Roman Concrete in Gaul

Comm. Hum. Litt. Vol. 128 139

The selection and use of lime mortars on the building sites of Roman Gaul

ArnAud CoutelAs

1. IntroduCtIon

It was Michel Frizot who undertook the first archaeological and technical studies of Gallo-Roman mortars in France. His research in the seventies focused on a large number of sites in Burgundy.1 His analytical protocol was based on the use of acid on the samples, with chemical analysis of the dissolved phase and in particular granulometric analyses of the residue. In spite of his remarkable work, it was not until the end of the eighties that the importance of lime mortar study became apparent, primarily in relation to medieval and modern building archaeology.2 Several works on medieval mortars then followed.3

Our work on ancient lime mortars began approximately ten years ago. It was based on identifying the composition of materials, and researching the geological origins of raw materials. This is why this study is based on a petroarchaeological approach, that is to say the data are obtained after analysis of materials in petrology (under a microscope). The results make it possible to establish a typology of the mortars, which is then linked with the chronology of a buildings construction phases, in order to see more clearly the evolution of a particular building.

Currently this research relates mainly to the identification of the technical legacy and expertise of Gallo-Roman masons and stucco workers; in particular it tries to establish which parameters will influence the final choice of mortar formula. Up to the time of writing, more than forty Gallo-Roman buildings have been studied.4

1 M. FrIzot, Mortiers et enduits peints antiques, tude technique et archologique (Centre de recherches sur les techniques grco-romaines 4), Dijon 1975.2 Ch. sApIn, Enduits et mortiers. Archologie mdivale et moderne (Dossier de documentation archologique 15), Paris 1991.3 B. pAlAzzo-Bertholon, Lapport des analyses chimiques ltude des enduits peints: lexemple des peintures murales gallo-romaines du boulevard Vaulabelle Auxerre, in Cl. AllAg (ed.), Peinture antique en Bourgogne (Revue Archologique de lEst, 21e supplment), Dijon 2003, 31-42. And more recently: s. Bttner, Ltude ptrographique des mortiers de gros uvre de labbaye Saint-Germain dAuxerre (Yonne). Apports la comprhension stratigraphiques des structures bties, Revue dArchomtrie 28 (2004) 117-25.4 A. CoutelAs, Ptroarchologie du mortier de chaux gallo-romain, essai de reconstitution et dinterprtation des chanes opratoires: du matriau au mtier antique (Ph.D thesis), Universit Paris I Panthon-Sorbonne 2003, http://tel.archives-ouvertes.fr/tel-00528508/. Moreover: A. CoutelAs, Les mortiers de chaux du sanctuaire de Ribemont-sur-Ancre (Somme), Revue Archologique de Picardie 3:4 (2003) 77-89; A. CoutelAs, Les mortiers de chaux gallo-romains en Bourgogne, Revue Archologique de lEst 54 (2005) [2006] 327-35; A. CoutelAs M. heIjMAns, Les mortiers de construction de la ville dArles (Bouches-du-Rhne) au Haut Empire, Revue Archologique de Narbonnaise 38-39 (2005-2006) 401-408.

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2. AnAlysIs

For the study of these archaeological materials, the choice of analytical methodology owes a lot to the authors experience in the field of Earth Sciences. Considering the mortars as reconstituted rocks, whose natural analogues would be sandstones with carbonated cement, we chose to employ the analysis methods common in geology, i.e. the petrography of sedimentary rocks.

Among these methods, one can mention the observation of rough samples with a magnifying glass, the observation of thin sections under a petrological microscope, and the examination by scanning electron microscopy (SEM) of thin sections or small rough fragments.

The advantages of this method lie in the fact that the observations offer a large number of criteria and data that are directly accessible to the analyst, and subject to his assessment. The plurality of optical analysis methods is a major asset, which makes it possible to analyse material on all scales.

In addition to aggregate identification, these observations make it possible to note the texture and internal structure of mortars, as well as their stratigraphy (for coatings). A lot of data can be obtained for the aggregate through the study of its appearance, which is important: this is the case for clay (especially illuviation clay brought with grains of sand, indicating the exploitation of precise geological locations), and also for the identification of badly burnt limestone fragments and for fragments of other mortars present as salvaged material or residues from the mixing process.

Moreover, the combination of these observations with physicochemical analysis techniques is one possibility frequently used to identify non-observable phases and to check and validate the observations. The principal techniques are:

Granulometry, for siliceous aggregate, after dissolution of the binder. X ray diffraction, to identify mineralized compounds in a few milligrams of

powder (useful for clay). Cathodoluminescence and electronic microscopy, very useful for the

identification, analysis and cartography of neoformation and hydraulic compounds.

Mass spectrometry. Multispectral micro-image processing.5

3. An exAMple oF A petroArChAeologICAl study: the gAllo-roMAn BAths oF le VIeIl-eVreux (eure, FrAnCe)

Let us consider the case of a complete petroarchaeological study of a Gallo-Roman building, the baths of Le Vieil-Evreux, in Normandy, excavated between 1996 and 2002

5 A. CoutelAs g. godArd ph. BlAnC A. person, Les mortiers hydrauliques: synthse bibliographique et premiers rsultats sur des mortiers de Gaule romaine, Revue dArchomtrie 28 (2004) 127-39.


by Laurent Guyard.6 It is a crucial study because this site has the double advantage of showing many accurately dated construction phases and of having benefited from a very complete analysis of its masonry mortars (more than two hundred samples were taken).7

3.1 The geological origin of raw materials

Le Vieil-Evreux is located on a chalky plateau covered with a kind of loess (argillaceous and silty surface formation). In the east and the north-east can be seen the current limits of erosion of tertiary limestones and sand formations (Lutetian limestone and Sables de Lozre). This plateau is framed by two rivers; the River Eure in the east and the River Iton in the west.

It is known that identifying the limestone used to produce lime is very difficult. Fortunately, many signs of lime manufacture were located in the construction layers, and in masonry mortars: lime nodules (white lumps), particles of lime which had been overburned, underburnt limestone, vitrified slag and pieces of lime kiln walls.

The observation of underburnt limestone can help us to identify the stone used to produce lime. Here, in fact, at least two sources can be noted: chalk, as expected, sourced two kilometres from the building site, but also Lutetian limestone, sourced seven kilometres away. It is important to point out that Lutetian limestone is the only one used for construction, in the opus reticulatum. Perhaps the same quarry was worked simultaneously for the stone and the lime.

Thanks to petrography, a large number of clues to the origin of sand could also be obtained. In the masonry mortars of the baths, the aggregate is mainly rounded quartz, with some flints, and there are many clay illuviation coatings, which had coloured the mortars. Moreover there are nodules (ochre nodules) which give us a perfect picture of the composition of the sand formation worked (Fig. 1).

By considering the whole, it became apparent that the sand formation worked was the Sables de Lozre, but in its altered version, rich in illuviation clay. This geological formation is, in fact, present on the whole of the plateau, at one or two meters depth, under the loess. Thus, in spite of its

6 l. guyArd s. BertAudIre, Les thermes de la ville sanctuaire du Vieil-Evreux, Les Dossiers dArchologie 323 (2007) 52-59.7 A. CoutelAs l. guyArd Ch. dAVId, Ptroarchologie de mortiers gallo-romains. Application de mthodes analytiques ltude des thermes du Vieil-Evreux (Eure), Les nouvelles de larchologie 81 (2000) 31-36.

Figure 1. Ochre nodule (sand and illuviation clay) in a lime mortar. Diameter: 1 mm. Transmission Optical Microscopy, plane polarized light. (A. Coutelas)

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high clay content, this formation was systematically worked during the Roman period for the construction of the public buildings of Le Vieil-Evreux; undoubtedly because this sand formation was the most abundant and the easiest to extract.

3.2 Typology

The typology of the mortars consists in the classification of the samples according to the study of their characteristic features. It is in a way the final aim of the petrographic analyses, and the basis of the archaeological study of these architectural binders. The cartography of the types of masonry mortar used enables us to reveal the various construction phases of a building. The baths of Le Vieil-Evreux, built in four periods between the beginning of the second century AD and the end of the third century AD, perfectly illustrate this process.

For the first construction phase, in the first half of the second century, a beige mortar called mortar number 1 (Mo1) is used for the foundations of the central baths only. After a first halt in the construction process, a more coloured mortar (Mo2) is used for the foundations of the palaestra, and for the last three courses of the baths foundations (Fig. 2). It ends this first great stage of building on the site, but the initial architectural project was not completely finished.

The architectural programme was completed towards the end of the second century. At that time, the Roman monument consists of a central building, made up of a double set of rooms placed on both sides of a central praefurnium. At the ends, two frigidaria with pools are preceded by an apodyterium communicating with the palaestra and the south yard.

A third mortar (Mo3), slightly pink orange, is seen in the first elevation courses of the baths. After another halt in construction, mortar number 4 (Mo4), pink orange, is used to complete construction of the baths and the palaestra, and then the south yard. Thus we see that this mortar is the principal construction material for the building.

At the beginning of the third century, the initial frigidaria are transformed into caldaria. New frigidaria have taken the place of the initial apodyteria. Cold pools are built with a masonry mortar rich in brick powder (Mo6). Mortar number 5 (Mo5), dark orange, is used for the addition of new apodyteria, circular changing rooms.

Several years later, in the middle of the third century, yellow mortars are used for a new extension, the addition of shops along the palaestra. Mortar Mo7 is only used for a few walls, with mortar Mo8 used for the major but unfinished phase of the extension.

These construction phases are now well recognised and dated. Thus, it appears that each type of masonry mortar can be dated with a great deal of precision. In concrete terms this means that, in these baths, any new wall discovered with one of these types of mortar can be dated with an accuracy of twenty-five years.

This large number of mortars probably indicates the scale of the building site. This is revealed by the mortar study due to the heterogeneity of the excavated sand formation, which involves variations in the material aspect, and to the obvious breaks in construction on the building site. The latter could be regarded as just seasonal pauses, but current research, linking the analysis of construction terra cotta and masonry mortars, tends to


show that the reasons for these pauses are more complex. They would be linked, at least at the start of construction (the transition between Mo1 and Mo2), to the end of a financing cycle and/or the end of a supply of building and other raw materials cycle.

This petroarcheologic approach to the study of lime mortars has been employed many times for the buildings of Roman Gaul. It was also tested on ancient sites in Roman provinces, such as buildings of the Italic peninsula. One can cite, for example, the shop frontages in the Casa del Salone Nero, in Herculaneum, the study results of which are of real historical significance.

Indeed a type of mortar named MS4 was used to block up three doorways, allowing passage between the shops and the domus. This discovery of the use of a single mortar

Figure 2. Baths of Le Vieil-Evreux (according to Mission Archologique Dpartementale de lEure) with surface cartography of the various types of masonry mortars. (Final CAD: A. Coutelas)

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made it possible to counterbalance the untidy appearance of the facings. This similarity of treatment thus allows us to conclude that the occupant of the house ceased to directly manage the shops located in the frontage, probably about 75 AD.8

4. lIMe MortAr In the hIstory oF ConstruCtIon teChnIquesThe study of lime mortars makes it possible to look at the craft and skills of masons in ancient times. To do this, we must consider that the composition of mortar results from both the choice of raw materials and the recipe (or formula). The nature of these choices gives us information about:

Traditions: i.e. the relationship between mortar function and its composition. Expertise (savoir-faire): the result, in particular, of practical experience, it

permits adaptations and flexibility of the operational sequence and allows for modifications and innovations.

External constraints: these constraints are essentially socio-economic or related to the geological environment.

In order to examine all these parameters, we will now examine the traditional craft of the Gallo-Roman mason. In particular, we will focus on the known relationship between the function and composition of lime mortar.

4.1 Masonry mortars

The first and principal function of a lime mortar is its use as masonry mortar. In Roman Gaul, as elsewhere in the Roman Empire, it is masonry with opus caementicium which predominates when walls are very thick. In addition, the mortar from caementicium is usually the same as that used for the facings. The mortar is made with lime and siliceous or calcareous sand. Examples of masonry mortar containing broken tile and brick are known, but are essentially used for the walls of a few small hydraulic structures such as pools.

8 n. MonteIx M. pernot A. CoutelAs, La metallurgia del piombo fra archeometria ed approcci classici, in p.g. guzzo M.p. guIdoBAldI (ed.), Nuove ricerche archeologiche nellarea vesuviana (scavi 2003-2006) (Studi della Soprintendenza Archeologica di Pompei 25), Roma 2008, 439-47.

Figure 3. Masonry mortar from the aqueduct of Le Vieil-Evreux (Eure, France). See the ochres nodules dispersed in the matrix. (A. Coutelas)


It is important to note the extent to which economic and environmental contexts influence the choice of raw materials for these construction mortars. By way of illustration, we may recall the case of Le Vieil-Evreux. Just as with the walls of the baths, the aqueduct masonries present very coloured, orange mortars, rich in ochre nodules (Fig. 3). These nodules are evidence of the working and use of very argillaceous sand. In spite of this presence of clay, and in contradiction with the writings of Vitruvius, which require a sand which does not leave stains or any particles of earth on a white garment (De Architectura 2.4), the builders decided to use this sand formation. This could be explained partly by the fact that the mortar obtained is of a sufficiently good quality but also by the fact that this sand formation is the most abundant and the easiest to extract, in a word the best choice from an economic point of view.

It appears, therefore, that for Gallo-Roman buildings, masonry mortars were often prepared with low quality raw materials, because accessibility and the volume of resources were the most important factors in the choice of the extraction site.

4.2 Refilled joints

Refilled or pointed joints are those where mortar fills gaps between the building stones of the facing (Fig. 4).9 Their composition is always different from masonry mortars. They are white, dense, and often seem like scored joints or tooled joints. Thus, occasionally they are confused with fragments of stucco when these elements are found during excavation. Moreover, the composition of the refilled joints is reminiscent of stucco. Indeed, they are characterized by a quasi absence of aggregate (Fig. 5), or by the presence of siliceous, fine and very pure sand. Thanks to such compositions the mortar is easier to work and to insert between stones.

Nevertheless, the quality is always very good and suited to its material function, which is to prevent early degradation of the facings, especially when the quality of the masonry mortar is poor.

9 r. gInouVs r. MArtIn, Dictionnaire mthodique de larchitecture grecque et romaine. Tome I. Matriaux, techniques de construction, techniques et formes du dcor, Paris 1985.

Figure 4. Refilled joints, in white mortar, between building stones of the facing. (A. Coutelas)

Figure 5. Thin section (3.6 x 2.4 cm) of a refilled joint. The mortar is characterized by a quasi absence of (siliceous) aggregate. (A. Coutelas)

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4.3 Wall paintings and plasters

In our opinion, there are two types of wall coatings in Roman Gaul. The first is plaster, whose main function is the protection of the facing. The second is wall painting, the main function of which is the decoration of walls.

Indeed, plaster is used for non-protected external faces (primarily the faade), and for non-residential and service rooms. In this case, only one mortar layer is observed, almost always made with a lime mortar rich in broken tile. Often this coating is painted (one can then speak of painted plaster), with ochre or black, but without intonaco (the typical finishing layer of wall paintings).

On the other hand, for wall painting several layers are applied in order to prepare for the decoration. The stratification of mortar depends in particular on the position in the building, and on the wall. It may be noted that the stratification sometimes changes between the lower zone of the wall and the upper zone. Among other explanations, one can suggest:

the need for good stability of the coating: the addition of a mortar layer on the lower zone of the wall

the need for good resistance to moisture (caused by capillary rise): the addition or use of a layer of lime mortar rich in broken tile and brick on the lower zone of the wall

the desire to keep quality raw materials for the more visible zones of the wall only: use of an intonaco rich in marble powder only on the lower and intermediate zones of the wall.

A large number of stratifications of Gallo-Roman wall paintings were identified a few years ago and results of the studies have been published.10 As expected, no coating shows a technique as elaborate as that described by Vitruvius, with seven mortar layers (the first is the coarsest, the three following ones are rich in sand, the three last are rich in crushed marble; De Architectura 7.3). It even appears, on the contrary, that the most abundant coatings are the simplest, made up of two layers only. The preliminary layer is made up of a good quality lime mortar with siliceous or calcareous sand. The finishing layer is also very often composed of lime mortar with good quality fine siliceous or calcareous sand.

It is noted finally that the aggregate of Gallo-Roman finishing layers is primarily composed of siliceous or calcareous sand, and secondly of marble sand (not powder) or one of its substitutes: vein of calcite or limestone with large calcite crystals.

However, it should be understood that the quality of the intonaco composition is always better than the quality of the other layers. This is obviously shown by the presence of marble powder, but it can also be apparent when the aggregate is natural sand. In the

10 A. CoutelAs, Les mortiers de support des peintures murales de Gaule romaine: premire synthse, in C. guIrAl pelegrn (ed.), Circulacin de temas y sestemas decorativos en la pintura mural antigua (Actes du IXe colloque international sur la peinture murale antique, Saragosse, 21-25 septembre 2004), Calatayud 2007, 505-07 et pl. 45.


example given in Fig. 6, the preliminary layer is composed of a mortar rich in flint and silt, whereas the aggregate of the finishing layer is only pure flint sand, probably selected for this use.

Clearly, it is not because of the lack of crushed marble in the intonaco that the quality of the wall painting is low. Moreover, it is unlikely that marble in intonaco always gives the result described by Vitruvius: a perfect, polished and possibly reflective surface (like polished marble). Indeed, to achieve such an effect it is necessary to include a lot of crystals within the layer of pigments, and of course hard work of polishing, and sufficient luminosity.

4.4 Hydraulic structures

One can divide hydraulic structures into two categories: those for waste water and those for clean water. In fact, there is no particular building material used for the construction of structures which allow the evacuation of wastewater. If mortar is involved, one finds it only in masonries, made with lime and natural sand.

The situation is quite different for structures related to the clean water supply. Indeed, one can observe the extensive use of mortars and concretes with broken tile and brick. But these materials are always used for coatings (on the walls or the floor). Examples of the use of lime mortar with broken tile in the walls of hydraulic structures are rare and always limited to small masonries (especially for small pools).

The question of the origin of the broken terra cotta often arises. In fact, the fragments in Gallo-Roman mortars almost always come from several sources: different types of bricks or tiles are present in the material. Thus, one must consider the likelihood that this aggregate is actually obtained by salvaging materials.

It is interesting to note that the results are similar for Italy, where we may take the example of the castellum aquae della Porta Romana in Ostia Antica11. Analyses have shown that the coatings inside the building, like those outside, are in opus signinum, otherwise called cocciopesto. To be more precise, and because the term opus signinum is unfortunately often misused, the plaster is made with a lime mortar rich in sand and broken tile and brick (Fig. 7).

11 e. BukowIeCkI h. dessAles j. duBouloz avec la collaboration de j. CArlut A. CoutelAs e. de senA M. leguIlloux g. poCCArdI, Ostie, leau dans la ville : chteaux deau et rseau dadduction, (Collection de lEcole franaise de Rome 402), Ecole franaise de Rome, Rome 2008.

Figure 6. Preliminary layer (b) and finishing layer (a) of a Gallo-Roman wall painting (Ribemont-sur-Ancre, Sommes, France). The two mortars are made with lime and flint (white crystals). The aggregate of the finishing layer is pure and less abundant. (A. Coutelas)

(a)

(b)

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4.5 Joints of tiles

The joints of tiles consist of mortar applied at the same time as the tiles are laid in order to protect the tiled roof against wind and gravity (Fig. 8). In Roman Gaul, the joints are mainly made of lime and siliceous sand; the absence of broken tile in the mortar is probably explained by the fact that impermeability of the roof is ensured by the tiles.

However, this mixture of lime and sand is not prepared randomly, the joint mortar quality is better than that of the masonry mortar: always dense, with an aggregate that is very fine and pure. This quality, which in particular indicates the preliminary layers of wall paintings, is obviously explained by the particular position of the material.

5. lIMe MortArs And puBlIC BuIldIng progrAMMesThus, there are a lot of principles in Roman Gaul. There is a particular lime mortar formula for each architectural use: in a word, each function requires an exact composition of materials, or at the least very specific technical know-how and expertise.

Moreover, recent work relating to the analysis of several public buildings that are part of a monumental unity (sanctuary, agglomeration) proves that the selection of a lime mortar recipe is not always restricted to one building only.12 Analysis appears to indicate that several buildings could show the same construction or decoration materials.

Thus, it is possible to check which buildings of a monumental unity share common phases of construction and decoration. This result is particularly promising, because it could enable us to better understand the various phases of city growth through the study of mortars, especially masonry mortars. In addition, it gives us new opportunities for understanding the involvement of local elites in providing labour and materials, and their importance as economic factors in urban development.

12 CoutelAs heIjMAns, cit. n. 4.

Figure 8. Type of joint of tiles used for two imbrices. Here, two of these joints and, between, one imbrex. (A. Coutelas)

Figure 7. Thin section (3.6 x 2.4 cm) of an opus signinum: plaster made with lime, sand and broken tile and brick (angular shape). Ostia Antica, castellum aquae della Porta Romana. (A. Coutelas)


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The building programme of the monumental complex of the Roman town Cassinomagus (Chassenon, Charente, France) provides an example. This immense complex is organized around a large temple. Other major constructions have been found: a theatre, smaller temples, baths and two aqueducts - the major and the secondary (Fig. 9).13 The south-western area, excavated by Ccile Doulan and Sandra Sicard between 2004 and 200814, is of particular interest. Here there are the major aqueduct, the secondary aqueduct, the peribolus of the temple and the south-western tower of the baths.

Analyses have shown the use of the same lime mortar for the construction of the two aqueducts and for the south tower of the baths. But the masonry mortar of the peribolus is different. This has enabled us to confirm that the monumental complex of Cassinomagus had undergone various building programmes, and that one of them included the two aqueducts as well as the south tower of the baths (a structure for the reception of water?), even perhaps the baths in their totality. The peribolus of the temple, and undoubtedly the large temple as a whole, probably belong to another building programme. Without going into great detail, it is clear that these variations of types of masonry mortars show different phases of city growth, and therefore various clients for these constructions.

6. ConClusIon: the teChnICAl ChAInIn conclusion, mortar analysis will provide information to reveal construction phases. It may give some details of the building process: for example the number of pauses in construction. Moreover, when several buildings are studied it seems that a city building programme can be identified.

The techniques of local builders, and to some extent their know-how, can be compared with established ideas at the time: essentially the relationship between the function of materials and their composition. This comparison provides information about technological skills and traditions: the study of the whole technical chain, as a relationship between environment, needs, knowledge and expertise, and authorities15, can then be reasonably considered.

Fig. 10 is a representation of the technical chain for Roman lime mortar. It is important to note that this is not the chane opratoire, translated as operational sequence, and described as the different stages of tool production from the acquisition of raw material to the final abandonment of the objects.

13 d. hourCAde p. Aupert ph. poIrIer, Chassenon Roman Baths, La Crche (F) 2004.14 s. sICArd j.-p. Bost A. CoutelAs th. MorIn s. soulAs, Longeas, commune de Chassenon (Charente), programme scientifique TherMoNat (2003-2006): fouille programme annuelle. Systme hydraulique entre temple et thermes du complexe monumental (Rapport de fouille), Poitiers 2005; C. doulAn Chr. BelIngArd A. CoutelAs th. lepAon s. sICArd s.soulAs, Systme hydraulique de lensemble monumental: tour sud-ouest des thermes et aqueduc secondaire entre temple et thermes. Longeas, commune de Chassenon (Charente), programme scientifique TherMoNat (2003-2006): fouille programme annuelle (Rapport de fouille), Poitiers 2006.15 s. ploux Cl. kArlIn, Le travail de la pierre au palolithique, ou comment retrouver lacteur technique et social grce aux vestiges archologiques, in B. lAtour p. leMonnIer (ed.), De la prhistoire aux missiles balistiques: Lintelligence sociale des techniques, Paris 1994, 65-82.


The box compositions is related to all the parameters. Choices are made by the builders; identification of the most frequently recurring choices enables us to characterize the technical traditions of this social group. As has been described above, many traditions are present in the craftsmanship of Gallo-Roman masons. Certainly these traditions are the response to architectural needs, but most are doubtless inherited from a long architectural legacy: they are widely used but the reasons are forgotten.

The geological environment and socio-economic constraints, and between them the treatment of raw materials carried out when this is really necessary and economically viable have, therefore, considerable influence on the compositions box. It is expertise (or practical experience) which allows for adaptations and flexibility of the operational sequence.

It seems that perfect knowledge of all the boxes of the technical chain can help us to obtain new data about the Roman building industry. Indeed, once all the traditions and expertise of the ancient craftsmen are fully understood, and when the constraints imposed by the geological environment are known, it is possible to tackle the question of external and mainly socio-economic constraints. Thus, these first factors allow us to develop research in the area of the organization of work and the supply of raw materials, notably through the study of pauses in construction and the study of material variation in the building.

Figure 10. Technical chain for Roman lime mortar. (A. Coutelas)

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Roman Concrete in Gaul