We are going to conclude this series of four articles about the main Roman masonry techniques.
Oppus Scutulatum, Oppus Alexandrinum, Oppus Sectile. With these names the Romans designated a series of cladding techniques characterized by the use of marble as a building material. Unfortunately, the findings of Roman marbles in Europe are not abundant because during the whole Middle Ages they were used as raw material for obtaining lime, hence they were systematically destroyed. The reason is Roman marbles had almost always very good quality, with few impurities, being obtained from its calcination an excellent lime.
Photo 1.- Detail of Oppus Alexandrinum in the orchestra of the theater of Metropolis (Turkey).
The differences between these three techniques are not too great, consisting mainly of the size, shape and color of the marble pieces. Otherwise the construction system is identical in all three cases, similar to that of the Oppus Spicatum and Reticulatum: covering a central core of Oppus Caementicum.
Photo 2.- Oppus Alexandrinum on the pavement of one of the main streets of the ancient Tripolis ad Maeander (Turkey).
Photo 3.- High quality Oppus Sectile in the Byzantine church of Hagia Sofia (Nicea/Iznik, Turkey).
Specifying a little more, it can be said that the Oppus Scutulatum used pieces of rhomboidal marble - with which the result was somewhat similar to that of the Oppus Reticulatum - while the Oppus Alexandrinum made use of circular and quadrangular designs (photos 1 and 2) . For its part, the most complex of the three techniques was the Oppus Sectile, characterized by a combination of pieces of marble of different colors, sizes and shapes placed in such a way that elaborate geometric compositions can be maked (photo 3).
Oppus Tessellatum, Oppus Vermiculatum. Known by the generic name of Oppus Musivum, it is the famous mosaic technique, so associated in the collective imagination with Roman culture and way of life.
To make a mosaic the first thing that had to be done was to prepare an appropriate support for that one. This support consisted of three well-differentiated layers, namely: a first bed of boulders called rudus, a second layer of fine pebble mixed with clay -the nucleus- and a third one named lectus, the authentic support of the mosaic, constituted by a lime mortar where the so-called tesserae were embedded -photo 4--. These tesserae were small pieces of different colors that, in the manner of architectural brush, allowed the drawing of all kinds of figures in order to form a scene chosen in advance. Manufactured in long strips that were then cut, the tesserae were carved both in stone and terracotta and, as of the third century AD, also in laminated glass paste with gold or silver (only for its visible side). Almost always they were of cubic geometry, being polished with care all the faces of the tesserae except for the one that had to come in contact with the lectus, which was left in rough in order to increase its adherence to this one.
Photo 4.- View of the three layers that constitute the support of a mosaic. From left to right (immediately below the tesserae): Rudus, Nucleus and Lectus. Roman villa of Carranque (Toledo, Spain).
There existed several techniques for the placement of the tesserae, combining elements of different materials in order to take advantage of not only their different color tones but also the different ways of reflecting light. In this way it was possible to generate very beautiful brightnesses and splendors, today not very appreciable given the great antiquity of the mosaics but that undoubtedly should have conferred its greatest attraction to this kind of cladding. The most used colors were green, blue and yellow; all three in all their ranges, although choosing especially the brightest and deepest. Black, white and different varieties of ocher were also widely used. For its part, highlighting in beauty over the rest of the mosaics, we can mention the so-called parietal mosaics, in which the tesserae were placed to different height (that is: some tesserae were longer than others). In this way specially beautiful light effects were obtained, although at the cost of making the cladding in question much more impassable, which undoubtedly explains the almost exclusive use of this variant in the vault mosaics.
Photo 5.- Beautiful mosaic of Oppus Vermiculatum. Roman villa of Carranque (Toledo, Spain).
The mosaic technique comes from Mesopotamian and Asia Minor lands, from where it would pass to the Greek world. The Romans, always worried about the embellishment of their temples, palaces and mansions, would take their development to the zenith at an early time, extending it to all the corners of their Empire. In fact, the invention of the tesserae as such, this is with its geometry and more or less careful finishing, is due to the Latin civilization since its precursor, the Greek, had not overcome the use of colored pebbles when making its mosaics. For its part, the oldest example of the use of Roman-type tesserae is found in Margantina, Sicily, with an estimated chronology between 250 and 270 BC.
Photo 6.- Roman Theater of Mérida (ancient Emerita Augusta, Badajoz, Spain). Detail of Oppus Vermiculatum.
There are numerous examples of mosaics located in the territories of the ancient Roman Empire, some of great extension and beauty, all of them directly related to the ruling classes of the population as it was an expensive and complex cladding, made exclusively by specialists. There is a lot of information about some of the main mosaic workshops because of the great dissemination of their models, sometimes on truly large geographical areas. These models were reproductions on a small scale, drawn on transportable cartons, of the mosaics offered by each workshop in particular, which were shown to the client so he could choose the one he liked most. And it is that any landowner who prided himself to be so had to choose the mosaic as decoration of the floors of his residence, covering also, in some cases of exceptional wealth, the walls of the building in question.
Photo 7.- Roman theater of Mérida. Detail of Oppus Tessellatum.
The dating of the mosaics does not usually represent too many problems since it is usual that, the more ancient they are, the smaller the size of their tesserae. Thus, older mosaics usually employ pieces less than one centimeter on each side: it is the so-called Oppus Vermiculatum (pictures 5 and 6), while in the golden age of the Empire (1 and 2 centuries AD) the centimeter is taken as a more or less homogeneous measure for all the tesserae. Later, when the decline of classical civilization began, the tesserae tended to increase in size (from 1 cm we are talking about Oppus Tessellatum, photo 7) until reaching sometimes much larger dimensions. The reason for this evolution obviously lies in an attempt to lower the cost of the finished work: we require fewer tesserae, if these are larger, to cover the same pavement, vault or wall surface.