A brief description of the main Roman masonry techniques. 1st. Part
Vitruvius wrote in the decade from 35 to 25 BC his masterpiece "De architectura", conceived as a summary of the architectural knowledge of his time and previous ones. Divided into ten books, the second one was chosen by the author to expose extensively the building materials and construction techniques used in his time. Enunciated all of them with the generic word "Opus", the second word serves to define each particular technique. Let's meet them...
Opus Quadratum. It is called by this name to the construction technique with ashlars --photos 1,1bis and 2--, well known before Rome, although it is under the Latin aegis when it reaches its zenith in Western Europe. This is evidenced, certainly, by the many examples of structures built in Roman times employing this technique: both civilian and military ones, residential, palatial or religious.
Photo 1.- Alconetar bridge. Garrovillas de Alconetar, Cáceres, (Spain). 2nd century AD. Opus Quadratum placed alternating headers and stretchers. Work of average quality, typically provincial.
Photo 1Bis.- Splendid example of high quality Opus Quadratum in the Roman theater of Miletus (Turkey).
Greek origin technique, the Hellenes raised the entire structure, what includes not only the outer walls but also the core, with ashlar. However the Roman architecture, more practical than the Greek, replaced from the beginning (end of the fourth century BC) the ashlar core by another one, much cheaper, of coarse masonry aglomerated with lime mortar in what is the first example of use of lime mortar as binder (until then the lime mortar had been used exclusively as a coating). This technique, known according to the classical sources by the name of Emplecton, spread quickly from Italy throughout the Roman Empire, having persisted in use until the final decades of the nineteenth century when the development of Portland cement and modern concrete withdrew it definitely.
Photo 2.- Ancient city of Cáparra. Villar de Plasencia, Cáceres (Spain). Splendid water well, zenith of the Opus Quadratum --curved ashlars--. 2nd century AD
The Roman Opus Quadratum was always dry-fitted (without mortar). The size of the ashlars was variable depending on the monumentality with which the future building had been conceived - although the height used to be fixed at 52 centimeters: it is the so-called Roman module--. In any case, it is easy to distinguish the roman quadratum from other ashlar techniques by the impeccable finish of edges and vertices. Normally the ashlars were placed by headers and stretchers -alternis curis said the Romans-, what means alternating blocks parallel to the wall with others perpendicular to it, in order to strengthen the stability of the wall.
Photo 3.- Ancient city of Cáparra. Villar de Plasencia, Cáceres (Spain). Cushioned ashlar used in the construction of the city wall.
Photo 4.- Ancient city of Ercávica. Cañaveruelas, Cuenca (Spain). Buttress of the cryptoporticus of the city. Cushioned ashlars. Centuries 1-2 AD.
A particularly expensive variant of this material were the so-called cushioned ashlars, characterized by being carved at least by one of its faces in high-relief, that is to say with the vertices and edges of the face or faces in question delayed with respect to the internal surface of the rectangle --photos 3 and 4--. Very used in the erection of city-walls and bridges, it can be said that, in general, the cushioned ashlar were suitable for any structure whose external walls had many possibilities of being attacked by especially aggressive external agents such as raging water, is the case of bridges, or enemy catapult projectiles, the one of city-walls. The reason for this convenience lay in the experience by which it was known that the weakest point of any ashlar masonry is always in the joints between blocks: elements that in the cushioned ashlar remain in a second plane with respect to the main surface of the block, much more sheltered by so much against any aggression.
Opus Vittatum. This material corresponds to the ashlar of small module and usually worse carving, although in the case of the buildings erected by Rome it is usually a better quality facing, with the prismatic blocks well cut and squared, precisely defining the core to be filled with the usual mixture of masonry, sand and lime mortar -photo 5--. That is why it is usually a dry-fitted technique, nice appearance and great longevity. In fact, if the opus vittatum is well executed, it does not need to seal the joints with mortar nor regulate the rows of blocks with rubble.
Photo 5.- Ancient city of Uxama Argaela. Burgo de Osma, Soria (Spain). Walls of Opus Vittatum alternating with Opus Quadratum.
Photo 5bis.- High quality Opus Vittatum located in the internal galleries of the Roman amphitheater of Frejus, the ancient Forum Iulii, in French Provence.
The Opus Vittatum was a very widespread technique in the Latin world in general, especially in the Gallic provinces where a master level in its use was reached , being in fact the constructuve technique that defines, with few exceptions, the personality of the Gallo-Roman architecture (photo 5bis).
Opus Siliceum. It is a technique of very ancient origin, known throughout the Mediterranean basin from before Rome, due to not in vain it comes directly from the cyclopean architecture characteristic of previous states of humanity. In fact, Oppus Siliceum employs large blocks of irregular stone, barely carved, but with edges sufficiently smooth to fit well between them and thus provide a certain harmony to the final wall -photo 6-. The normal thing was that this type of factory constituted only the exterior walls, being filled the inner core with rubble and sand.
Photo 6.- Hijovejo Fortress. Quintana de la Serena, Badajoz (Spain). 1st century BC Example of Opus Siliceum.
Rarely used by the Latin world except in its early days given the excessive rudeness of the technique in question, there are few examples of Opus Siliceum outside Italy made by Roman hands.
Opus Signinum. This is the name given by the Romans to the most advanced of their coatings. Composed basically of lime, sand - in different degrees of fineness - and tile or brick crushed, the resulting product was a reddish colored mortar with a high waterproofing capacity, which made it especially useful for the lining of reservoirs, tanks and other structures for the storage of liquids - picture 10--. Likewise, the frequent use of fines from pozzolanic clays conferred on the final mortar an additional virtue as valuable as that of hydraulicity, ie the ability to continue with its setting even in conditions of continuous presence of water. On the other hand, regardless of its advantages in liquids storage, this kind of coating was also very useful for the general plastering of walls, mangers - photo 11 - and pavements - photo 12 - due to its great hardness and strength: much higher to that of conventional lime mortar or white mortar. It is easy to understand, in view of all these benefits, because the Roman historian and naturalist Pliny the Elder described the Opus Signinum as "one of the most spectacular inventions of Humanity".
Photo 10.- Roman city of Lacimurga. Navalvillar de Pela, Badajoz (Spain). Pool waterproofed with Opus Signinum. 1st century BC.
Photo 11.- Ancient city of Regina. Casas de Reina, Badajoz (Spain). Opus Signinum lined casket.
Photo 12.- Lacimurga. Opus Signinum pavement. 1st century BC.
Material of Roman origin as far as its use in the western Mediterranean is concerned (in the East it was known from Hellenistic times at least), multiple examples of it can be found given the great extent of its use. In fact, its degree of employ never declined in antiquity, passing the technique from the classical world to the Muslim one through the Byzantine or Eastern Empire: undoubtedly the main depositary of knowledge of Rome after the demise of the Western Empire in 476 AD.