MONOGRÁFICO: Materiales, transporte y producción. Pósters del Workshop Internacional de Arqueología de la Construcción V, (Universidad de Oxford, 11-12 Abril 2015 /
MONOGRAPH: Materials, transport and production. Posters of the 5th International Workshop on the Archaeology of Roman Construction, (University of Oxford, April 11-12, 2015)

Quantitative analysis of brick-faced masonry: examples from some large imperial buildings in Rome

Análisis cuantitativo de paramentos de ladrillo: ejemplos de algunos grandes edificios imperiales en Roma

Maura Medri^a, Valeria Di Cola^a, Samuele Mongodi^b e Giorgia Pasquali^a

^aUniversity Roma Tre
^bUniversity Roma “Tor Vergata”

e-mail: maura.medri@gmail.com; valeria.dicola@uniroma3.it; samuele.mongodi@gmail.com; lilythvd@gmail.com

ABSTRACT
This paper aims to present some preliminary results of an ongoing study which started from the analysis of the external brick facing of the Aurelianic Walls. The methodology employed was to analyse the brick facing by sampling the brick wall surfaces, drawing consistent areas of 1x1 m² in CAD, based on rectified photographs. These samples were then measured in detail to make a quantitative-statistical analysis of the elements constituting the brick facings, in order to yield standard parameters which could meaningfully describe their features. In this first part of the work, we tried to compare some of the most important imperial brick buildings, each with its own characteristic features. Sampling has covered the Aurelianic Walls extensively, focusing on the early periods of Aurelian and Honorius. After were sampled brick-faced walls from a Neronian building facing the Valley of the Colosseum; the substructures of the Baths of Trajan; and some sectors of the Domus Tiberiana, towards the north slope of the Palatine Hill.
KEYWORDS: Aurelianic Walls; Palatine Hill; Baths of Trajan’s; Domus Tiberiana.

RESUMEN
El objetivo de este documento, es presentar resultados de un estudio en curso, comenzando con el análisis de la cortina externa en ladrillos de los muros Aurelianos. Como metodología de estudio, las capas de ladrillos fueron inspeccionadas a través de muestras de sus superficies, seleccionando partes de 1x1m² dibujados en CAD, utilizando fotos sin distorsión. Dichas muestras, fueron medidas en detalle, para generar análisis de estadísticas cuantitativas de los elementos constituyentes de éstas cortinas, para obtener standars, que pueden describir sus características. En esta primera fase, tratamos de comparar algunas de los más importantes fabricantes de ladrillos imperiales, cada una con características especificas. El muestrario ha cubierto extensivamente los muros aurelianos, concentrándose en los periodos de Aureliano y Honorio. Luego, fueron estudiadas paredes de ladrillos de edificios de época neroniana, frente al valle del Coliseo; subestructuras de las termas de Trajano; sectores de la Domus Tiberiana, en dirección a la falda norte del Palatino.
PALABRAS CLAVE: Muralla Aureliana; Palatino; Termas de Trajano; Domus Tiberiana.

CONTENIDOS

INTRODUCTIONTop

This study started from the analysis of the external brick facing structures of the Aurelianic Walls in Rome, focusing on the early periods of Aurelian (271-275 d.C.) and Honorius (400-402 e 417 d.C.). The main purpose was to argue whether the towers and curtains of the Walls were built using new or recycled bricks. Scholars who have dealt with the Aurelianic Walls have not agreed: Lanciani (1892Lanciani, R. 1892: Le mura di Aureliano e Probo, Bullettino della Commissione Archeologica Comunale di Roma, 20, 1982, pp. 87-111.) stated that new bricks were used, Richmond (1930Richmond, I. A. 1930: The City Wall of Imperial Rome. An account of its architectural development from Aurelian to Narses, Oxford 1930.) that new and recycled bricks were mixed, and Heres (1982Heres, Th. L. 1982: Paries. A proposal for a dating system of late-antique masonry structures in Rome and Ostia, Amsterdam 1982.) and then Mancini (2001Mancini, R. 2001: Le mura aureliane di Roma: atlante di un palinsesto murario, Roma 2001. ), that only recycled ones were used. Given this uncertainty and lack of agreement, based merely on superficial aspects, we have tried to find a safer way for evaluating the data more objectively. For this purpose, we have created a rigorously analytical sampling system, which would allow brick-faced walls to be compared in detail [1].

ROMAN BRICKS: SIZE AND PIECESTop

As is known, brick thickness has been considered indicative of the chronology since the first pioneering quantitative studies by Giuseppe Lugli. Together with the thickness of the mortar beds it constitutes Lugli’s «modulo», corresponding to the thickness of 5 bricks+5 mortar beds (Lugli 1957Lugli, G. 1957: La tecnica edilizia romana con particolare riferimento a Roma e Lazio, Roma 1957.); moreover, the length of pieces visible in the wall facing should allow the size of the bricks originally made for the construction to be determined (Bukowiecki 2010Bukowiecki, E. 2010: La taille des briques de parement dans l'opus testaceum à Rome, in S. Camporeale, H. Dessales, A. Pizzo (edd.), Arqueologia de la costruccion II. Los procesos constructivos en el mundo romano: Italia y provincias orientales, Madrid - Mérida 2010, pp. 143-151.), that is to say the three standard sized bricks produced in Rome since the Neronian-Flavian period: bessalis, sesquipedalis and bipedalis.

Completely new, instead, are the evaluation parameters introduced through measuring the rate of coverage of the wall surface, divided into constitutive material, i.e. bricks, and binding material, i.e. mortar. Moreover, as verified, the estimate of the two mentioned parameters must be correlated to the reconstructed number of pieces in the sampled 1sqm area, because only by comparing both values is the quality of the building as it has been projected really perceivable.

METHOD OF SAMPLINGTop

The method of sampling (Fig. 1) consists of taking measurements from a uniform 1sqm section surface of the wall. Afterwards, the sample is drawn in CAD, by digitising the details from rectified scaled photographs. For each sample, an Excel-sheet is drawn out recording the following numerical data:

Fig. 1. Aurelianic Walls, brick faced external structures, quantitative analysis of 1 sqm sample; left, rectified scaled photograph and CAD drawing; right, estimate of uncut and cut pieces. [View full size] [Descargar tamaño completo]

A. Estimate of pieces uncut by the sample frame

• Number of uncut pieces in the facing
• Thickness and length, measured as distances between midpoints of opposite sides, and CAD area
• Total area of uncut pieces
• Thickness and median length of uncut pieces, median area of uncut pieces

B. Estimate of pieces cut by the sample frame:

• Numbers of pieces cut by the frame
• CAD area of single cut pieces
• Total area of cut pieces

C. Sample global estimate for 1 sqm of masonry:

• Reconstructed number of pieces for 1 sqm (= area of cut pieces : medium area of uncut pieces)
• Total number of bricks (= number of uncut pieces + reconstructed number)
• Brick area of coverage in the facing
• Mortar area of coverage in the facing

Finally, elements compared are:

• The thickness and lengths of only uncut pieces visible in the facing;
• The rate of coverage between bricks and mortar;
• The reconstructed total number of pieces used in 1 sqm.

AURELIANIC WALLSTop

For the Aurelianic Walls (Fig. 2), many sectors were sampled choosing particularly from those masonry parts where evidence of the two principal building phases had been already detected (see Dey 2011Dey, H. W. 2011: The Aurelian Wall and the refashioning of imperial Rome, A.D. 271 - 855, Cambridge 2011.): the Aurelianic phase, less well-preserved, was sampled in sectors A, B (Fig. 3 a-b), and C, the Honorian phase in the sectors G, J and K (Fig. 4 a-b), where it is clearly visible over the Aurelianic structures (sectors by Mancini 2001Mancini, R. 2001: Le mura aureliane di Roma: atlante di un palinsesto murario, Roma 2001. ).

Fig. 2. The circuit of the Aurelianic Wall with the sectors from Richmond 1930Richmond, I. A. 1930: The City Wall of Imperial Rome. An account of its architectural development from Aurelian to Narses, Oxford 1930.. [View full size] [Descargar tamaño completo]

Fig. 3. Aurelianic Walls, sampling of tower B17. Right: general view; left: right side, samples 01-02. [View full size] [Descargar tamaño completo]

Fig. 4. Aurelianic Walls, sampling of tower K07. Right: general view; left: central side, samples 01-04. [View full size] [Descargar tamaño completo]

IMPERIAL BUILDINGS IN ROMETop

Data gathered from the structures of the Aurelianic Walls were compared with ones collected from other buildings, different in chronology and function but all belonging to large imperial projects, yielding the possibility to analyse how the quantitative parameters would have performed in different contexts. For now, the buildings faced in brick which have been compared to the Aurelianic Walls are: two different rooms, A03 (Fig. 5) and A04, from a Neronian building towards the Palatine Hill north slope, facing the valley of the Colosseum; the substructures of the Baths of Trajan’s (Fig. 6); the Hadrianic substructures alongside the Via Tecta, a building dating to the same period (Fig. 7), and another of the Flavian age, all in the area of the Domus Tiberiana.

Fig. 5. Palatine Hill north slope, facing the valley of the Colosseum, Neronian building, room 03, sample 01. [View full size] [Descargar tamaño completo]

Fig. 6. Baths of Trajan’s, substructures, samples 01-04. [View full size] [Descargar tamaño completo]

Fig. 7. Palatine Hill north slope, Domus Tiberiana, the Hadrianic substructures alongside the Via Tecta, samples 01-03. [View full size] [Descargar tamaño completo]

This comparison has yielded some significant data; nevertheless, continuing research will be necessary to broaden the database to possibly confirm these statements.

QUANTITATIVE ANALYSIS: CONCLUSIONSTop

About the bricks, it is possible to affirm that (see Figs. 8, 9, 10):

Fig. 8. Quantitative analysis. 1. Thickness of pieces: Aurelianic sample of tower B17 in comparison with Flavian and Hadrianic structures. 2. Thickness of pieces: Honorian sample of the tower K07 in comparison with Flavian and Hadrianic structures. 3. Length of pieces: Aurelianic sample of tower B17. 4. Length of pieces: Honorian sample of the tower K07. 5. Thickness of pieces: structures of the Bath of Trajan in comparison with Aurelianic sample of tower B17, and Honorian sample of the tower K07. 6. Length of pieces: structures of the Bath of Trajan. [View full size] [Descargar tamaño completo]

Fig. 9. Length of pieces: all samples, except structures of the Bath of Trajan. [View full size] [Descargar tamaño completo]

Fig. 10. Quantitative analysis, reconstructed number of pieces for 1 sqm, bricks area and mortar area of coverage in the facing. [View full size] [Descargar tamaño completo]

• for both the projects, Aurelianic and Honorian, the brick materials are recycled and perhaps chosen according to the length of brick pieces to be used in the facing, mainly in the Honorian structures, which seem to reach a greater homogeneity in the construction (Fig. 8 .4);
• the analysis of the thickness has proved to be a real criterion for arguing whether the structure are built with new or recycled materials, because in the recycled pieces we observed widely spread thickness measurements (i.e. with high variance), often with varying peak values, whereas new ones have a single peak value, coinciding with the mean, around which all observations are distributed with relatively small variance (Fig. 8.1-2);
• the length of pieces in the wall facing seems to yield the same range of variability in all the structures analysed (Fig. 9): the dimensions of the pieces visible in the facing concentrate between >14 cm and <26 cm, perhaps in relation to the use of cuts which are smaller than the ones currently considered standard (see above); at the time, the only exception is the Baths of Trajan where the average length of the brick facing is = or < 24 cm (Fig. 8.6).

Generally, about the Walls it is possible to state that (see Fig. 10):

• the quality of the Aurelianic masonry is different in each sector;
• in the structures of the Aurelianic phase the systematic use of small-sized pieces has presumably produced areas of collapse, mainly in the lower part of the Wall facings, which appear to be particularly reshaped (Fig. 10.1);
• it is still not certain whether the rate of coverage of the constitutive material, which is less than 60%, was provided by the Honorian builders in order to save materials or to reduce the strain on the underside wall, or both; nevertheless, at present this appears to be a recurring element (Fig. 10.2).

Generally, in relation to brick masonries of large imperial buildings in Rome (Fig. 10.3), it might be said that:

• the rate of the constituent material, i.e. bricks, which is greater than 75%, appears to be an extraordinary value; it needs to be verified, through other samplings, whether there are other structures where the use of brick reaches the same rate;
• the quality of a masonry can be judged only by matching both the parameters: number of pieces used and materials rate of the constitutive material.

In this regard, the difference observed between structures made with a medium of 75 pieces/sqm visible in the facing in order to have a coverage of about 70% - as in the Baths of Trajan - and those that reach or exceed the value, but using up to 90 pieces/sqm in the facing, clearly emerges. This difference is obviously in the size of the brick cuts, but also in how far the triangular pieces go into the masonry concrete core, which affects considerably the amount of material used (Fig. 11).

Fig. 11. Ideal samples. Left: 1 sqm with 52,5 pieces 29 x 0,40 cm, 61,7% coverage of bricks, needed 6,5 sesquipedalis. Right: 1 sqm with 68 pieces 22,2 x 0,40 cm, 59,6,3% coverage of bricks, needed 4,5 sesquipedalis. [View full size] [Descargar tamaño completo]

ACKNOWLEDGEMENTSTop

Aurelianic Walls: Sovrintendenza di Roma Capitale, directors Francesco Giovanetti, Gabriella Cimino.

Baths of Trajan: Sovrintendenza di Roma Capitale, director Rita Volpe .

Valley of the Colosseum: Clementina Panella, University Sapienza, Rome, Italy.

Domus Tiberiana: Soprintendenza Speciale per il Colosseo il Museo Nazionale Romano e l'Area Archeologica di Roma, director Mirella Serlorenzi, with the collaboration of Stefano Camporeale, University of Siena, Italy.

NOTESTop

BIBLIOGRAPHYTop