Evaluation of Portable Raman for the Characterization of Salt Efflorescences at Petra, Jordan

This art icle was downloaded by: [ Com plut ense Universit y of Madrid] , [ Dr Miguel Gom ez- Heras] On: 13 February 2012, At : 07: 05 Publisher: Taylor & Francis I nform a Lt d Regist ered in England and Wales Regist ered Num ber: 1072954 Regist ered office: Mort im er House, 37- 41 Mort im er St reet , London W1T 3JH, UK Spectroscopy Letters: An International Journal for Rapid Communication Publicat ion det ails, including inst ruct ions f or aut hors and subscript ion inf ormat ion: ht t p: / / www. t andf online. com/ loi/ lst l20 Evaluation of Portable Raman for the Characterization of Salt Efflorescences at Petra, Jordan Paula López-Arce Heras a a b a , Ainara Zornoza-Indart , Mónica Alvarez de Buergo a a , Carmen Vázquez-Calvo & Raf ael Fort a , Miguel Gomez- a Inst it ut o de Geociencias (CSIC-UCM), Madrid, Spain b Depart ament o de Pet rología y Geoquímica, Universidad Complut ense de Madrid, Madrid, Spain Available online: 17 Oct 2011 To cite this article: Paula López-Arce, Ainara Zornoza-Indart , Carmen Vázquez-Calvo, Miguel Gomez-Heras, Mónica Alvarez de Buergo & Raf ael Fort (2011): Evaluat ion of Port able Raman f or t he Charact erizat ion of Salt Ef f lorescences at Pet ra, Jordan, Spect roscopy Let t ers: An Int ernat ional Journal f or Rapid Communicat ion, 44: 7-8, 505-510 To link to this article: ht t p: / / dx. doi. org/ 10. 1080/ 00387010. 2011. 610411 PLEASE SCROLL DOWN FOR ARTI CLE Full t erm s and condit ions of use: ht t p: / / www.t andfonline.com / page/ t erm s- and- condit ions This art icle m ay be used for research, t eaching, and privat e st udy purposes. 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The publisher shall not be liable for any loss, act ions, claim s, proceedings, dem and, or cost s or dam ages what soever or howsoever caused arising direct ly or indirect ly in connect ion wit h or arising out of t he use of t his m at erial. Downloaded by [Complutense University of Madrid], [Dr Miguel Gomez-Heras] at 07:05 13 February 2012 Spectroscopy Letters, 44:505–510, 2011 Copyright # Taylor & Francis Group, LLC ISSN: 0038-7010 print=1532-2289 online DOI: 10.1080/00387010.2011.610411 Evaluation of Portable Raman for the Characterization of Salt Efflorescences at Petra, Jordan Paula López-Arce1, Ainara Zornoza-Indart1, Carmen Vázquez-Calvo1, Miguel Gomez-Heras1,2, Mónica Alvarez de Buergo1, and Rafael Fort1 1 Instituto de Geociencias (CSIC-UCM), Madrid, Spain 2 Departamento de Petrologı́a y Geoquı́mica, Universidad Complutense de Madrid, Madrid, Spain ABSTRACT The advantages of using portable Raman spectrometer equipment, such as avoiding sampling and providing a higher number of results, are contrasted with some of its shortfalls that make other analytical techniques necessary to characterize salt efflorescences on historic buildings. In-situ analyses of salt efflorescences were carried out with a portable Raman at both the so-called ‘‘Silk Tomb’’ and ‘‘Monastery’’ rock-cut façades at the Archaeological Park of Petra (Jordan). Samples were also taken to be analyzed in the laboratory with X-ray diffraction (XRD) and Environmental Scanning Electron Microscope with Energy-Dispersive X-ray spectroscopy and Cathodoluminescence (ESEM-EDS-CL). This research shows the pros and cons of these analytical techniques—and how they complement each other—to identify the occurrence and determine the origin of soluble salts, which are deeply damaging these rock-cut monuments by salt crystallization processes. KEYWORDS cathodoluminescence, nondestructive testing, portable Raman, salt efflorescence, stone decay INTRODUCTION This submission was presented during the CORALS-2 Meeting on Micro-Raman Spectroscopy and Luminescence Studies in the Earth and Planetary Sciences, which was held between May 19th and 21st, 2011, Madrid, Spain. This is an invited paper for a special CORALS-2 GEO-SPECTROSCOPY issue of Spectroscopy Letters. Received 22 June 2011; accepted 6 July 2011. Address correspondence to Paula López-Arce, Instituto de Geociencias (CSIC-UCM), C= Jose Antonio Nováis 2, Madrid 28040, Spain. E-mail: plopezar@geo.ucm.es The ancient city of Petra, which is in the Shera Mountains of Southwest Jordan, was built by the Nabateans, with the influence of other civilizations, around 2000 years ago. Hundreds of monuments were carved mainly in the mid and upper Cambrian Umm Ishrin Sandstone and the Ordovician Disi Sandstone,[1] also known as Nubian sandstone (quartzarenite). Regarding the soils derived from Nubian sandstone, Singer and Amiel[2] mention that these can contain carbonates and soluble salts, including gypsum. The weather in this region is very dry, and the rain is seasonal and torrential in winter.[3–5] Signs of water runoff and alveolar weathering caused by rainwater, wind, and salts can be observed on the outcrops at the mountains of the archaeological park of Petra (Fig. 1a–b). Most of the rock-cut monuments in Petra are affected by salt crystallization processes causing spalling, flaking, and salt efflorescences on the surface of the rocks (Fig. 1c–f). There are many studies on the decay processes of the rock-cut monuments of Petra.[3,5,6] However, there is a need for deepening the knowledge of 505 Downloaded by [Complutense University of Madrid], [Dr Miguel Gomez-Heras] at 07:05 13 February 2012 FIGURE 1 Archaeological Park of Petra (Jordan). (a) Traces of water runoff on the rock; (b) Alveolar weathering; (c) ‘‘Silk Tomb’’; (d) ‘‘The Monastery’’; (e) and (f) Spalling and flaking on these monuments. (color figure available online.) weathering processes and the origin of salts, so strategies for preventive conservation to slow down weathering will become more efficient. One area still in development in the case of Petra is the use of data obtained from portable nondestructive analytical techniques (NDT), as they could allow, without any sampling or damage of structures, detection of on-site patterns of salts variability and better-informed decisions based on a wider breadth of data. In-situ monitoring of salts using portable Raman is also a useful tool for the assessment of desalinization or cleaning treatments of artworks, since it may help monitoring the progressive salts removal and deciding when to stop the treatment.[7] Assessment of stone deterioration by salt crystallization or identification of inorganic degradation products of stones, mortars and wall paintings can also be carried out with portable Raman equipments.[8,9] Portable Raman is a useful technique for approaching nondestructive analyses in situ on heritage buildings. Although arguably Raman is not the most suitable technique to analyze salts (as ionic bonds are frequent in many types of common salts), this paper shows this is still a useful technique for a quick in situ approximation to the composition of salt efflorescences with ionic-covalent bonds present on heritage buildings, being the absence of sampling the most important advantage of this technique, which is of paramount importance when considering structures where any sampling means an irreparable artistic and cultural loss. OBJECTIVE The aim of this research is to evaluate the use of portable Raman spectrometer in case studies of P. López-Arce et al. the rock-cut monuments of Petra (Jordan). This paper shows its pros and cons in comparison to other analytical techniques and how they can complement each other to identify the occurrence and determine the origin of salt efflorescences, which are deeply damaging these monuments. MATERIALS AND METHODS Over 100 in-situ measurements were made with a portable Raman (Inspector RamanTM, Delta Nu1, USA) with a 785 nm diode laser for excitation, output power of 120 mW, and integration times between 5 and 20 s (depending on the type of materials and substrate). These measurements were carried out on the first 2 m from the bottom of the ‘‘Silk Tomb,’’ one of the façades facing West forming part of the Royal Tombs complex, which is situated on high ground at the widest and most exposed area of the main valley (Fig. 1c); and the ‘‘Monastery,’’ which is located in a secluded area up in the mountains (Fig. 1d) of the Archaeological Park facing southwest. Three scans were recorded for each spectrum to improve the signal-to-noise ratio. The resolution is 8 cm
1 in the range 200–2000 cm
1. In-situ analyses were compared to a purpose-made database of commercial salts: Potassium nitrate (Niter, KNO3) (221295 – Sigma–Aldrich Quimica SA, Spain) and Calcium sulfate dihydrate (gypsum, (CaSO4 2H2O)) (102161, Merck, Germany) with a purity of at least 99%, and measure conditions were optimized through analyses of 22 samples of salt efflorescences and substrate collected in a previous field campaign and characterized with X-Ray Diffraction (PW 1752, PHILIPS, The Netherlands) CuKa 506 Downloaded by [Complutense University of Madrid], [Dr Miguel Gomez-Heras] at 07:05 13 February 2012 radiation, continuous step scanning from 2 to 68 2h, scan step size 0.02 and 2=min, 40 kV, and 30 mA . Before in-situ monitoring in the field, different integration times, to reduce background noise and increase signal=noise ratio, were tested in the laboratory with the portable Raman on the most frequent common salts found in the monuments. Six samples with the most diverse array of salts found at the studied sites were also analyzed with Environmental Scanning Electron Microscopy (ESEM) (Quanta 200 FEI, The Netherlands) with Energy-Dispersive X-ray Spectroscopy (EDS) (model 7509, Oxford Instrument Analytical–Inca, UK) and Cathodoluminescence (CL) (MonoCL3, Gatan, USA). RESULTS Niter (KNO3) and gypsum (CaSO4  2H2O), together with halite (NaCl) and sylvite (KCl), are the most abundant salts at the Silk Tomb, whereas gypsum is the most abundant in the Monastery as the XRD and portable Raman results show. Figure 2 shows the results of the preliminary tests for different integration times of spectra acquisition obtained with the portable Raman in the laboratory. These results show that the higher integration times were used, the better spectra were acquired when measuring on commercial salts. However, in stone flakes or fragments was the opposite: At lower integration times, better spectra were acquired. Raman spectra obtained in the laboratory on samples of salt efflorescences show the main bands of niter (710, 1049, 1345, and 1360 cm
1) and gypsum (415, 490, 622, 668, 1008, and 1135 cm
1) with lower intensity and more fluorescence than the spectra obtained in the commercial salts (Fig. 3b). The Raman spectra of these same efflorescences analyzed in situ display better defined patterns than the spectra obtained in the lab. The efflorescences with the most diverse array of salts were found in the internal chamber of the Silk Tomb (Fig. 4). The XRD (Fig. 4a) shows a mixture of quartz (SiO2), calcite (CaCO3), dolomite CaMg(CO3)2, kaolinite (Al2Si2O5(OH)4), and niter, together with polyhalite (K2Ca2Mg(SO4)4  2(H2O) and syngenite (K2Ca(SO4)2  (H2O). No Raman spectra were FIGURE 3 Raman spectra of pure commercial salts and salt FIGURE 2 Raman spectra of commercial salts obtained with portable Raman in the laboratory using different integration times (IT): (a) Potassium nitrate; (b) Gypsum. 507 efflorescence samples obtained with the portable Raman in the Lab and in situ in the monuments of Petra. (color figure available online.) Evaluation of Portable Raman for the Characterization of Salt Downloaded by [Complutense University of Madrid], [Dr Miguel Gomez-Heras] at 07:05 13 February 2012 FIGURE 4 Efflorescence in the internal chamber of the ‘‘Silk Tomb.’’ (a) XRD; (b) Raman. (color figure available online.) obtained in this collected sample that was analyzed in the laboratory. However, a Raman spectrum was obtained in situ, where three main bands can be observed (Fig. 4b). One band may correspond to niter (1051 cm
1), and the other bands (984 and 1010 cm
1) may correspond to syngenite, in agreement with the higher amount of both phases detected by XRD. Halite (NaCl) and sylvite (KCl) are also very abundant at the studied monuments, especially as ‘‘rockmeal’’ crusts on the surface of the stone at the Monastery. Their identification with Raman was not possible, since these salts are not sensitive to Raman vibrations due to their strong ionic bond, nevertheless they were easily identified with XRD (Fig. 5a) and also observed with ESEM and analyzed with EDS (Fig. 5b) and cathodoluminescence (Fig. 5c and 5d). DISCUSSION Salt efflorescences of gypsum (CaSO4  2H2O) and niter (KNO3) were quickly and easily identified with micro-Raman spectroscopy in most of the in-situ analyses of the buildings. FIGURE 5 Examinations of salts. (a) XRD of a salt efflorescence sample; (b) ESEM-EDS of the same sample; (c) Cathodoluminescence (CL) spectra of sodium and potassium chlorides; (d) Pancromatic map showing luminescence of these salts. P. López-Arce et al. 508 Downloaded by [Complutense University of Madrid], [Dr Miguel Gomez-Heras] at 07:05 13 February 2012 Laboratory and in-Situ Raman Analyses More and better spectra—with less fluorescence— were acquired in the field than in the laboratory. There are several reasons for this; firstly, the possibility of measuring the surroundings of the previously collected samples was higher and hence the broader array of salts found in the field. Secondly, measuring conditions had been optimized beforehand when measuring in situ. Moreover, the temperature and relative humidity at which the measurements are made affect the acquisition of the spectra. This adds up to the challenges mentioned by Vandenabeele et al.[10] on the transition from a laboratory environment to in-situ Raman investigations in museums or in the field. The presence of halite (NaCl) and sylvite (KCl) is also a difficulty, as they do not present vibrational bands when pure due to their ionic bonds. These are abundant salts in Petra, and their presence in the field was inferred precisely due to this absence of spectra in measurements made on salt efflorescences. It should be noted then, when working on efflorescence found on building stone, that the absence of clear Raman spectra might not mean the absence of salts, but the presence of ionic salts. XRD, ESEM-EDS-CL It is common to find salts on buildings prone to phase changes due to hydration and dehydration. This is an issue when sampling at certain environmental conditions and analyzing, for example with XRD in the lab, at others. Some salts as halite, sylvite, gypsum, and niter can be easily detected with XRD. However, other hydrated salts, such as polyhalite, syngenite, and gypsum could undergo phase transitions from the environment of the monument to the environment in the lab. In-situ Raman monitoring is, therefore, an advantage to detect different states of hydration of the salts on site. In addition to being nondetectable with portable Raman, some samples show halite and sylvite whose compositional differences sometimes cannot be detected with ESEM in backscattered mode (Fig. 5b). In these cases EDS analyses show crystals with different composition related to KCl, NaCl, and CaSO4. However, the panchromatic CL map allows distinguishing clearly the areas of NaCl (with the stronger luminescence signal compared to 509 the less luminescent KCl and gypsum) (Fig. 5d). CL spectra of halite, according to the literature, could be related to the presence of Mn2þ, which substitutes Naþ in the crystal lattice.[11] Luminescence phenomena of halite and alkali halides with simple structure are also explained as a thermoluminescence phosphors attributed partially to different centre types.[12] CONCLUSIONS In-situ measurements with portable Raman spectrometer in the context of cultural heritage are an advantage (i) to avoid sampling, especially in the case of monuments or artworks, (ii) to avoid the influence of environmental changes that could affect hydrated salt, and (iii) to provide faster and a higher number of analyses than those that could be obtained through sampling and posterior analyses. However, when possible, optimization of the measuring conditions in the laboratory with selected samples from the object to study saves time in the field and increases the chances of obtaining a wider array of data. Some salt efflorescences, which are frequently found in buildings, such as chlorides, cannot be detected with portable Raman spectroscopy, so it is important to note that, although portable Raman does reduce dramatically the need of sampling and laboratory testing, it cannot substitute for them completely and that additional techniques, such XRD or CL, are necessary for their identification. Other salts easily identified with Raman, such as gypsum, show less luminescence signal compared to chlorides or are not detected with ESEM-EDS as N (nitrogen) in Niter. In relation to the origin of salts, the most accessible monuments, such as the Silk Tomb, display higher rates of more complex salts (polyhalite (K2Ca2Mg (SO4)4  2(H2O))) and syngenite (K2Ca(SO4)2  (H2O)), and specially niter (KNO3) related to human and animal activity. The most secluded monuments, such as The Monastery, display less presence of the mentioned salts and higher rates of gypsum (CaSO4  2H2O), related to the geological origin of the rock. These salt effloresces are easily identified with portable Raman in the field. ACKNOWLEDGMENTS The authors acknowledge the financial support of PCI-AECID (A=025170=09), GEOMATERIALES Evaluation of Portable Raman for the Characterization of Salt (S2009=MAT-1629), CONSOLIDER-TCP (CSD20070058), and JAE CSIC Programme (PL). The authors thank L. Tormo, A. J. Garcı́a, and M. Furió (MNCN, CSIC) for technical support with ESEM-EDS-CL analyses and I. Serrano for technical support with XRD analyses. Special thanks go to Eng. Fawwaz R. Ishakat, from Hashemite University (Jordan), without whose help this work would not have been possible. Downloaded by [Complutense University of Madrid], [Dr Miguel Gomez-Heras] at 07:05 13 February 2012 REFERENCES 1. Jaser, D.; Barjous, M. O. Geotechnical Studies and Geological Mapping of Ancient Petra City; Natural Resources Authority: Amman, Jordan, 1992. 2. Singer, A.; Amiel, A. J. Characteristics of Nubian sandstone-derived soils. Journal of Soil Science 1974, 25(3), 310–319. 3. Paradise, T. R. Petra revisited: An examination of sandstone weathering research in Petra, Jordan. In Stone decay in the architectural environment. Geological Society of America Special Paper 2005, 390, 39–49; Turkington, A. V. (ed.). 4. Abu-Taleb, A. A.; Alawneh, A. J.; Smadi, M. M. Statistical analysis of recent changes in relative humidity in Jordan. American Journal of Environmental Sciences 2007, 3(2), 75–77. P. López-Arce et al. 5. Eklund, S. Stone weathering in the monastic building complex on Mountain of St Aaron in Petra, Jordan. Master of Arts Thesis, University of Helsinki, 4, 2008. 6. Heinrichs, K. Diagnosis of weathering damage on rock-cut monuments in Petra, Jordan. Environmental Geology 2008, 56, 643–675. 7. Martı́nez-Azcarazo, I.; Sarmiento, A.; Maguregui, M.; Castro, K.; Madariaga, J. M. Portable Raman monitoring of modern cleaning and consolidation operations of artworks on mineral supports. Analytical and Bioanalytical Chemistry 2010, 397, 2717–2725. 8. Kramar, S.; Urosevic, M.; Pristacz, H.; Mirtic, B. Assessment of limestone deterioration due to salt formation by micro-Raman spectroscopy: application to architectural heritage. Journal of Raman Spectroscopy 2010, 41(11), 1441–1448. 9. Pérez-Alonso, M.; Castro, K.; Martinez-Arkarazo, I.; Angulo, M.; Olazabal, M. A.; Madariaga, J. M. Analysis of bulk and inorganic degradation products of stones, mortars and wall paintings by portable Raman microprobe spectroscopy. Analytical and Bioanalytical Chemistry 2004, 379, 42–50. 10. Vandenabeele, P.; Weis, T. L.; Grant, E. R.; Moens, L. J. A new instrument adapted to in situ Raman analysis of objects of art. Analytical and Bioanalytical Chemistry 2004, 379, 137–142. 11. Gorobets, V. S.; Rogojine, A. A. Luminescent Spectra of Minerals; All–Russia Institute of Mineral Resources (VIMS), Moscow, 2002; 300. 12. Krbetschek, M. R.; Goètze, J.; Dietrich, A.; Trautmann, T. Spectral information from minerals relevant for luminescence dating. Radiation Measurements 1997, 27(5=6), 695–748. 510