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.
Reflections on Decay Patterns
The reflection on probable material intrinsic and environmental factors that might cause, contribute or accelerate the two predominant decay patterns was important in order to gain a better understanding on their damaging potential and the rates of degradation.
It was assumed that on one hand, stone-intrinsic properties and style of quarrying have caused and accelerated these kinds of decay. The distinctive stratification of the stone varieties A1 and A3 combined with the unsuitable positioning of the steles – the layers are orientated at a right angle to the ground – might have favoured delamination. The vertically aligned layers can be a wear point and initiate cracking and fissuring.
It is also likely that the granite quarried was not totally sound. The hardness of the material makes it very difficult to quarry, especially when structural defects are lacking. So it is possible that pre-weathered stone slabs or boulders from the surrounding were used in first place for the erection of the complex monument. This was also underpinned by the inspection of the surrounding area which indicated no traces of possibilities for mining or quarrying. The pre-weathering in turn can adversely affect the subsequent degradation rate of the stone (Rodrigues 1980).
On the other hand, the hypothesis is proposed that thermal loads and temperature gradient play a significant part in the decay process.
The evaluation of meteorological and climate data from a weather station approx. 150 km south of the respective site (Saikhan-Ovoo, Dundgovi) showed that in summer maximum temperatures reach around 35 °C, while in winter temperatures can drop to minus 25 °C. Throughout the year the temperature difference between day and night is approximately 15 °C. The area counts almost 250 sunny days a year, often with clear cloudless skies. Combined with the lack of trees and therefore shadow in the steppe, this direct sunlight leads to a quick heating up of the stone. This in turn is assumed to result in a considerable temperature gradient within the stone and thermal stress due to a differential expansion of the rock minerals. Both factors might accelerate physical weathering and delamination (Rodrigues 1980).
In order to examine this assumption, the surface temperature on the top and the bottom of all faces of the main stele was measured in 2019 hourly with an infrared thermometer (Bosch Thermodetektor PTD 1) from sunrise to sunset. It is notable that particularly after sunrise (between 6:30 and 7:30) the surface of the east face heated up verifiably faster than the shaded west face. While 32 °C were measured on the top of the east face, the top of the west face had only 11.5 °C. Further, a perceptible difference of up to 4.1 °C could be measured from 8:30 to 14:30 between the top and the bottom of the East and South face. Over the measuring period the surface temperature varied from the minimum of 7.7 °C to the maximum of 44.6 °C.
Although the measurements only reflect heating and cooling during an average summer day, it is obvious that there have been thermal loads and a measurable temperature gradient between the front and back face of the stele since its erection 1,300 years ago. Particularly the gradient on the top might play a role in the delamination which is particularly distinctive in this area.
106Freeze-thaw-cylces, which considerably contribute to stone deterioration at our (European) latitudes, could be widely excluded as cause of damage in Mongolia. From September to November and middle of February to middle of April successive temperature transitions over the freeze point have to be expected in the Mongolian steppe. As there is hardly precipitation in these periods (less than 5 mm), damage caused by freeze-thaw-cycles can be neglected (Ruedrich, Kirchner and Siegesmund 2011).
Summary
The project provided the unique opportunity for the Austrian and Mongolian conservators and conservation scientists to study a stone monument from the 8th century, which has been continuously exposed to weathering.
Condition surveys together with extensive analyses of taken samples and three non-destructive field tests enabled to characterize the extent and type of decay. The findings showed a clear correlation between material instrinsic properties and prevalent decay patterns: while the stone varieties with layering mainly suffered from delamination, those without layering were rather affected by disintegration. They further revealed that particularly the human- and animal-shaped sculptures are, different to what was expected, heavily decayed and suffer from advanced disintegration.
Surface temperature measurements and the evaluation of meterological and climate data suggested that thermal loads and temperature gradients rather than freeze-thaw-cyles enhance decay and weathering of the site.
Based on the findings, the conservation treatment concept was developed and implemented. While the emphasis at the animal- and human-shaped sculptures was on the injection of fissures and partial consolidation, backfilling and micro pointing were carried out at the two steles.
Monitoring, survey, assessment and conservation of the heritage site have been carried out in a collaborative model-like approach to be utilized as a best practice example for future preservation efforts. In addition, the field tests, which were carried out for the first time at the respective site, provided important measurement data on pulse velocities and water absorptions of local stone varities to be used for comparative studies at similar sites in Mongolia.
References
Hoffmann A. 2006. Naturwerksteine Thailands: Lagerstättenerkundung und Bewertung. Dissertation Georg-August-Universität, Göttingen.
Kotwicz W., Samoïovitch A. 1928. Le monument turc d’Ikhe-khuchotu en Mongolie centrale. Rocznik Orientalistyzny, vol. 4, pp 60–107.
Rodrigues J. D. 1980. Decay of granite, Advanced study course, Science and technology of the environment for sustainable protection of cultural heritage. Relim.
Ruedrich J., Kirchner D., Siegesmund S. 2011. Physical weathering of building stones induced by freeze-thaw action: a laboratory long-term study. Environmental Earth Sciences, Volume 63, Issue 7-7, pp 1573–1586.
Svahn H. 2006. Non-Destructive Field Tests in Stone Conservation, Field and
