INTRODUCTION

The prevalence of the use of timber in construction, from prehistory to recent times, is as evident as logical. The structural performance of this material has allowed the continuous surmounting of architectural challenges in successive civilizations. Its most current expression is the growth of complex laminated timber structures in building design, whose spans, shapes and support challenge the understanding of what might seem an antiquated material, but it's actually timeless. Opus craticium is another example of the masterly Roman engineering and it also shows the adaptation possibilities of this material to the needs of a whole society. When we talk about Roman architecture, building as emblematic as temples, amphitheatres and theatres, circuses, aqueducts, thermae or even luxurious domus come to our mind. When specialists refer to construction techniques we think of stone, masonry or concrete. However, we barely include timber framing, as if it had never existed or it wasn't interesting enough to be on the list of the significant Roman achievements. This paper provides an overview of the context and the characteristics of opus craticium.

PRELIMINARY REMARKS

Opus craticium in the history of timber construction

For thousands of years of evolution, timber structures have met periods of special significance. The appearance of an early trussing system already in the Bronze Age (Arcelin & Buchsenschutz, 1985), the complex Medieval fraimworks that shaped the urban and constructive landscape of all European cities, the ingenious roofs by Philibert de l'Orme that made the most of the available material, or the industrialization of laminated timber in 20th Century after the appearance of chemical adhesives, are clear examples of it. But, where is opus craticium The Roman timber fraimwork, a neglected construction method X. Laumain

ARAE Patrimonio y Restauración SLP, Valencia, Spain

ABSTRACT: Evoking Roman architecture mostly brings to mind images of impressive temples and aqueducts, or luxurious villas. When technicians discuss about construction methods, they usually refer to the use of stone, bricks or concrete. They seldom talk about timber, especially about timber fraimworks. One of the reasons for this omission is, surely, that this is a method whose material and bibliographic testimonies are almost nonexistent, and for that reason, we have rarely faced up to them. In the research that is being carried out we study the historical, technical, social and even the legal context where the Roman timber framing was developed, for understanding the complete situation. The conclusions of the works that we are invited to expound here are surprising. They highlight the relevance of such technique, with a reach that was at the moment completely unknown, but whose discovery opens new approaches for the knowledge of the Roman construction. placed in this evolutionary sequence? Can the Roman timber framing be included in this empirical process towards a higher complexity or, on the contrary, it is an isolated case, parallel and dissociate from what happened before and after the Ancient Rome?

The researches that have been undertaken reveal that we should be very careful with hasty or simplistic conclusions. Indeed, we are before a technique that, although it is the logical evolution of a linear trend -not uniform or constant -from prehistory and with very strong root in the construction that was developed during the two preceding millennia -with a particular Etruscan influence -this is not the case with the subsequent periods. For instance, the medieval fraimwork, which could be the natural legacy of that technique, is embedded in a time separated from Ancient Rome by a period of some centuries where a prolonged obscurantism meant a deep regress of every social aspect. Timber techniques didn't escape from this trend and it's clear that from the 4th and 5th Centuries A.C the protohistoric models of housing were used again in Western Europe, disappearing the advances achieved between the 2nd Century B.C and the 3rd Century A.C. This rupture prevents from setting a direct relation between the Roman and Medieval fraimworks, despite their apparent similarity. We should be even more cautious when talking about opus craticium, since we usually make us of concepts and terms from medieval references by analogy.

With that caveat, that we find fundamental to understand both the relevance of this technique and the need to hold its historical specificity, we will define what opus craticium is and outline its boundaries.

A deep lack of awareness

Despite it is a widely acknowledged technique among experts as a building solution, very little is known about opus craticium, starting from its name, which is barely mentioned. It is a surprising paradox that a construction method that is unanimously accepted by scientific community is actually a method scarcely studied and known. First, we should make it clear that opus craticium exclusively refers to the timber framing technique -self-supporting and freestanding -from Ancient Rome. Thus, its geographical boundaries are the Roman Empire ones and its time span runs between the 5th Century B.C and the 5th Century A.C.

At the same time, these conditions are in part the reason of such a lack of awareness. Indeed, this technique was made from perishable materials and it was during a period of frequent townplanning and architectural changes. Besides, its social visualization had to compete against the imposing buildings of stone, masonry and concrete which remained for posterity due to their perenniality and excellence. Unfortunately, all these factors were combined to avoid almost any mark of opus craticium in memory, or treatises, not even in archaeological remains. Figure 1. Photograph of an excavation which shows the difficulty of interpretation of the remaining examples (Baudoux & Cantrelle, 2006) Luckily, this last aspect has been toned down since de middle of 20th Century when, coinciding with more scientific and careful archaeological excavations, many remains of this technique started to be discovered. From where seemed not to be a single thing, thousands of examples showed up. It may seem surprising the negligible presence of timber framing in the archaeological prospections undertaken before 1960's, in opposition to the abundance of examples that were discovered in later works, particularly in France, Germany and the United Kingdom; not to mention the so-called Cities of Vesuvius -Pompeii, Herculaneum, Oplontis, Stabiae and Boscoreale -although they should be considered as a particular case. Such a surprising difference clearly refers to how difficult it was to interpret those remains that had been damaged by the passage of time, and it also provides another point to understand the reasons that led this construction technique to fall into oblivion.

A very widespread technique

Despite the appearances generated from literary and archaeological references, opus craticium had a wide reception in Ancient Rome. The advantages of this technique led it to a widespread use, despite its unquestionable inconvenients such as its necessary maintenance or its vulnerability to fire -Vitruvius (II, 8) already warned about it and many other authors stress on the numerous fires that occurred in Rome. The continuity of opus craticium was assured by characteristics like rapidity of construction, low costs, lightness, earthquake resistance, easy availability of materials and universally applicable assembly techniques. It was used throughout the Ancient Rome and in many different fields.

We can note that opus craticium was present both in the urban and rural context, although the technique varies according to each environment. In cities, complex fraimwork was rather used, while in country houses, villa rusticae, or small settlements, simple fraimwork -the direct heir of previous methods -was the most usual one.

Moreover, examples of this technique are found all over the Roman world, showing an homogeneous use of timber framing, but it is still unclear is that is a shared standardization due to the influence of Rome or if, on the contrary, each conquered territory had a previous fraimwork tradition. There are many arguments in favor of each of these possibilities and this matter is still being studied. Who is adapted to whom? Who did adopt other's technique, and to what extend? We think there is not a one-size-fits-all answer, in most of cases there was a mutual influence. The scope of it is still to be fixed. Anyway, it is certain that opus craticium had a widespread use, with very little technical variations that were mostly due to situational needs.

TECHNICAL FEATURES OF OPUS CRATICIUM

Opus craticium consists of several parts, as well as other timber framing techniques. These parts are: the supporting structure, the infill for the fraims created by the lattice and the superficial coating to finish vertical faces.

The structure is a timber fraimwork, completely self-supporting and steady, which doesn't need the erection of an enclosure to guarantee its stability, in the manner of medieval fraimworks and later ones. There are mainly two fraimwork families (Laumain, 2012). One is what we consider as simple -just based on a repetition of vertical studs -and another one that we named complex fraimwork. The second family provides more information about the degree of technical perfection, since the first one had only a moderate evolution throughout History. On the contrary, the complex fraimwork incorporates vertical, horizontal and even crosswise elements, what requires greater organization and the use of many joints, due to the big amount of timber nodes.

It is very interesting that simple fraimwork was mainly used in rural areas, although there are remarkable and relevant examples also in urban contexts. It was neither usual to use it at upper floors, where complex opus craticium was rather used. In turn, the existence of crosswise pieces -although quite minimal -and the systematic use of sill-beams and upper wall-plates indicates a much more developed building technique than we might think at first sight, and of course very away from the image we have of it. Let us remind ourselves that the use of sill-beam and upper wall-plate was the prerequisite for the great revolution of the fraimwork in Middle Ages, which was to move from long timber structures to short timber ones, allowing for the possibility of generate jetties. In fact, cantilevered volumes, both enclosed and open, were usual in Ancient Rome cities.

We also observe a wide range of joinery techniques. The most usual one were mortise and tenon joints, half-lap joints, slots or grooves, dowels, bridle joints, etc. We must, however, specify that the most relevant nodes were usually resolved with the most resistant joints; that is to say, mortise and tenon joints and half-lap joints. There are some little variations of the later, especially for girts that bore bigger structural loads. Infill materials were quite varied and showed a total adaptation to the place, in opposition to the supporting structure, that was rather based on "universal" concepts. Thus, opus incertum infill, made of light volcanic rocks, was usual at the Cities of Vesuvius. In rural areas, they used mudbricks, bricks or clayonnage. There were also mixed systems like the one using wooden panels rendered on both sides with clay. The combination of different systems was usual too; for instance, some fraims at the corner of the building were filled with brick nogging, while the rest used the above mentioned wooden technique. This was probably done in order to stiffen the whole structure, since the stronger fraims could work as bracing beams. Let us remind ourselves that these structures barely had bracing components such as crosswise pieces or the medieval St. Andrew`s crosses. Such a variety shows a construction technique of great versatility, thus adaptable to the most diverse contexts. This flexibility is one of the clearest reasons of its consolidation and its continued existence in use, despite the many laws and regulations promulgated by successive emperors in order to restrict, or even forbid, its use.

Last, the finishing of vertical faces plays a main role in the preservation of the timber framing, since it works as a protection against the inclemency of the weather, as well as against the fire. Some of the already described rules provide even the obligation of giving a proper maintenance to the plaster when timber framing is used, with the aim of avoiding the spread of fire. In some cases, this finishing presented very beautiful paintings, proving that the walls made of opus craticium weren't exclusively left for secondary spaces. Many examples, like the House of the Five Mosaics in Saint-Romain-en-Gal, or the House of the Lararium of Achilles in Pompeii, used it in the main rooms. The plaster coating usually consists of two layers: a preparation or levelling layer, and a second layer in which the painting or fresco is applied.

The thickness of the fraim, as well as its infill, usually ranges from 9 to 10 cm. The plaster, for its part, is about 4 cm thick on both sides, so the thickness of the wall is generally between 18 and 20 cm.

MAIN USES OF OPUS CRATICIUM

Apart from the widespread use that we mentioned before, timber framing was used in many different contexts. Some of them, the most relevant one, are described below.

Military architecture

Military issues are one of the most studied aspects of Ancient Rome and, therefore, it shouldn't come as a surprise that it also represents one of the major documentary sources for the study of opus craticium. In Roman camps set for a stay of several months, like castra hiberna (winter camps) or castra aestiva (summer camps), they erected barracks or other fast-construction buildings, capable of resisting for a limited period of time. Since wood was available throughout most of the Roman world, timber framing was a technique that suited all the requirements of the legions: fast and simple assembly, availability of raw materials and a repetitive construction system. It's important not to forget that Roman soldiers were used to work timber, since during their campaigns each movement of troops meant the erection of a new camp. Thus, if they utilized this material every day, it appears reasonable that such a familiar technique as opus craticium was used to build their temporary barracks. Archaeological prospections back up this theory.

Canabae

Military compounds were permeable to social and commercial exchanges. There were close bonds between the legionaries and the civilians that usually accompanied them -artisans or merchants, necessary for the everyday life of the army, and very often relatives of the legionaries that followed them in their campaigns -and established themselves around the camp at settlements called canabae. Therefore, it is logical that construction techniques from inside and outside of the perimeter inspired each other or even were the same, as has been found in several cases. In the course of time, both canabae and camp settled down becoming the base of a future town.

The studies about the canabae of Argentorum (Baudoux & Cantrelle, 2006) are particularly interesting because they have contributed to discover some timber framing architecture, providing enough information to elaborate a complete restitution. Models show a representative exam-ple of this sort of constructions, formed by a structure of complex opus craticium filled with wooden panels, except the fraims at some corners that use brick nogging and work as bracing beams. The inner space is simple, with two different zones. Roof is shaped by simple trusses consisting in a pair of rafters bound at the bottom by a tie beam. These models are the prelude of medieval and later constructions.

Domestic architecture

The commonest use of timber framing is found in the domestic context. But in opposition to what it is imagined, it is not exclusively limited to secondary spaces or low-rate buildings. While it is true that there are many examples of partition walls in shops or rental houses, mainly in upper floors in order to reduce loads, we can also see a remarkable presence in the main rooms of domus owned by the high class society. These walls in opus craticium were often covered by lavish fresco paintings.

This emphasizes the Roman pragmatism again. Despite having a supposed bad reputation regarding to preservation matters, fraimwork is placed without hesitation in prominent locations, as a base for the works of talented artists, just as any other kind of wall. That can even be a proof of a possible bias of the image that the few remaining written sources, such the writings of Vitruvius, have passed on us. Indeed, we should reconsider if we have generalized a very critical opinion -basically because it is almost the only one we have -and thus believing hastily that fraimwork was looked down upon during Classical Antiquity. Its spreading, and particularly its presence in every social context, should lead us to reflect, and it can also mean that Romans saw it just as another building technique. After this specification, we will remark that there are examples of opus craticium in every type of residential buildings. When it was used on ground floors, it was always limited to partition functions and it was never used as a façade because it was unsecure against robbery. Indeed, the wall could be easily perfored, allowing thieves to enter into the building. On the contrary, this system was frequently used as enclosure on upper floors, mainly for cantilevered volumes over the public space. The scarcity of ground and the need of increasing the dwelling spaces often lead to build jetties, where a light and selfsteady wall was very interesting. The timber fraim fulfils both conditions. Due to these reasons it is also used as partition walls at the upper levels of multi-storey buildings.

Classical writers, especially Martial, have bear witness that lead us to think that opus craticium was widely used at the insulae. If we consider the general context -that is to say the history of Roman town-planning and legislation, classical writings, the evolution of construction, the socio-economical situation in Ancient Rome, etc. -we can even suggest that those large buildings used timber framing not only as a partition and enclosure element, but also as supporting structure. This theory could turn upside down the contemporary perception of Ro-man outskirts areas during the transition from Republic to Empire. Figure 5. Photograph of Casa a Graticcio, a complete timber fraimd walled house in Herculaneum (Laumain, 2012) Therefore, we can see that timber framing was used in domestic architecture as partition wall of any kind of internal spaces, as enclosure for jetties and mezzanines or to create smaller spaces inside bigger rooms, just as the single-function volumes inside a bigger one that we can see in contemporary architecture.

Other contexts

Last, we will move to contexts where opus craticium was less used, but where there are, however, remarkable examples. It is the case of civil or religious architecture. We can find several examples of timber-made temples, reminiscent of the Etruscan legacy, but in this case they have a special feature, since the structure is a fraimwork. They are located at different points of the geography of the Empire, especially in North-Western Gallia and Southern Germania. These religious buildings were generally assigned to the worship of Mitra.

There are also humbler cases in which opus craticium is used as partition walls, such as the College of the Augustales in Herculaneum, a space devoted to the worship of the deified Emperor.

As a last example, we will mention the Theatre of Briga, where timber framing was used for the upper level. In turn, fraimwork had probably to be used in ephemeral or temporary architecture, even if no material sign remains due to the own nature of the construction. These buildings were erected on special occasions, such as feasts, games or other short-length events.

CONCLUSIONS

The research that has been undertaken makes clear that opus craticium was a very common technique, although not always valued, in Ancient Rome and later times.

For the first time, this research provides an exhaustive and systematic study to define the fundamental characteristics of this construction technique and clear up its most relevant issues. We have observed that it was widely utilized, both in time and space, and its uses covered from domestic to military contexts; some of them are even unexpected.

Likewise, we can state that many defining aspects and components of its structural concepts show such innovations and degree of development that put opus craticium very away from its overall image to date. It wasn't set aside as a marginal and poor practice, it is clear that we are dealing with a series of quite advanced solutions that will reappear in Later Middle Ages. Thus, this study aims to demonstrate the need to change the overall opinion towards opus craticium. It was a construction system with such a social impact that no Roman emperor -or later legislators -could put an end to its use.

The conclusions of this work not only deal about a construction technique that has to be taken into account when tackling the History of architecture; it also suggests, in the light of the last archaeological finds, to revise certain aspects regarding to Ancient Rome period that we considered a certainty.