Different treatments (consolidation and water-repellent) were applied on samples of marble and granite from the Front stage of the Roman Theatre of Merida (Spain). The main goal is to study the effects of these treatments on archaeological stone material, by analyzing the surface changes. X-Ray Fluorescence and Laser-Induced Breakdown Spectroscopy techniques, as well as Nuclear Magnetic Resonance have been used in order to study changes in the surface properties of the material, comparing treated and untreated specimens. The results confirm that silicon (Si) marker tracking allows the detection of applied treatments, increasing the peak signal in treated specimens. Furthermore, it is also  possible to prove changes both within the pore system of the materialand in the distribution of surface water, resulting from the application of these product

Alberghina, M.F.

Bustamante Montoro, Rosa

Fontana, D.

Pérez Ema, Natalia

Archivo Digital UPM

NDT for the detection and characterization of superficial treatments on stone material from archaeological sites of Merida (Spain)

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IND 73 
NDT FOR THE DETECTION AND CHARACTERIZATION OF 
SUPERFICIAL TREATMENTS ON STONE MATERIAL FROM 
ARCHAEOLOGICAL SITES OF MERIDA (SPAIN) 
N. Perez-Ema1,4,5, MF. Alberghina2, D. Fontana2, A. Longo2, M. Marrale2, L. Tranchina3, M. 
Brai2,3 R. Bustamante4, M. Alvarez de Buergo5 
1 CEI Campus Moncloa. UCM-UPM. Edif. Real Jardín Botánico Alfonso XIII, Avda. Complutense, 
s/n 28040 Madrid. Spain 
2 Dipartimento di Fisica e Chimica – Università degli Studi di Palermo – Viale delle Scienze, Ed. 
18, 90128  Palermo, Italy 
3 Laboratorio di Fisica e Tecnologie Relative - UniNetLab  – Università degli Studi di Palermo – 
Viale delle Scienze, Ed. 18, 90128  Palermo, Italy 
4 Escuela Tecnica Superior de Arquitectura, ETSAM, Universidad Politecnica de Madrid. Avda. 
Juan de Herrera, 4. 28040 Madrid, Spain. 
5 Instituto de Geociencias, IGEO (CSIC, UCM). C/ José Antonio Novais, 12. 28040 Madrid. Spain 
 
Abstract  
Different treatments (consolidation and water-repellent) were applied on samples of marble 
and granite from the Front stage of the Roman Theatre of Merida (Spain). The main goal is 
to study the effects of these treatments on archaeological stone material, by analyzing the 
surface changes. X-Ray Fluorescence and Laser-Induced Breakdown Spectroscopy 
techniques, as well as Nuclear Magnetic Resonance have been used in order to study 
changes in the surface properties of the material, comparing treated and untreated 
specimens. The results confirm that silicon (Si) marker tracking allows the detection of 
applied treatments, increasing the peak signal in treated specimens. Furthermore, it is also 
possible to prove changes both within the pore system of the materialand in the distribution 
of surface water, resulting from the application of these products.   
 
Introduction 
The work described arises from the collaboration between different research groups, under 
the framework of the PICATA programme (CEI Moncloa):   AIPA Research group (ETSAM, UPM),   Petrology applied to Heritage Conservation, Instituto de Geociencias, IGEO (CSIC, 
UCM),   Laboratorio di Fisica e Tecnologie Relative- UniNetLab (UNIPA).  
The overall goal of the project is the study of the effects of conservation treatments applied 
on stone material from selected archaeological sites of Merida (Spain), in terms of 
superficial changes and effectiveness. In this sense, one of the first premises is 
characterizing the surface of the treated and untreated material in order to determine 
changes in physical and chemical properties.   
Objectives 
Selected archaeological sites of Merida, the Roman Theatre (Figure 1) and the House of 
Mitreo, have been subject of several restoration intervention, where different treatments 
were applied to natural stone material, including artificial stone materials - mortars, wall 
paintings and mosaics -. Some of these interventions have been accurately registered, some 
others not. In this sense one of the objectives of this work is to test the effectiveness of 
different analysis techniques to characterize the surfaces of new treated stone material. 
This will also allow us subsequently working on the detection of treatments applied on site, 
by using portable equipments. 
 
     
Figure 1. Overview of the Roman Theater and Mitreo’s House 
 
Materials and methodology 
Samples of white marble (Figure 2) and granite (Figure 3) which are present  in the Front 
stage of the Roman Theatre (1), have been subject of different treatments documented in 
past interventions (2):  Consolidative treatment with ethyl silicate TEOS (Estel 1000©)> S  Siloxane water-repellent treatment (Tegosivin HL©)> H  Consolidative + Water-repellent treatments (double application)> S+H  Some other samples have been left as a reference with no treatment> N 
 
    
Figure 2 and 3. White marble specimens (4x4cm) mainly used in cornices and 
capitals. Granite specimens (5x5cm) are found in the basements of the Front stage. 
 
In all cases two lateral sides of specimens were free of treatment, intended to imitate the 
characteristics of the pieces preserved in situ, where only some faces are exposed. 
Two techniques were used for the characterization and detection of superficial treatments: 
X-Ray Fluorescence (XRF instrument-Bruker AXS, mod. ARTAX 400). XRF 
measurements allow us to reveal the presence of treatment by monitoring the presence of 
silicon (Si) marker. The acquisition parameters used were voltage-40kV, X-ray tube 
current at 700 µA with an acquisition time of 200 seconds, under a flow of helium gas. 
LIBS enables to analyse the stratigraphic sequence of different layers starting from the 
treated surface up to the bulk, thus, obtaining elemental information, not just the surface, 
but also in the inner layers (3, 4). The portable experimental apparatus for the LIBS 
measurements is the MObile Dual-pulse Instruments for LIBS material analysis (MODì) 
from Marwan Technolgy. It integrates a double Q-Switched Nd:YAG laser (Lotis, mod. 
LS-2131D) emitting, with pulsed emission, two collinear laser pulses at 1024 nm, whose 
energy can be set by varying the lamp flash voltage. The pulsed energy, focused on the 
sample surface by a 100 mm focal length Plano-convex lens, is within 50-120 mJ with a 
maximum repetition rate of 10 Hz and a reciprocal delay adjustable from 0 to 60 μs. The 
lateral spatial resolution of the LIBS measurements corresponds to the dimensions of the 
microcrater left by the laser on the sample surface. The microcrater diameter, whose 
dimensions are related to the laser energy, does not exceed 100 microns. Acquisition 
parameters selected for LIBS measurements were: GPD-2 µs, GPW-1 µs, and laser pulse 
energy-18J. 
Besides, with Nuclear Magnetic Resonance the water content in the surface of the sample 
(depth of 2mm-5mm), and its distribution in the porous system have been measured. It 
allows the determination of the dimension of the pores in the surface material, and the 
interconnection between them (5). NMR measurements were performed by a single-side 
relaxometer mq-ProFiler (Bruker Biospin ®, Italy) operating at about 15 Mhz. Spin-spin 
relaxation time T2 of water protons were assessed by CPMG-8000 sequences with an echo 
time TE of 44 microsec.  Each measurement is the average of 1024 accumulations acquired 
with a recycle delay of 2s. Specimens where vacuum-sealed and then saturared? with 
distilled water in order to fill the pores. All measurements were performed at room 
temperature and covering the sample by a film in order to avoid any leak of water. We 
have analysed for each sample the spin-spin relaxation trends of the NMR signal recorded 
and then calculated the distributions of the spin-spin relaxation time (T2) for the various 
samples studied.  
All these equipments are part of the UniNetLab within Palermo University. 
Measurements were performed in different points of three diverse faces of the samples 
(Figure 4), taking an average of three shots per point regarding XRF and LIBS (27 
measurements per specimen), and two measures per face in NMR case.  
 
 
 
 
   
 
 
 
Figure 4. Schematic drawing in which the measured points in the marble and granite 
samples are indicated 
 
 
 
Face A (weathered treated 
face) 
Face E (non-
treated face) 
Face B 
(fresh-cut 
treated face) 
1
5
4
 Results 
XRF 
General results regarding marble samples confirm that the amount of Si varies increasingly 
when comparing treated and untreated samples, being purely clear only when confronted 
with S+H, and moderately distinguishable between untreated, and treated with S or H 
(Figure 5). If we analyze changes between the different faces of the specimen we can see 
that the Face A, being exposed to weathering, already has a natural silicon content, due to 
the rock impurities, which however increases in treated surfaces. Another observation is 
that the treatment has penetrated, or has been crept during application, in the free 
treatments sides (Face E), been very similar to treated Face B, and therefore can be 
considered that the entire specimen is treated. 
 
 
Figure 5. Box-and-Whisker graphs showing differential analysis of marble specimens 
(Si marker): (a) comparison of samples with data gathered from all faces; (b) 
weathered faces data, (c) freshly-cut inner faces data and (d) non-treated faces. 
 
Regarding granites, the compositional heterogeneity of the grains is much higher than the 
difference between treated and untreated areas. Therefore the XRF analysis is 
unrepresentative in this sense, however it allows us to specify the majority of the element 
of grains (three grain types are distinguished) (Figure 6). 
 
a b 
c d 
 Figure 6. Principal Component Analysis (PCA) of XRF granite data from 
measurements of the principal macroscopic grains identified. Elements constituting 
grains can be associated to quartz, biotite and calcium 
 
 
LIBS 
LIBS measurements confirm what XRF showed, presenting higher presence of Si on 
treated surface. In addition, the Si content decreases as deepens the surface layer (sequence 
up to 15 shots) (Figures 7 and 8). 
 
0 2 4 6 8 10 12 14 16 18 20 22
0
3
6
9
12
15
18
21
24
 MTM30_NT
 MTM30_S
 MTM30_H
 MTM_30_S+H
I(
S
i)
/I
(S
i 
N
T
 b
u
lk
)
A
         
287.0 287.5 288.0 288.5 289.0
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
In
te
n
s
it
y
Wavelength (nm)
 MTM27-3(SH)_B1_S3
MTM27-3(SH)_B1_S14
 
 
 
 
 
NMR 
Results of NMR measurements performed on granite sample are shown in figures 9, 10 
and 11, where comparison of treated and untreated specimens have been carried out 
analyzing the distribution of the spin-spin relaxation time (T2).   
 
 
 
 
 
 
 
Figure 7. LIBS signal comparison among 
MTM30 samples undergone to different 
treatments. The  I(Si)/I(Si NT bulk) ratio 
is reported for each specimen as function 
of laser shot number 
 
Figure 8. LIBS spectra shows the intensity 
of Si peak detected by LIBS at different 
points at the stratigraphic sequence: 
MTM27-3(SH) sequence 3 vs sequence 14. 
 
  
 
 
 
 
 
 
 
 
 
 
Figure 9. The treatment with a water-repellent product involves a reduction of the 
presence of water for T2 values smaller than 370 ms.  
 
 
 
 
 
 
 
 
Figure 10. The effect of the silicate-based treatment consists in a reduction of the 
microporosity in favour of mesoporosity. The presence of TEOS reduces the 
accessibility of water to micropores in granite specimens.  
0.00E+000
1.00E-001
2.00E-001
3.00E-001
4.00E-001
5.00E-001
6.00E-001
7.00E-001
8.00E-001
9.00E-001
1.00E+000
1.00E-0021.00E-0011.00E+0001.00E+0011.00E+0021.00E+0031.00E+004
T2(ms)
MTG16N vs MTG16SH
MTG16_8_N
MTG16_6_SH
 
Figure 11. The combined effect of TEOS and water-repellent treatments involves a 
reduction of the average pore size because both obstruct the mesopores and the 
percentage of smaller pores increases respecting the untreated specimens.  
 
0.00E+000
1.00E-001
2.00E-001
3.00E-001
4.00E-001
5.00E-001
6.00E-001
7.00E-001
8.00E-001
9.00E-001
1.00E+000
1.00E-0021.00E-0011.00E+0001.00E+001.00E+0021.00E+0031.00E+004
T2(ms)
MTG16N vs MTG16S
MTG16_8_N
MTG16_4_S
0.00E+000
2.00E-001
4.00E-001
6.00E-001
8.00E-001
1.00E+000
1.20E+000
1.00E-0021.00E-0011.00E+0001.00E+0011.00E+0021.00E+0031.00E+004
MTG16N vs MTG16 H
MTG16_8_N
MTG16_5_H
T2(ms)
 Conclusions 
Regarding the identification of treated surfaces, we have found that only the mineralogical 
homogeneity of marble allows differentiation between the treated and untreated specimens. 
It is due to, not only this homogeneity, but to its high granular compaction, which 
provoked the treatment to remain in surface, while in the case of granite intergranular 
spaces are considerable, allowing greater penetration of the treatment and, therefore, the 
lower superficial retention (this is confirmed by NMR tests). Taking into account that Si is 
the marker for the detection of treatments; another limitation is the presence of Si on the 
exposed surface of the stone material (Face A on marble samples). It can be concluded 
that, despite these considerations, increasing of Si is distinguishable between treated and 
untreated samples, though being only unmistakable when comparing N and S+H 
specimens. However, when this comparison is made over recently cut faces the differences 
in Si content become much clearer, enabling to distinguish between the three different 
treatments and the untreated specimens, both in the case of XRF as in LIBS analysis. 
 Comparing the hydric behaviour of treated and untreated material, some general 
conclusions are: 
- Water-repellent treatments, in most cases, tend to homogenize the presence of water in 
the pores of varying size and reduce the overall presence of water. 
- Silicate based treatments  involves a variation in the distribution of water between pores 
of varying size but does not significantly change the presence of water . 
- The heterogeneity of each sample (and therefore its water absorption capacity) must be 
considered, varying when considering one face or another of each specimen.   
- In all cases, the water permeability of the treated samples markedly reduces. 
 
References 
(1) Pizzo, A. (2010). “Las técnicas constructivas de la arquitectura pública de Augusta 
Emerita”, Anejos de AEspA Vol. 56, Madrid. ISBN: 978-84-00-09181-1. 
(2) VV.AA (1996) Teatro Romano. Proyecto de intervención en el escenario. Mérida. 
Consejería de Cultura, Junta de Extremadura. 
(3) Maravelaki-Kalaitzaki, P. Anglos, D. Kilikoglou, V. Zafiropulos, V. "Compositional 
characterization of encrustation on marble with laser induced breakdown 
spectroscopy". Spectrochimica Acta, part Â, 56(6), 2001, pp. 887-903.  
(4) Maravelaki-Kalaitzaki, P. Kallithrakas-Kontos, N.  Gioutantis, Z. Maurigiannakis, S. 
Korakaki, D. "A comparative study of porous limestones treated with silicon-based 
strengthening agents, Progress in Organic Coatings". Organic Coatings, 62, 2008, pp. 
49–60. 
(5) Capitani, D., Di Tullio, V., Proietti, N. “Nuclear Magnetic Resonance to characterize 
and monitor Cultural Heritage”. Progress in Nuclear Magnetic Resonance 
Spectroscopy. Vol. 64, pp. 29-69, July 2012 
 
 
 
Acknowledgements 
This research was supported by the PICATA program from Campus of International 
Excellence of Moncloa. Our acknowledgements to GEOMATERIALES programme 
(S2009-MAT1629) within Instituto de Geociencias (CSIC, UCM), to AIPA programme 
(ETSAM, Polytechnic University of Madrid) and to the Research Group financed by the 
Complutense University of Madrid "Alteration and Conservation of heritage stone 
materials” (ref. 921349). We thank the Consorcio de la Ciudad Monumental de Merida, for 
its collaboration in this research. 
 
 
 


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NDT for the detection and characterization of superficial treatments on stone material from archaeological sites of Merida (Spain)

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