Monument Future

Conclusion

Lede stone at Ghent and Berlare were both deteriorated and contained gypsum crusts. The crusts were more elaborate in Ghent with overall a higher content of soluble salts. 16S rRNA gene amplicon sequencing revealed potential stone deteriorating nitrogen and sulphur prokaryotes, especially on the City Hall. This suggests a positive correlation with air pollution. However, they were only present with a low concentration in some samples and absent in the isolates. Currently, a strong effect of prokaryotes on crust formation is not expected. Furthermore with decreasing sulphur pollutions, the gypsum is mainly a result of historical accumulation. on Lede stone, prokaryotes can have an impact on the appearance of the monuments as Arthrobacter agilis successfully discoloured Savonnières limestone.

Acknowledgements

This work was funded by Research Foundation Flanders (Research grant number 11D4518N). We would like to thank Sarah Williamson and the lab technicians of CMET for their help with the experiments.

References

Bai, Y., Thompson, G. E. and Martinez-Ramirez, S. (2006) ‘Effects of NO2 on oxidation mechanisms of atmospheric pollutant SO 2 over Baumberger sandstone’, Building and Environment, 41(4), pp. 486–491. doi: 10.1016/j.buildenv.2005.02.007.

Bionda, D. (2005) RUNSALT – A graphical user interface to the ECOS thermodynamic model for the prediction of the behaviour of salt mixtures under changing climate conditions. Available at: http://science.sdf-eu.org/runsalt.

Camuffo, D., Del Monte, M. and Sabbioni, C. (1983) ‘Origin and growth mechanisms of the sulfated crusts on urban limestone’, Water, Air, and Soil Pollution. Kluwer Academic Publishers, 19(4), pp. 351–359. doi: 10.1007/BF00159596.

Castanier, S., Le Métayer-Levrel, G. and Perthuisot, J.-P. (1999) ‘Ca-carbonates precipitation and limestone genesis — the microbiogeologist point of view’, Sedimentary Geology, 126(1–4), pp. 9–23. doi: 10.1016/S0 037-0738(99)00028-7.

Dewanckele, J. et al. (2014) ‘Neutron radiography and X-ray computed tomography for quantifying weathering and water uptake processes inside porous limestone used as building material’, 100Materials Characterization. Elsevier Inc., 88, pp. 86–99. doi: 10.1016/j.matchar.2013.12.007.

Doehne, E. and Price, C. A. (2010) Stone Conservation, An Overview of Current Research. 2nd ed. Re. Los Angeles, CA: Getty Conservation Institute.

Greene, A. C., Patel, B. K. C. and Yacob, S. (2009) ‘Geoalkalibacter subterraneus sp. nov., an anaerobic Fe(III)- and Mn(IV)-reducing bacterium from a petroleum reservoir, and emended descriptions of the family Desulfuromonadaceae and the genus Geoalkalibacter’, INTERNATIONAL JOURNAL OF SYSTEMATIC AND EVOLUTIONARY MICROBIOLOGY, 59(4), pp. 781–785. doi: 10.1099/ijs.0.001537-0.

Hunter, C. N. et al. (eds) (2009) The Purple Phototrophic Bacteria, Photosynthetica. Dordrecht: Springer Netherlands (Advances in Photosynthesis and Respiration). doi: 10.1007/978-1-4020-8815-5.

De Kock, T. et al. (2015) ‘Lede Stone: A potential “Global Heritage Stone Resource” from Belgium’, Episodes, 38(2), pp. 91–96. doi: 10.18814/epiiugs/2015/v38i2/004.

Krumbein, W. and Gorbushina, A. (2009) ‘Some Aspects of Biological Weathering and Air Pollution Some Aspects of Biological Weathering and Air Pollution’, in The Effects of Air Pollution on Cultural Heritage. Boston, MA: Springer US, pp. 127–145. doi: 10.1007/978-0-387-84893-8_5.

Li, Q. et al. (2016) ‘Distribution and diversity of bacteria and fungi colonization in stone monuments analyzed by high-throughput sequencing’, PLoS ONE, 11(9), pp. 1–17. doi: 10.1371/journal.pone.0163287.

Mansch, R. and Bock, E. (1996) ‘Simulation of Microbial Attack on Natural and Artificial Stone’, in Microbially Influenced Corrosion of Materials. Berlin, Heidelberg: Springer Berlin Heidelberg, pp. 167–186. doi: 10.1007/978-3-642-80017-7_13.

Mansch, R. and Bock, E. (1998) ‘Biodeterioration of natural stone with special reference to nitrifying bacteria’, Biodegradation, 9(1), pp. 47–64. doi: 10.1023/A:1008381525192.

De Muynck, W. et al. (2009) ‘Evaluation of strategies to prevent algal fouling on white architectural and cellular concrete’, International Biodeterioration & Biodegradation, 63(6), pp. 679–689. doi: 10.1016/j.ibiod.2009.04.007.

De Paepe, K. et al. (2017) ‘Inter-individual differences determine the outcome of wheat bran colonization by the human gut microbiome’, Environmental Microbiology, 19(8), pp. 3251–3267. doi: 10.1111/1462-2920.13819.

Price, C. A. (2000) An expert chemical model for determining the environmental conditions needed to prevent salt damage in porous materials. London: European Commission Research Report No 11 (Protection and Conservation of European Cultural Heritage). Archetype Publications.

Rosenberg, E. et al. (2013) The Prokaryotes, The Prokaryotes: Prokaryotic Physiology and Biochemistry. Edited by E. Rosenberg et al. Berlin, Heidelberg: Springer Berlin Heidelberg. doi: 10.1007/978-3-642-30141-4.

Schröer L., De Kock T., Cnudde V., Boon N. 2020. Differential colonization of microbial communities inhabiting Lede stone in the urban and rural environment. Science of the Total Environment 733: 139339.

Villa, F. et al. (2015) ‘RNA-based molecular survey of biodiversity of limestone tombstone microbiota in response to atmospheric sulphur pollution’, Letters in Applied Microbiology. John Wiley & Sons, Ltd (10.1111), 60(1), pp. 92–102. doi: 10.1111/lam.12345.

Vlaamse Milieumaatschappij (2019) Jaarrapport Lucht Emissies en concentraties van luchtverontreinigende stoffen.

De Vrieze, J. et al. (2018) ‘The active microbial community more accurately reflects the anaerobic digestion process: 16S rRNA (gene) sequencing as a predictive tool’, Microbiome. NLM (Medline), 6(1), p. 63. doi: 10.1186/s40168-018-0449-9.

Williams, T. J. et al. (2012) ‘Magnetospira thiophila gen. nov., sp. nov., a marine magnetotactic bacterium that represents a novel lineage within the Rhodospirillaceae (Alphaproteobacteria)’, INTERNATIONAL JOURNAL OF SYSTEMATIC AND EVOLUTIONARY MICROBIOLOGY, 62(Pt 10), pp. 2443–2450. doi: 10.1099/ijs.0.037697-0.

101

STONE CULTURAL HERITAGE IN MONGOLIA – MODEL-LIKE STUDY AND CONDITION ASSESSMENT OF THE SITE OF IKH KHÖSHÖÖT

Martina Haselberger¹, Marija Milchin^1*, Katharina Fuchs¹, Galbadrakh Enkhbat², Tserendorj Tsolmon², Johannes Weber³, Gabriela Krist¹

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 Institute of Conservation, University of Applied Arts Vienna, Austria

2 National Center for Cultural Heritage, Ulaanbaatar, Mongolia

3 Institute of Art & Technology, University of Applied Arts Vienna, Austria *corresponding author

Abstract

The monument of Turegs Kul’Chur at Ikh Khöshööt is among the numerous funerary monuments spread over the Mongolian countryside dating back to the Turkic period. It comprises steles with inscriptions, humanand animal-shaped sculptures and remnants of a sarcophagus all made of different varieties of local granitoids. Since 2016, Austrian-Mongolian conservation training workshops have been held there. The aim was to preserve the site and achieve a better understanding of physical and chemical properties of the used material, weathering and decay processes and the impact of the climatic conditions and environment to the site, in-depth research that is only at the beginning in Mongolia.

Two decay patterns common for granites, delamination and disintegration, pose a considerable threat to the original substance. It is supposed that thermal loads, temperature gradient, stone-intrinsic properties including stress release (Winkler 2013) and quarrying may have caused and accelerated the decay.

Visual inspections in situ were combined with petrographic studies of samples using instrumental analysis to determine composition, structure and texture of the stone and to describe decay patterns. The combined use and evaluation of three non-destructive field tests – two tests on water absorption and ultrasonic pulse velocity measurement – enabled to characterize the degree of weathering and decay of the different stone elements. Results were put in context with surface temperature measurements to characterize heating and cooling behavior and meteorological and climate data. Thus, the hypotheses on environmental and material intrinsic factors that accelerate and contribute to decay could be partly supported. Based on the condition survey underpinned with extensive analyses, a conservation treatment concept was developed and carried out.

The Complex

Over centuries the Mongolian nomads had influenced art, culture and religion in Central Asia significantly and left a remarkable legacy including numerous funerary monuments. They are spread over the area of today’s Mongolia, often remote and far away from settlements. Among them is the monument of Ikh Khöshööt, situated in the Delgerkhaan soum (sub-district) of Tov Province in 102Eastern Mongolia. It is acknowledged as tomb for a Turkic military leader from the beginning of the 8th century. The complex comprises 15 main stone sculptures arranged over an area of many square metres. They include two rectangular steles, whereby the larger shows Turkic runic inscriptions on three sides. The other animal- and human-shaped sculptures are present in pairs, as already observed by Kotwicz and Samoïovitch (1928: 69). While the animal-like ones probably depict lions and sheeps (Kotwicz and Samoïovitch 1928), the human-likes appear as simple torsos.

In addition fragments of a former sarcophagus are lying dispersed on the West side of the complex. The original arrangement and positioning of the single objects is no longer comprehensible as the objects have been repeatedly dislocated in the framework of expeditions and site visits. Nevertheless, a linear arrangement is assumed following an axis running along the former sarcophagus and the steles with the remaining sculptures arranged alongside this straight. Research on the complex site was pursued since the beginning of the 20th century, whereby emphasis was placed on documentation (visual and descriptive) and transcription of the inscriptions. Conservation scientific research on the site has started in 2016 in the framework of the collaboration between the University of Applied Arts Vienna and the National Center for Cultural Heritage (NCCH) in Mongolia. On site campaigns in 2016, 2018 and 2019 co-funded by the Eurasia-Pacific Uninet and the NCCH were dedicated to the documentation, the condition survey, non-invasive tests, the implementation of conservation measures and monitoring as well as to the training of Mongolian colleagues. In parallel, analyses of taken samples were carried out in Vienna.

Condition, Damage and Decay Patterns

The sculptures comprising the complex monument are made of three varieties of granitoids, which were identified on the basis of petrographical studies of samples using thin-section microscopy (transmitted and reflected light) and scanning electron microscopy (SEM-BSE): a fine-grained lithotype A1 (c. 1.5 mm grain size), a medium-grained lithotype A2 (c. 4 mm grain size) and a coarse-grained lithotype A3 (c. 5 mm grain size). Beside deviations in grain size, they also slightly differ in colour and accessory mineral content. Two of them (A1 and A3) show a more distinctive layering (stratification) and were both used for the two steles at the site. This choice is probably based on the stone intrinsic property and the resultant sheeting of the rock deposit, which facilitated the quarrying in the form of relatively thin layers. Another possibility is that the sheets were already available as separate boulders. The other variety (A2) is more bulky and was used for the human- and animal-shaped sculptures. All sculptures were probably processed and shaped by use of different chisels, whereby only some remnants of tool marks could be detected on the bottom side of one of a torso.

Figure 1: Monument of Ikh Khöshööt, 2019. © Institute of Conservation, University of Applied Arts Vienna.

Considering the age of the monument (almost 1,300 years), the harsh environment as well as the absence of previous large-scale conservation treatments and regular maintenance, the condition of the monument can be described as relatively good. The engraved inscription on the main stele, with a presumable original depth of some millimetres, is still visible in large parts, which indicates a comparably slow weathering and erosion of the upper surface layers.

Nevertheless, several damage and decay patterns are evident ranging from fractures to heterogeneous deposits comprising remnants of animal rubbings and bird droppings, which have partly converted to compact, thick, hardly removable crusts. Among 103them particularly two decay patterns common for granites, delamination and disintegration (Rodrigues 1980), attracted the attention and concern of the conservators and conservation scientists.

Characterizing the Decay – Non-Destructive Tests and Investigations

Already at the first inspection it was noted that the individual sculptures are affected to a varying degree by delamination and disintegration.

As a first step, level and extent of both decay patterns were thus assessed visually and sensorily as well as recorded in descriptive form and visualized in mappings. It revealed that the two steles are mainly affected by delamination while the human- and animal-shaped sculptures suffer from disintegration. This already suggests a link and dependence between the stone variety and the main decay pattern. Further, the survey showed that particularly the upper third of each of the steles is affected by delamination.

On the main stele the east face shows a more extensive scaling parallel to the surface than the opposite. The human- and animal-shaped sculptures show an even roughness on all sides. Surfaces are hardly sanding, which is likely due to a past consolidation treatment as communicated by NCCH.

Figure 2: Detail of the delamination in the upper third of the main stele, 2018. © Amarsanaa.

In a second step, a set of non-destructive field tests were carried out in order to further characterize and evaluate the degree of decay and weathering. Multiple ultrasonic pulse velocity measurements using the PUNDIT^® PL-200 (Proceq) with p-wave transducers 250 kHz (and 350-fold receiver gain) were done at all objects in different directions to detect cracks, voids and cavities within the materials more precisely. Further, inhomogeneity or disintegrated areas, which are an indication of enhanced degradation, were determined. Figure 3 illustrates the obtained measuring results at the main stele. In the upper two thirds a signal was received only very sporadically. The measurements perpendicular to the layering (i. e. from east to west) showed an average pulse velocity of 1,769 m/s in the lower third. Parallel to the layering an average pulse velocity of 2,050 m/s was measured in the upper third.

At the small stele signal were received only sporadically perpendicular to the layering (i. e. from north to south) with an average pulse velocity of 949 m/s. Parallel to the layering an average pulse velocity of 3,116 m/s was measured whereby signals were detected over the whole height. At four of the animal- and human shaped sculptures (no. 2, 6, 7, 11) no signal was received regardless of the direction. At the others a transmission of the pulse was partly possible. Altogether the average pulse velocities were lower than those at the two steles. The Karsten tube and the contact-sponge were used to assess the Water Absorption Behaviour (WAB) of the stone materials in order to further describe the porosity and degree of weathering (Svahn 2006: 21; Vandevoorde et al. 2009; Vandevoorde et al. 2011). Of particular interest were striking deviations in the WAB among the individual objects.

The contact-sponge method was applied on all objects. Results revealed that the average water absorption (WA) of the stone varieties with layering is 0.76 g/ m²s (A1, main stele) and 0.59 g/ m²s (A3, small stele) respectively. Only a roughened surface 104on top of the west face of the main stele showed a maximum WA of 5.38 g/ m²s. The third stone variety (A2, human- and animal-shaped sculptures) shows a slightly higher average WA of 1.12 g/ m²s, whereby four sculptures (no. 6, 9, 11 and 14) stand out due to a noticeably higher WA.

Figure 3: Mappings of the ultrasonic pulse velocity at the main stele (no. 1); measurements at the front perpendicular to the layering (left) and at the side parallel to the layering (right).

Due to time constraints only the two steles and five human- and animal-shaped sculptures (No. 2, 4, 7, 9, 11) could be exemplarily tested with the Karsten tube method. Results obtained from this field test widely correspond to the results of the contact-sponge tests: the two steles showed very low to no WA over a period of 45 minutes. Among the animal- and human-shaped sculptures no. 9 and 11 had the highest WA, equivalent to the results given by the contact-sponge tests. The others showed a constant but slower water uptake.

Interpretation of Results

In general the measured ultrasonic velocities were rather slow compared to the other values of granitoids given in literature (Hoffmann 2006: 80; Vasconcelos et al. 2008: 27), which indicate an advanced degree of deterioration (Svahn 2006: 21–23). This assumption was supported by the WA measured with the contact-sponge and Karsten tube method. Although it was not very high (compare Vandevoorde et al. 2009), it proves a moderate weathering of the respective material.

The lack of signals in the upper third of the main stele (in right angle to the layering) confirmed the visually determined heavy decay. It showed that delamination is advanced and cracks and cavities run through the entire material. Also the smaller stele seems to suffer from advanced delamination as the pulse was only partly transmitted from bottom to top (in right angle to the layering). As the steles absorbed little to no water it can be assumed that they are hardly affected by disintegration or fissures.

Figure 4: Water absorption measured with Karsten tube (4 ml, contact time up to 60 min) at the two steles (no. 1 and 10) and the sculptures (no. 2, 4, 7, 9, 11).

The failed or partial transmission of the pulse at all of the animal- and human shaped sculptures 105indicated that the material suffered from advanced disintegration. This assumption was confirmed by the water absorption behaviour of the stones. Interestingly, this finding contradicted the macroscopic examination, which indicated a rather good condition (hardly cracks and no sanding). Particularly the sculptures no. 9 and 11 with the highest water absorption (contact-sponge and Karsten tube) showed a rather slow or no signal during ultrasonic measurements. In general, sculptures with a comparatively higher water uptake also had a slower or no transmission of the pulse.

The combined use of the three non-destructive fields test and the comparative analyses of results helped to gain a better understanding on the degree of decay among the individual sculptures. Results mutually support and confirm each other. Thus a clearer picture of the state of condition could be gained.