Petrographic study of lime-based mortars and carbonate incrustation ...

Tapete et al._periodico 21/01/14 15:58 Pagina 503

DOI: 10.2451/2013PM0030

Periodico di Mineralogia (2013), 82, 3, 503-527 PERIODICO di MINERALOGIA established in 1930

An International Journal of MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, Archaeometry and Cultural Heritage

Petrographic study of lime-based mortars and carbonate incrustation processes of mural paintings in Roman catacombs

Deodato Tapete1,*, Fabio Fratini1, Barbara Mazzei2, Emma Cantisani1 and Elena Pecchioni3

Istituto per la Conservazione e la Valorizzazione dei Beni Culturali (ICVBC), Consiglio Nazionale delle Ricerche (CNR), Via Madonna del Piano 10, 50019 Firenze, Italy 2 Pontiﬁcia Commissione di Archeologia Sacra (PCAS), Via Napoleone III 1, 00185 Roma, Italy 3 Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via G. La Pira 4, 50121 Firenze, Italy 1

*Corresponding

author: [email protected], [email protected]; currently at Department of Geography, Durham University, Durham, UK

Abstract

The Roman catacombs of St. Tecla, Domitilla and St. Mark, Marcellian and Damasus located south of the centre of Rome were extensively investigated in this research, to study the technology of lime-based mortars and techniques of mural painting used by the ancient fossori to execute the decorated surfaces. The integrated minero-petrographic and microchemical approach exploited here and including thin section observations, X-ray diffraction analyses and diamond anvil cell FT-IR spectroscopy, provided evidence of the technical variability in terms of materials, mortar stratigraphy and application methods. Scientific data suggest potential environmental and operational explanations for certain technical solutions used by fossori in the second half of 4th - early 5th century A.D. We also discuss a wide range of surface carbonate crystallisations, which represent one of the alteration processes affecting the inner surfaces in Roman catacombs, with direct impacts on the preservation and promotion of the painted heritage. The petrographic examination of the textural properties confirms that the morphologies with which the crystallisations occur depend on the hypogean microclimate of exposure. Strategies of preventive conservation should be based on long-term monitoring of the critical microclimate parameters, coupled with periodic diagnostics of the surfaces. Key words: ancient mortars; mural paintings; lime; calcite; crystallization; catacombs.


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Periodico di Mineralogia (2013), 82, 3, 503-527

Introduction

Roman catacombs constituted the subterranean cemeteries of the Christian communities since late 2nd - early 3rd centuries A.D., and nowadays extend for kilometres underneath the ground level, mainly within the tuff strata in the surroundings of Rome, south of the Aurelian Walls (see Fiocchi Nicolai et al., 2009, for a more detailed historical and archaeological background). Besides their religious importance, the catacombs preserve a huge heritage of painted and plastered surfaces, which testify the pictorial abilities of a clerical guild active in Rome in the 2nd - 5th centuries A.D., i.e. the so-called fossori (Latin fossores). They actually were more than mere diggers of underground cavities and corridors for funerary purposes. They were organized according to their specializations including, among others, pictores imaginarii and pictores parietarii, i.e. those who firstly planned the painting and those who afterwards transferred the design onto the wall, respectively. Although an old conception of history of art classified the catacomb paintings as the decline of the classical art, recent studies recognized the artistic skills of these artifices (Mazzei, 2010a), undoubtedly coupled with a high expertise in mortar preparation and plastering. The materials and methods used in the catacombs enhance a technology which built upon the Roman tradition of lime-based mortars (cf. Adam, 1994 for a comprehensive overview of the Roman mortar technology). Nevertheless, fossori developed technical solutions different from those considered ‘usual’ in Roman Age mural paintings, and that was aimed to respond satisfactorily to the specific operational conditions due to the hypogean environment, as well as to organisational needs linked to the funerary destination of the catacombs (specific comments and examples can be found in Mazzei, 2002; 2005) . In this regard, for instance, a

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further proof is provided by the sequence of different phases of plastering discovered by Patrizi et al. (2012) in the catacombs of Domitilla. On the other side, the microclimate parameters typically characterising a hypogeum system have frequently contributed to trigger one of the most common phenomena observable in the catacombs, i.e. carbonate crystallization processes, which in some cases reach up to thick incrustations with optical and structural impacts on the painted surfaces. Crystal growth affects the readability of the pictorial details, and can alter the former mortar composition and structure. Therefore issues rise for the implementation of sustainable strategies of preventive conservation in environments only ideally isolated and confined, as well as of costeffective and selective cleaning treatments. Several tests and campaigns of laser-based cleaning have been recently conducted in catacombs (Patrizi et al., 2010; 2012) obtaining successful results. Nevertheless, there is still the need of assessing the microclimate control on these crystallization phenomena which are actually widespread in different geographic contexts, such as the Jewish catacombs in Rome (Borrelli and Laurenzi Tabasso, 1996), prehistoric caves (Chalmin et al., 2007), North Korean (Mazzeo et al., 2006) and Etruscan tombs (Pallecchi et al., 2009). In the latter case a beneficial effect was attributed to the compact and transparent layer of calcite found over the painting, since this crystallization preserved the colours and enhanced the brightness of the painted layer. Anyway, carbonate crystallisations represent a conservation challenge, starting from their study and consequently to prevent their formation. Concerning the origin, there are also researches that proved biomineralization of carbonates by bacteria in hypogean environments (e.g., Sanchez-Moral et al., 2003; 2004). These two topics, i.e. the technology of lime-


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Petrographic study of lime-based mortars and ...

based mortars and carbonate incrustation processes are dealt with jointly in this paper, through the discussion of the microscopic evidence collected and the reflections drawn after years of investigation. Diagnostic campaigns were carried out in 2008, 2010 and 2012 across three different catacombs Domitilla, St. Tecla and St. Mark, Marcellian and Damasus (Figures 1 and 2) - selected as representative sample of Roman catacombs preserving hypogean wall decoration dated to late 4th - early 5th centuries A.D. The data presented are therefore the result of a cream-off from a wider sample set and relate to a longstanding research that complemented the studies and restorations carried out by the Pontifical Commission for Sacred Archaeology (PCAS), who is responsible for the protection, surveillance, scientific excavation and exploration of the Roman catacombs since 1852. The first part of the paper gives background information about the three catacombs investigated and indicates the location of the samples, while concise reference to the methods and instrumentations is reported in the analytical methods section. The latter also includes a brief description of the monitoring system currently acquiring microclimate data in the catacomb of St. Mark, Marcellian and Damasus, part of which are commented in the discussion section with specific regard to the topic of the paper. Data from the petrographic study form the core of the paper and are presented by each layer of the ‘virtual’ stratigraphy that an archaeologist or conservator can find in catacombs of the period considered here. The technical aspects of the decorative surfaces in Roman catacombs span from simple thin lime wash without any aggregate (the so-called dealbatio) and white finishing layers (marmorino) to proper mural paintings with colours mostly applied onto wet plaster (thereby classifiable as a fresco technique). Various range of morphologies of carbonate crystallisations were also described,

with reference to their mineralogical, textural and stratigraphical features as appreciable on thin section under the polarised light microscope, which in the present work was used as the main diagnostic technique to investigate these alteration processes. Case studies and sampling

Samples of mortars and mural paintings were taken as fragments including the whole stratigraphy (mostly up to the tuff substrate) from walls and vaults of cubicles belonging to different catacombs. We discuss here the following three case studies sited south of the centre of Rome (Figure 1a), as being quite representative of a wide spectrum of different technical solutions used by fossori: 1 the double cubicle P in the catacombs of St. Tecla (late 4th - early 5th centuries A.D.), currently underneath the civil building located in Via Silvio D’Amico, 42 (Figure 1b, c and 2a, c); 2 the Bakers’ Cubicle (i.e. Cubicolo dei Fornai or dei Pistores; second half - end 4th century A.D.) in the catacombs of Domitilla (Figure 1d, e and 2d, f); 3 the cubicles Ai and Af of the catacombs of St. Mark, Marcellian and Damasus (second half 4th century A.D.) (Figure 1d, e and 2g, l). The latter two are located close to Appia Antica (Appian Way), with the second one belonging to the broader complex of St. Callixtus (Figure 1d, e). In particular we sampled un-restored surfaces, prior to any treatment (such as the plasters and mural paintings of the cubicles in the catacombs of Domitilla and St. Tecla, which were subsequently subject to laser cleaning interventions; see Mazzei, 2010b; Ortolan, 2010; Patrizi et al., 2010; 2012) and without historical superimpositions, to avoid any interference on the obtained results with regard to the former


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Figure 1. a) Aerial view of the southern suburban areas of Rome, Italy, with indication of the Roman catacombs investigated in this study (i.e. St. Tecla, Domitilla, and St. Mark, Marcellian and Damasus; modiﬁed from BingMaps; ©2013 Microsoft Corporation). b) Zoomed view and c) geological map 1:10,000 for the area of St. Tecla, and d), e) the corresponding ones for the catacombs of Domitilla and St. Mark, Marcellian and Damasus. The geological maps are derived from Funiciello and Giordano (2005).

condition. We also chose the catacombs of St. Mark, Marcellian and Damasus, since they are currently closed to the public and we could assume the absence of anthropogenic influence on the physic-chemical processes which favour the growth of carbonate crystallisations, extensively covering the decorated surfaces and naked rock (Figure 2b, c and h, l). Table 1 summarises the collected mortar and wall paint samples, with indication of their respective locations.

Analytical methods

Stereomicroscopy (STEREO) and optical microscopy (OM) Morphological and surface features of all the hand samples, as well as the structural relationship between the tuff substrate and levelling layers and that among the mortar layers and marmorino/painted surface, were observed under a ZEISS Stemi 2000-C stereomicroscope (STEREO), equipped with a 10x eyepiece lens




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Figure 2. a) Plan of the catacombs of St. Tecla with the location of the double cubicle P (red square), and b, c) details of its vault and left wall respectively, covered by carbonate incrustations before the laser cleaning. d) Detail of the plan of the catacombs of Domitilla with indication of the Bakers’ Cubicle (red circle) and e, f) views of the mural paintings before the restoration. g) Plan of the regions A and C of the catacombs of St. Mark, Marcellian and Damasus with indication of the cubicle Af, where part of marmorino detached from the vault (h), and the cubicle Ai, i.e. the cubicle of the Twelve Apostles (i), where the mural paintings show crystallization processes (l).


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Table 1. Description of the mortar and wall paint samples collected from the Roman catacombs investigated in this work.

Sample

Material/description

Catacombs

Location

DOM_M5

dealbatio on tuff substrate

Domitilla

Bakers’ Cubicle, arcosolium no. 2

DOM_M6

plaster on tuff substrate

Domitilla

DOM_M7

plaster on tuff substrate

Domitilla

ST1 ST4 ST6 MMD_1 MMD_2

plaster composed of two mortar layers over tuff substrate, with discontinuous incrustation plaster composed of a mortar layer and painted surface, with incrustation and pigments plaster composed of a mortar layer and painted surfaces, with incrustation marmorino over one coarse mortar layer and tuff substrate

marmorino over two coarse mortar layers and tuff substrate

St. Tecla

Bakers’ Cubicle, fallen fragment from the wall Bakers’ Cubicle, fallen fragment from the wall Exterior double cubicle P, right wall, above the arch on the left

St. Tecla

Double cubicle P, left wall, centre of the arcosolium entrance

St. Tecla

Double cubicle P, centre of the vault

St. Mark, Marcellian and Damasus St. Mark, Marcellian and Damasus

and objective magnifications from 0.65x up to 4.5x, and a NIKON DXM1200F digital camera for image acquisition. STEREO was also used to examine the whole stratigraphy and measure the total thickness, by observing the thin sections (30 μm thick). Further investigations at higher magnification were performed on both hand samples and uncovered thin sections (and respective crosssections) with an Olympus BX51M optical microscopy (OM) in reflected light, with 10x eyepiece lens and objectives up to 100x. Pictures were acquired by means of an Olympus DP70 digital scanner.

Cubicle Af (collapse), fragment fallen from the vault plaster

Cubicle Ai (Twelve Apostles), fragment from the vault, left wall, close to the arcosolium

Polarised light microscopy (PLM) Minero-petrographic and fabric features were assessed by observing the uncovered thin sections under a ZEISS AxioScope A1 polarised light microscope (PLM), equipped with 5 Megapixel resolution digital camera and dedicated software Axio Vision, both in plane (PPL) and cross-polarised (XPL) light.

X-ray diffraction (XRPD) Principal mineralogical composition of the powdered samples was determined with a PANalytical diffractometer X’Pert PRO, equipped with X’ Celerator multirevelatory and High Score data acquisition and interpretation


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software, and operating with the following instrumental and measuring conditions: radiation CuKa1 = 1.545Å; 40 kV/30 mA; range 2θ between 3-70°; step size 0.03° and time per step 60.32 s. Mineral identification was performed using ICDD (JCPDS PDF-2) database. Polymorphs of the surface carbonate crystallisations were recognized on selective samples collected by scratching the incrustations under STEREO and avoiding the collection of unrelated material from the substrate. Selective XRPD measurements were also performed on microsamples taken from the different plaster layers, to characterize the mortars at their respective stratigraphic position.

the ongoing pilot study HYPOGEA. This project is aimed, among others, to create a long time series of microclimate data to correlate the environment of exposure with the surface crystallization processes observed and monitored on simulated mortars (Tapete et al., 2012). A full description of the monitoring system and its concept is reported in Tapete et al. (2013) and Cuzman et al. (in press). It is worth mentioning that, differently from previous microclimate monitoring campaigns conducted within other Roman catacombs (cf. Sanchez-Moral et al., 2005a, b), the present one is designed to cover continuously a longer temporal interval of a few years.

Microclimate monitoring system Microclimate data reported in the discussion section were collected by means of a monitoring system currently acquiring air temperature (Ta), relative humidity (RH) and air CO2 content at a sampling frequency of 30 minutes within the cubicle Ai of the Twelve Apostles, in the catacombs of St. Mark, Marcellian and Damasus, since October 2012. This monitoring network was installed based on the outcomes of the research presented in this paper and it is part of

The results of the petrographic study are presented below, following the sequence of the main layers that can be found in a hypogean decoration of late 4th - early 5th centuries A.D., i.e. from inner to outer: Rock substrate Dealbatio Mortars and painted layer Marmorino Surface carbonate crystallisations The rationale of choosing this type of data presentation rather than describing each sample for each catacomb of provenance lies on the purpose to provide the reader with a comprehensive knowledge of the stratigraphic scheme typical of this mortar and plaster technology (a sort of ‘virtual’ stratigraphy reconstructed from information retrieved from a representative set of samples). Therefore the mural painting samples selected from a wider set collected during different investigation campaigns (cf. Table 1) are used here accordingly, to discuss the features of each stratigraphy layer. To ease the link between the transversal discussion across the samples and the specific stratigraphic features of the samples

Diamond anvil cell (DAC) FT-IR spectroscopy Chemical analyses were carried out complementarily to the mineralogical ones by using a Perkin Elmer Mod. System 2000 Fourier Transform Infrared spectrometer (FT-IR), associated to the dedicated software Spectrum One. We investigated the medium IR region 4000 - 370 cm-1, obtaining IR spectra with spectral resolution of 2 cm-1. A Hellma Italia miniature diamond anvil cell (DAC) allowed us to specifically analyse carbonate crystallisations, selectively sampled from the investigated surfaces and located within the sample chamber under STEREO. 32 scans per spectrum were acquired to increase SNR and obtain clearer spectral bands.

Results and discussion


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themselves, the latter are summarised in Tables 2, 3 and 4, referring to the catacombs of Domitilla (DOM), St. Tecla (ST) and St. Mark, Marcellian and Damasus (MMD), respectively. The final paragraph reports some specific comments about the hypogean microclimate, as inferable from the analysis of the first year of data retrieved from the microclimate monitoring currently active in the catacomb of St. Mark, Marcellian and Damasus.

Rock substrate The natural substrate used by fossori to apply plasters and mural paintings is composed of volcanic tuffs (ignimbrites), formed from the deposition of volcanoclastic falls from the Colli Albani volcanic complex (de Rita and Giampaolo, 2006; Giordano and the CARG Team, 2010). The geological map 1:10,000 by Funiciello and Giordano (2005) clearly distinguishes the tuff strata within which the studied Roman catacombs were dug (Figure 1c and e). The area of the catacombs of St. Tecla is characterized by Pozzolane Rosse ignimbrite (RED; Figure 1c), while Domitilla and St. Mark, Marcellian and Damasus catacombs by Villa Senni Unit litofacies Occhio di Pesce (VSN2; Figure 1e). Referring to Giordano and the CARG Team (2010) for full petrographic details of these two lithostratigraphic units, it is worth noting that these tuffs contain horizons the exposed surfaces of which can be frequently from moderate to highly friable, and easily break into small pieces and powder. Hence they are nowadays a critical conservation issue. On the other side, this negative aspect was formerly a positive factor which facilitated fossori to dig cavities following the strata more easily to excavate. Fossori themselves indeed seem to have been aware of these properties of the local rocks, as also testified by the different tools used during excavations (Testini, 1980). Such influence of the lithological variety is particularly appreciable in the catacombs of St.

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Mark, Marcellian and Damasus, where fine- and coarse-grained tuffs can be recognized (Figure 3a, d), thereby confirming what also observed by Sanchez-Moral et al. (2005a) in the catacombs of Domitilla and St. Callixtus. The corridors and cubicles were dug within fine-grained tuffs, while the ceilings show a coarser surface texture with whitish-greyish coloured crystals of analcime highly visible with the naked eye. An extensive sampling from different cubicles, and in particular from the cubicle Ai and Af (Figure 2h, l), led us to obtain a clear distinction of the two varieties in terms of mineralogical composition retrieved from XRPD analyses (Figure 3e). Fine-grained tuffs are mainly constituted by phyllosilicates (e.g., mica) and kfeldspars (sanidine) and secondarily by pyroxenes, while analcime and pyroxenes predominate together with phyllosilicates in the coarse-grained tuffs. Presence of calcite is evidently due to re-crystallisations occurred over the surface and/or within the inner pores, the latter following dissolution/crystallization processes of water infiltration and percolation from the overlying terrains. Although no porosity measurements were performed, since they were outside of the scope of this study, in situ inspections confirmed the presence of secondary porosity in the finegrained tuff, up to channels with circular section reaching the rock surfaces, due to fluid circulation and penetration of plants roots from the gardens at the ground level. Such channels likely played a relevant role to trigger the detachment of the vault plaster in the cubicle Af, as suggested by the close observation of the exposed surface (Figure 2h).

Dealbatio One of the simplest finishes characterizing the inner surfaces of the Roman catacombs is the socalled dealbatio (plural dealbationes), i.e. a thin whitewashing applied directly to the rock substrate and made of pure aerial lime prepared

!

4 mm

coarse-grained tuff (phyll, k-f, Px, An)

rock substrate

3.6 mm 400-600 µm

1 cm max. 50 µm few µm

microsparitic calcite coarse-grained tuff (phyll, k-f, Px, An) micritic lime wash not homogeneous micritic to microsparitic aerial lime + pozzolana micritic calcite microsparitic calcite

Cx

rock substrate

dealbatio

mortar layer

lime with tuff powder

200 µm

1 cm

n.a.

–

–

A

n.a.

n.a.

–

–

inner traces due to application

Interfaces

shrinkage cracks, rounded pores with re-Cx

abundant, with re-Cx within pores

–

–


Porosity

–

n.a. –

–

distinguished based on fabric and interfaces

n.a.

fine tuff powder (50- clearly distinguished 150 µm) from the mortar layer

sub-angular to sub- superficial enrichment rounded, coarse (100 of micritic lime (max. 1/1 - 1/2 µm - 2 mm) volcanic 300 µm) due to slags and Px application pressure

n.a.

n.a.

n.a.

widespread porosity

very few, with regular shape

shrinkage cracks, rounded pores with re-Cx


–

–

sub-angular to sub- superficial enrichment rounded, coarse (100 of micritic lime (max. very few, one long fracture 1/1 - 1/2 µm - 2 mm) volcanic 300 µm) due to parallel to the surface slags and Px application pressure

n.a.

n.a.

B/A

Notation: phyllosilicates (phyll); k-feldspar (k-f); pyroxenes (Px); analcime (An); binder-aggregate ratio (B/A); aggregate (A); carbonate crystallizations (Cx) and re-crystallizations (re-Cx); not applicable (n.a.); absent or negligible (–).

micritic calcite including tuff powder

mortar layer

micritic lime wash

dealbatio not homogeneous micritic to microsparitic aerial lime + pozzolana

max. 1.2 cm

coarse-grained tuff (phyll, k-f, Px, An)

rock substrate 400-600 µm

max. 100 µm

Fe oxides, tuff powder

deposit / finishing layer (?)

incrustations

few µm

micritic lime wash

dealbatio

50-100 µm

Thickness

Composition

Stratigraphy


DOM_M7

DOM_M6

DOM_M5

Sample

Table 2. Summary of the petrographic data for the samples from the catacombs of Domitilla (DOM). Strata are listed from bottom to top.



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ST6

ST4

ST1

sparitic calcite (2) containing silicates and oxides

sparitic calcite (1)

no organic binding medium

brown and red ochre mixed with coarse grains of glauconite and celadonite, applied over a lime-paint carbon black

mostly 50-100 µm (up to 200300 µm due to coarse grains) 400600 µm 20-250 µm

4.8 mm

not homogeneous aerial lime (part micritic and part microsparitic) + pozzolana

n.a.

n.a.

1/2

n.a.

–

sub-angular, bimodal (100-200 µm; 500-1000 µm) pozzolana, rare Px, leucite

n.a.

sub-rounded, bimodal (30-50 µm; 200-400 µm) glauconite and celadonite grains; 20 µm red-coloured oxide grains

sub-angular, bimodal (50-150 µm; 800-1500 µm) pozzolana, rare Px, leucite

n.a.

lampblack deposit separates the paint from Cx

contiguous with the mortar layer well distinguished based on fabric and interfaces traces of smoothing near the surface thin Ca-rich layer runs under the black paint

calcite layer between the two mortars

sub-angular, bimodal (40-80 µm; 400-600 µm) pozzolana, rare Px, leucite sub-angular, tri-modal (40-100 µm; 400-600 µm; 1-1.5 mm) pozzolana, rare Px, leucite

n.a.

Interfaces

A

–

–

abundant, with irregular shape

–

–

few, with irregular shape pores with irregular shape abundant, with irregular shape

Porosity


incrustations

painted layer

mortar layer

up to 50 µm

30-40 µm

very compact microsparitic nearly palisade calcite (2) micritic to microsparitic calcite (3), including oxides

n.a.

50-100 µm

micritic calcite (1) including brown ochre particles

painted layer

incrustations

1/2

1/2

7.2 mm

3 mm

2 mm

not homogeneous microsparitic aerial lime + pozzolana

homogeneous microsparitic aerial lime + pozzolana lime lumps homogeneous micritic aerial lime + pozzolana lime lumps

Thickness B/A

mortar layer

2nd mortar layer

1st mortar layer

Composition

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Sample Stratigraphy

Table 3. Summary of the petrographic data for the samples from the double cubicle P in the catacombs of St. Tecla (ST). Strata are listed from bottom to top. Notation is the same as that of Table 2.


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up to 300 µm

microsparitic calcite (1)

0.5-0.7 cm

5 mm

3-4.5 mm

600-800 µm

120-300 µm

fine-grained tuff (phyll., k-f, Px)

not homogeneous micritic and microsparitic aerial lime + pozzolana it fills the tuff concavity

not homogeneous micritic and microsparitic aerial lime + pozzolana

homogeneous micritic aerial lime + grinded calcite veins

opaque microsparitic calcite

rock substrate

1st mortar layer

2nd mortar layer

marmorino

incrustations

incrustations 10-50 µm

1.2-1.3 mm

not homogeneous micritic aerial lime + re-used marbles and secondarily grinded calcite veins

microsparitic to sparitic calcite (2)

3.5 cm

mortar layer

marmorino

Thickness

Composition

not homogeneous micritic and microsparitic aerial lime + pozzolana lime lumps

Stratigraphy

n.a.

1/3

1/2 1/3

1/2 1/3

n.a.