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
