Monument Future

Materials and Methods

Montesclaros is located approximately 140 km from Fuente de Cibeles and approximately 15 km north of Talavera de la Reina (Toledo). The marbles are very coarse to coarse-grained (crystal size up to 5 mm) and predominantly white, white-bluish in colour. In addition, there are gray, blue-gray, cream, pink and cream marbles.

Alterated marble stones have been extracted from a historic Montesclaros quarry. Samples have been extracted at 5, 10 and 15 mm from their surface (M1, M2, and M3 respectively). These samples have been cut to measure Hg intrusion porosimetry, colorimetry and for thin sections.

Petrographic Microscopy (PM)

Two thin sections of M1, M2 and M3 were observed under a polarisation microscope Leica DM 4500 P, equipped with a digital FireWire Camera Leica DFC 290 HD that worked with the Leica application suite software LAS 4.

Linear microcrack density (LMD) was calculated as the number of microcracks per millimetre. It was found for the first 5 mm from the surface to the inside of another thin section (M4) by counting the number of microcracks intersecting a line of 5 mm.

Mercury Intrusion Porosimetry (MIP)

The micropore size distribution was measured by mercury intrusion porosimetry (MIP; Pascal 140 and Pascal 440 from Thermo Scientific) using irregular samples (ca. 1.0 g).

The analysis was run on M1, M2 and M3 samples oven-dried to a constant weight at 70 °C.

249Colour

After oven-drying at 70 °C to a constant weight, 15 colour measurements were taken in each sample, and the mean for each sample were calculated. Afterwards each sample was submerged in water for an hour and 20 colour measurements were taken again in the wet samples. The colour was assessed with an X-Rite colorimeter (model 964), with 45°/0° geometry and specular component included, D65 illuminant and 8 mm aperture. Colour was expressed using the three chromatic coordinates of CIE-L*a*b* system. The colour difference (ΔE*) between the dry and wet state was calculated for each sample.

Results and Discussion

Petrographically, Montescaros marble is dolomitic with crystals visible to the naked eye with coarse equigranular blasts and granoblastic texture. The blasts’ boundaries are very sinuous, and microcrystals fill cracks and blast boundaries (Fig. 6).

Figure 6: Montesclaros marble. a: Altered surface. b: Parallel Nicol microscopic image. c: Crossed Nicol microscopic image.

The porosity is very low. Fig. 7 shows the pore size distribution for the three samples M1 = 0.09 %, M2 = 0.29 %, M3 = 1.46 %.

Figure 7: Montesclaros marble. Pore size distribution determined by MIP for three tested Montesclaros marble samples (M1, M2 and M3).

LMD decresses with the depth. LMD = 17 in the first millimeter and LMD = 9 in the fourth millimeter. The rhombohedral crystals that M4 presents on the surface are due to the alteration, which accentuates the rhombohedral exfoliation, decreasing the equidistance between the microcracks (Fig. 8, red ellipse).

The alteration produces a reduction in the luminosity of the marble. Marble becomes more yellow and red (table 1). The colour difference (ΔE) between dry and wet state has been 18.0, 14.1 and 12.3 for M1, M2 and M3 respectively.

Table 1: Colour parameters of M1, M2 and M3 L*: lightness; a*: red-green value; b*: blue-yellow value.

Conclusions

Knowledge of stones, historic quarries, modifications suffered in the monuments and causes of 250stone decay are necessary for conservation interventions, especially for reintegration and replacing the original stone with compatible materials.

Figure 8: PM micrograph mosaics of Montesclaros marble (M4 sample) above: parallel Nicols. A 5 mm line indicates the LMD. below: crossedNicols. A red ellipse indicates the tightly rhombohedral exfoliation in the most superficial part of the sample.

Fuente de Cibeles was originally at ground level, protected by 20 granite bollards.

All water-spouts and bollards were removed in 1862, and a perimeter cast-iron fence was installed.

The monument was moved, rotated and raised in 1895 and the rocky platform was expanded to add two putti.

A hand, the keys, the sceptre and the nose of the goddess Cybele were damaged and restored in 1931. The left lion lost its snout and suffered damage to the left front leg and tail in 1936.

The perimeter fence was removed after the Spanish Civil War and the perimeter of the fountain was landscaped. Two granite basins with cascading water from the upper basin to the new external basins were added and the flint rock covering the base of the rocky promontory was removed in 1968. The last restoration was in 2016.

The dissolution of the smaller calcite crystals that border the dolomite crystals causes increased porosity, colour change and superficial disintegration of the blasts.

Tightly rhombohedral exfoliation is in the most superficial part of the alterated sample.

Acknowledgements

Stimulus of Scientific Employment Individual Support 2017. CEECIND/03568/2017. Fundação para a Ciência e a Tecnologia of Portugal (FCT).

References

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De Wever P., Baudin F., Pereira D., Cornée A., Egoroff G., Page K. 2016. The importance of Geosites and Heritage Stones in Cities. Geoheritage doi: 10.1007/s12371-016-0210-3.

López de Azcona M. C., Fort González R., Mingarro Martín F. 2002. Conservation of the stone in Cibeles Fountain, Madrid (Spain). Materiales de construcción doi: 10.3989/mc.2002.v52.i265.345.

Luque A., Cultrone G., Mosch S., Siegesmund, S., Sebastian, E., Leiss B. 2010. Anisotropic behaviour of White Macael marble used in the Alhambra of Granada (Spain): The role of thermohydric expansion in stone durability. Eng Geol doi: 10.1016/j.enggeo.2009.06.015.

Ondrasina J., Kirchner D., Siegesmund S. 2002. Freeze–thaw cycles and their influence on marble deterioration: a long-term experiment, Geological Society, vol 205. Special Publications, London, pp 9–18.

Siegesmund S., Török A. 2011. Building stones. In: Siegesmund S, Snethlage R (eds) Stone in architecture-properties, durability. Springer, Berlin, Heidelberg. Pp. 11–96. Doi: 10.1007/978-3-642-14475-2_2.

251

WATER CONTENT ESTIMATION USING NON-DESTRUCTIVE TOOLS APPLIED TO ARCHAEOLOGICAL MATERIALS

Oriol Sánchez Rovira^{1, 2, 3, 4}, David Giovannacci^{1, 2}, Jean-Didier Mertz^{1, 2}, Jérôme Wassermann³, Béatrice Ledésert⁴, Ronan Hébert⁴, Yannick Mélinge³

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 LRMH, Laboratoire de Recherche des Monuments Historiques, Ministère de la Culture et de la Communication, 29 rue de Paris, 77420 Champs-sur-Marne, France

2 Centre de Recherche sur la Conservation (CRC), Muséum national d’Histoire naturelle, CNRS, Ministère de la Culture, 36 rue Geoffroy Saint Hilaire, 75005 Paris, France

3 L2MGC ; Laboratoire de Mécanique et Matériaux du Génie Civil-EA4114, CY Cergy Paris Université, 5 mail Gay Lussac, Neuville sur Oise 95031 Cergy-Pontoise, France

4 GEC, Laboratoire Géosciences et Environnement Cergy, EA 4506, CY Cergy Paris Université, 1 rue Descartes, Neuville sur Oise 95031 Cergy-Pontoise, France

Abstract

Water content in stone is of primary relevance for the preservation of cultural heritage. High water content promotes the development of microorganisms and causes mechanical or physico-chemical alterations by swelling/shrinkage or dissolution/recrystallization of salt. The identification and then the control of the water transfer remain important to assess the risk of damage. A good prediction of the water content helps to develop some preventive action dedicated to the conservation of Heritage.

The most accurate methods to measure water content require sampling and can only exceptionally be used. Few methods are non-destructive, but their accuracies are often limited.

The aim of this study concerns the application of Non-Destructive Technics (NDT) to determine the water distribution within the building masonry, in particular the infrared imaging and electrical method.

The electrical method enables to image the spatial and/or temporal variation of the electrical properties related to the water content distribution in the materials while infrared thermography provides the boundary limit conditions by the means of surface thermograms. Those methods are performed for several samples used as building materials in the archeological site of Vaux de la Celle (Genainville, France). The site concentrates various structures built between the 2nd and 4th century AD. The temple structure is constructed at the lowest part of the valley with its foundations in direct contact with the vadose zone where the water table fluctuates. Reproducibility and reliability are also provided through several experimental configurations.

Introduction

The damage of the stone building materials is closely related to change in the equilibrium between the stone and the atmosphere. Thus, instabilities introduced by the environmental variations are the driving force of stone damage. In such cases, the biggest threats to the stone are related to those 252cyclic factors (Benavente et al., 2008), which are related to water and heat transfer.

The patterns of stone deterioration (ICOMOS ISCS, 2008) depend on the nature of the material and the weathering processes and are mainly linked with water content in its different aspects as its spatial and temporal distribution.

The most common methods used to characterize water content and distribution within the porous materials need sampling in order to determine it by gravimetry (EN 16682, 2017). However, for heritage building, due to the invasive nature of the direct measurements approach it is better to use indirect non-destructive methods to characterize the water content.

Indirect methods analyze the variation of a physical property and/or quantity of the materials which can be exploited to characterize moisture content. Nuclear magnetic resonance (NMR), evanescent-field dielectrometry (EFD) (V. Di Tullio et al., 2010), infrared thermography and electrical resistivity surveys are the most common indirect non-destructive methods of sensing the water content of porous media.

Infrared thermography allows to image the temperature map of the surface of the materials. Depending on endogenic or exogenic conditions, the temperature of the damped areas may vary from the dried parts. These thermal behaviors make the infrared thermography a powerful imaging method to make qualitative measurements of water content distribution. However, because the relationship between temperature and water content is highly affected by the material properties and the environmental conditions, quantitative measurements need calibration curves generated through controlled laboratory conditions (Grinzato et al., 2011).

Resistivity methods are mostly used in geophysical survey for geological and archaeological applications. However, since they monitor the resistance of the material to the passage of an electric current, it can be applied to characterize water content in porous building materials. Indeed, this resistance is directly influenced by water content, its salinity, its temperature, as well as by its distribution within the pore network (Hassine et al., 2018). As for the infrared imaging method, due to the complex relationship between the different parameters affecting the resistivity measurements, quantitative analysis needs prior calibration data.

Thanks to resistivity imaging methods providing volumetric information, and infrared thermography providing information from surface, the combination of the two methods can provide complementary information to characterize water content and its distribution.

The work presented in this paper is dedicated to highlight the complementarity of the infrared and electric imaging methods. Such methods are used to characterize water content variations in limestone from an archeological site. The final goal is to establish the basis of a non-destructive method-based water content characterization protocol in situ.

Materials

The stone samples used in this research come from a Gallo-Roman temple which is part of the greater archaeological complex of Vaux-de-la-Celle located in the bottom of a valley at 60 km at the north west of Paris.

Figure 1: Archaeological site location.

The main structures of this archaeological site (theatre, sanctuary, basins…) were erected during the 2nd century A. D. (Vermeersch, 2009). The particular hydrogeological context characterizing this archaeological site is the presence of a water table that appears to be at or near the ground surface level in the lower topographical area of the valley where the sanctuary complex is erected (BRGM, 2531974). Since the foundations of the Temple might be in direct contact with the ground water, the erected parts of the structure are affected by capillary rise phenomena. Thus, from all major structures it has been chosen to analyze the limestone material from the Temple, which is the most representative building material of the walls from the architectural complex constituting the sacred area.

In order to understand the flow properties of the water through the porous media the petrophysical properties have been characterized (porosity, permeability and pore size distribution). Obtained results are presented in the table 1 and in the figure 2.

Table 1: Limestone petrophysical properties results.

Figure 2: Pore size distribution of the limestone sample.

The limestone samples from the temple are characterized by a high porosity and permeability and a bimodal pore size distribution showing a major peak corresponding to a macropore radius of 10 µm.

Methodology

Considered the specificity of the studied heritage, non-invasive techniques have been investigated in the project with the use of infrared thermography and electric resistivity measurements. Those technics help us to investigate the water content in a porous media at real scale and to prevent against damages of the structures.

To illustrate the methodology, the focus is done with the use of IR thermography.

The technic is firstly calibrated and then is used reversely to characterize a specific situation.

IR Thermography calibration

With the aim to highlight a functional link between water content and surface temperature, a FLIR microbolometer (sc655, spectral range of 7.5–13 µm and ±2 °C (or ±2 % of reading) accuracy is used and vertically positioned in an apparatus to ensure the calibration of the focal length and the orientation of the camera. A parallelepiped sample is positioned near a perfect reflector on a table. The apparatus is at room temperature with a restriction of the light noise. The selected geometry of the sample is done to prevent against optic default.

Dimensions of the limestone samples are given as follow (38.9 ±0.4 mm width, 62.9 ±0.4 mm height and 10.5 ±0.6 mm thick).

The samples have been dried in a stove at 65 °C until mass became stable. Then partial saturation and homogenization of the sample have been done from ≈0 % to ≈100 %. The lowest saturation corresponds to the amount of absorbed water at environmental conditions while the maximum water content is represented by the samples completely saturated using the 48 h porosity protocol.

The partially saturated samples are then placed in a chamber with a relative humidity of 100 % for a duration of 15 days to allow the diffusion of the water within the volume of the sample to obtain a homogeneous distribution.

Illustration of the IR Thermography measurement is given in the figure 3. For each sample, several thermograms are registered in order to statistically determine its relevance. In the thermal scene, the perfect reflector (crumpled aluminum-foil) has a predictable emissivity that allows to estimate and filter the environmental noise affecting the measurements. Moreover, such a reference is used to 254calibrate the apparent temperature delivered by the infrared camera.

Figure 3: Thermogram of the sample and the crumpled aluminum-foil (reference) showing the different ROI tested.

The statistical relevance of the data is determined by the temperature analyses of each region of interest (ROI) of all the samples. Such analyses are done over ten images of the thermal scene. The statistical moments (mean, standard deviation, Skewness and Kurtosis) are calculated. The analysis of those parameters allows to determine the best ROI that will be used to define the calibration function representing the thermal response over the water content variation.