A brief description of the main Roman masonry techniques. 2nd. Part

Let's continue with the description of the various Roman masonry techniques started in the previous publication of this web A brief description of the main Roman masonry techniques. 1st. Part

Oppus Caementicum. By this name, germ of our word "cement", is known the main contribution of the Roman civilization to the history of architecture: true responsible of the magnificence and longevity that characterize to Roman works. Very similar in its conception to modern concrete, Oppus Caementicum was made using either a wooden formwork or a sort of external walls of ashlar/masonry as a formwork lost, which were filled in both cases with a fairly homogeneous mixture of aggregate -the caementa- and lime mortar. After, this mixture was macerated until its full solidification. Sometimes the aggregate was thrown first, still dry, then mixed with the mortar inside the formwork itself. In any case the Roman layers - this material was placed by layers as in today's world - rarely exceeded one meter in height.

Photo 1.- Cerro de la Muela, Carrascosa del Campo, Cuenca. Oppus Caementicum wall with very marked layers.

Photo 2.- Segóbriga. Saélices, Cuenca. Oppus Caementicum wall in very good condition. 1st century AD.

Once the lower layer had set -photos 1 and 2- the formwork of the upper one was placed, taking advantage of the already raised wall, repeating the procedure in this way until reaching the desired height. The result, if the processes for the elaboration of this technique indicated by experience had been properly respected, was a very resistant, solid material and what is better: suitable for the elaboration of any architectural geometry. Needless to say that this advantage, capital without a doubt, opened the door for the massive use of structures such as arches, domes and vaults, until then of scarce use given its ominous cost, because they had to be necessarily manufactured in carved stone. Besides, this technique provided the possibility of erecting resistant and lasting walls while economic given the relative low cost of its components as well as the high speed of execution that characterized it. All this being said, it seems evident the real revolution that involved the use of this kind of material, whose only drawback lay in the rather coarse aspect that conferred to the final structure, reason because the Oppus Caementicum was often coated with a decorative cladding.

Photo 3.- Ercávica (Cañaveruelas, Cuenca). Ruins of a large monumental building edified with Oppus Caementicum (medium size aggregate).

Vitruvius provides us, in his second book, abundant data on this kind of material, massively extended throughout the Empire from its development in Rome around 2nd century BC. According to Vitruvius, to knead a good binder mortar it was necessary to use one part of lime for every three of sand or two for every five in case the sand used was only of medium quality. The quality of the sands was estimated by observing their physical characteristics. Ideally, a good sand had to be loose, clean, crunchy when pressed, and, the most important thing, not earthy since in case of being the resulting mortar would present a poor resistance against moisture. Because of these requirements, the sand extracted from the mines was preferred to the sand from the rivers, especially if this last one had rounded corners due to the action of the current. That of the beaches was only used in case of lack of the previous two. In terms of the quality of the lime mortar, Romans were always highlighted by the conscientious selection of the best limestones: white and free of impurities, what allowed to make a good quality basic malt which was then kneaded everything necessary until confer it the ideal consistency to be mixed with sand and gravel. It is worth adding that this meticulousness, together with the proper placing of the materials, undoubtedly constitutes the main reason for the extraordinary, almost legendary, quality of the Roman buildings.

Photo 4.- Valeria. Valera de Abajo, Cuenca. Oppus Caementicum walls delimiting the sanitation cisterns of the city.

Photo 5.- Roman city of Regina. Casas de Reina, Badajoz. Water canalization built in Oppus Caementicum.

In addition to sand, the Oppus Caementicum works also incorporated larger size aggregates. Depending on the quality required for the future structure, we can observe the use of a wide variety of gravels, both rounded and angled, being possible to establish the upper limit of this system in what has been called cyclopean concrete: a form of Oppus Caementicum -photo 6- characterized by the use of very large aggregates (small masonry in fact) and that was typical of the large masive structures where the enormous amount of material needed advised the reduction of the costs by means of increasing the aggregates size. This is the case, for example, of the core of Roman gravity dams. However, it must also be said that the Romans were unaware of the advantages of using continuous granulometries, so that whatever the size of the aggregate used in the manufacture of an Oppus Caementicum, this one remained uniform in its granulometry inside that structure.

Photo 6.- Alcantarilla Roman dam. Mazarambroz, Toledo. Cyclopean concrete constituting the bulk of the dam.

Last but not least, the Romans also resorted to the use of additives in order to improve the characteristics of the Oppus Caementicum structures, especially at the building phase. Effectively, we have proof of the use of additives such as egg white, blood, lard, fig milk or rye paste. Some elements, such as egg white, served to improve the binding capacity of lime mortar, while others, such as blood, increased the plasticity of the lime mortar with similar advantages to those provided by modern additives. On the other hand, the use of cooked clays and pozzolans served to confer hydraulic characteristic to the mortar, essential at the time to raising walls in conditions of continuous presence of water.

Oppus Incertum. Fairly economic construction technique, in which the external walls, verified in uncarved masonry except for a slight smoothing by the visible side, were embedded in a previously elaborated Oppus Caementicum core -photo 13-. Although, as it was said, the masonry used to be roughly manufactured, however it was usually of a more or less similar size (small / medium size normally) and placed approximately in rows in order to give the finished work a certain architectural presence. It is known that the more modern is an Oppus Incertum structure -within a Roman chronology, of course- better is usually the treatment of its masonry; hence it is possible to date any structure following this principle.

Photo 7.- Medellín (the ancient Metellinum, in Badajoz province), Roman city-wall. Oppus Incertum work (observe the finish of the masonry, slightly smoothed) embedded in a core of Oppus Caementicum.

Photo 8.- Roman dam of Alpuebrega. Polán, Toledo. Oppus Incertum in mass.

This construction technique differs from the traditional masonry technique in that the core is the first part to rise, not constituting the external masonry walls any class of bearing structure as, on the contrary, it occurs in traditional masonry: either heterogeneous and in mass, agglomerated with mud, or carved slightly in order to build two parallel walls which to use as formwork lost. Anyway, it should be noted that the masonry in mass (traditional masonry), agglomerated with lime mortar -photo 8- and not with mud as it was in pre-Roman times, was used also in Roman times and not little, being called also Oppus Incertum in most specialized texts. The weak point of the structures built using this constructive techique was always in the corners: lack of firmness to withstand the thrust of the walls. That is the reason because the corners of this structures were usually built with ashlar or brick.

Oppus Spicatum. Constructive technique in what the pieces -often of small size- arrange themselves obliquely to each other, fitting at their ends with the consequent development of a long succession of right angles inclined alternately to the left and right. It is also known by the name of "Fishbone" technique: term that comes directly from the latin word Spicatum, translatable as "in thorn".

Photo 9.- Roman city of Sisapo. La Bienvenida, Ciudad Real. Oppus Spicatum pavement. Centuries 2-3 AD.

Constructive technique characteristically roman, the Oppis Spicatum was conceived for the first time in the Italian peninsula. However, it was then used with great profusion, especially in the construction of pavements since, unlike other techniques such as Oppus Quadratum, Vittatum, Caementicum or Testaceum and like the aforementioned Incertum, this is not treated of a structural technique but one system destined to the final covering of the structure in charge of sustaining the loads, normally Oppus Caementicum, or, in its case, to cover the nucleus of the pavement in question -almost always simple lime mortar-.

Oppus Reticulatum. Also called Oppus Certum, it is also a revestiment technique for walls and pavements. That is why we must start from a previous structure, in most cases Oppus Caementicum or lime mortar for the case of pavements. The objective of this technique is the creation of a kind of mesh or reticle, hence its name, formed by small squares -also can be done with rhombuses- of stone or brick. This was achieved by carving the pieces of the reticle in the form of a pyramid and then nailing them by the tip in the mass of Caementicum before its complete setting. In this way the base of the pyramids remained visible, square or in rhombus, being possible to generate the reticle in question -photo 10-. Sometimes pyramids carved in stone of different colors were alternately used in order to obtain a better aesthetic finish.

Photo 10.- Ercávica. Detail of a cistern revested with Oppus Reticulatum.

The first testimonies of the use of this technique in the western provinces of the Roman Empire go back to the 1st century BC. It was quite usual from that moment in Roman constructions, especially in its pavement mode, since the use as a wall covering soon fell to the benefit of brick. The Oppus Reticulatum is considered an evolution of the Oppus Incertum, where the aesthetic sense advised to align better and better the masonry, at the same time better worked, till to form a grid. This last subclass of technique, as we said precursor of Oppus Reticulatum, was known in the classical world by the name of Oppus Quasi Reticulatum.

The temporal evolution of this technique can be followed in the progressive increase of the size of the pyramids -cubilia in latin-, without a doubt with the objective of cheapening the revestiment. Thus the works from the middle of the first century BC show pyramids of only 5-6 cm on each side, number that goes up to 9 cm during the reigns of Tiberius and Caligula (second quarter of the first century AD). Thereafter the size gets stabilize around the 8 centimeters. Obviously this is very helpful when we need to date structures covered with this constructive technique.