or simple mud mortar. According to the written sources it was a constructive system very used in Roman time, mainly in the construction of buildings of domestic character. Unfortunately, no example has survived to this day due to the scarce durability of wood. Although the truth is that it is easy to find moderately recent structures executed with this technique, what indicates that the Oppus Craticium survived by long to the Roman times.