Monument Future - Siegfried Siegesmund

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properties

      Based on its ultrasonic velocity (Tab. 2) and due to the low values of surface hardness, the natural stone used for the Geghard Monastery can be classified as a low bound stone material with an expected compressive strengh of around 10–20 N/ mm2 (Wedekind et al. 2016). Due to its pore size distribution and the high amount of micropores (Fig. 2 g and h), as well as due to the high S-value, the rock material seems to be sensitive to ice- and salt crystallization (Hirschfeld 1912).

      The high hydric dilatation as well as the micoporosity of both rock varieties speak for a certain proportion of swellable clay minerals (Wedekind et al. 2013). Both values correlate with each other (compare Fig. 2 below and Fig. 4). Also, the significant decrease in ultrasound velocity under water-saturated conditions suggests the presence of swellable clay minerals and a softening of the rock structure.

       Experimental conservation

      Because of the onsite observed weathering forms and the measured values of hydric dilatation, the two rock varieties were treated with a swelling inhibitor and then their hydric dilatation was measured again. After the treatment, the clastic material shows a plain reduction of the hydric dilalation of around 40 %. In the case of the RC variety, a reduction of the anisotropy of the hydric dilatation to nearly zero is remarkable (Fig. 5). Furthermore, the fine-grained material (RF) shows a reduction of more than 50 % (from 0.53 mm/m to 1.8 mm/m) in the Z direction (perpendicular to bedding), whereas the XY-direction shows a lower reduction reaching 0.093 mm/m.

      During the consolidation test with the silica sol, gel formation sometimes occurs on the sample surface. This can also be attributed to the pretreatment with the swelling inhibitor. Silica sols are ion-sensitive and can therefore only be used to a very limited extent on saline substrates. And, a swelling inhibitor is actually a saline solution. Consolidation tests with the silica acid esther showed a lower consolidation effect, but also a low darkening of the sample material.

      It is striking that there is only a very slight increase in strength, in the case of both consolidants to the two directions parallel to the stratification (XY) (Tab. 2). The increase is around 8 % for the RF by using KSE and 7.5 % using the silica sol. The 262RC variety reaches less than 1 % in the case of KSE and around 3 % for the silica sol. The strengthening effect perpendicular to the bedding (Z) is much higher for both rock types and consolidants (Tab. 2). This attains 16 % for the RF and 30 % for the RC variety by using KSE. By using the silica sol, a consolidation effect can be established in the Z-direction for RF with 19 % and for RC at 30 %.

      Figure 4: Hydric dilatation of the two rock varieties.

      Figure 5: Reduction of the hydric dilatation after the treatment with the swelling inhibitor.

       Conclusions

      Treatment with the swelling inhibitor turned out to be a successful conservation strategy, which reduces the hydric dilatation as well as the anisotropic behavior of the rock in the case of hydric swelling. The consolidation was able to stabilize the cohesion of the material and significantly reduce the anisotropic properties in the case of ultrasonic velocity.

       Acknowledgements

      We would like to thank the Administration of Heritage Preservation of the Geghard Monastery as well as the Armenian Apostolic Church for their friendly cooperation.

      This work was generously supported by the Volkswagen Foundation (AZ93919).

       References

      Hirschwald, J. (1912) Die Prüfung der natürlichen Bausteine auf ihre Wetterbeständigkeit. W. Ernst & Sohn, Berlin.

      Karapetian, SG., Jrbashian, RT., Mnatsakanian, AK. (2001) Late collision rhyolitic volcanism in the north-eastern part of the Armenian Highland. J Volcanol Geoth Res 112(1), p. 189–220.

      Meliksetian, K., Savov, I., Connor, C., Halama, R., Jrbashyan, R., Navasardyan, G., Ghukasyan, Y., Gevorgyan, H., Manucharyan, D., Ishizuka, O. (2014) Aragats stratovolcano in Armenia-volcano- stratigraphy and petrology. In: Geophysical Research Abstracts, vol 16, EGU2014-567-2

      Pötzl, Chr., Siegesmund, S., Dohrmann, R., Koning, JM, Wedekind, W. (2018) Deterioration of volcanic tuff rocks from Armenia: constraints on salt crystallization and hydric expansion, Environmental Earth Sciences 77:660, https://doi.org/10.1007/s12665-018-7777-8

      Wedekind, W., Poetzl, C., Doncel-López, R., Siegesmund, S. (2016) Surface hardness testing for the evaluation of consolidation of porous low bound stones. In: Hughes, J. J., Howind, T. (Eds.) Science and Art: A Future for Stone. Proceedings of the 13th International Congress on the Deterioration and Conservation of Stone. University of the West of Scotland, Paisley 6th to 10th 2016, Volume I, p. 491–499.

      Wedekind, W. López-Doncel, R., Dohrmann, R., Kocher, M., Siegesmund S. (2013) Weathering of volcanic tuff rocks used as natural building stone caused by moisture expansion. p. 1203–1224, 2013. DOI: 10.1007/s12665-012-2158-1.

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       THE THREE FUNERARY STELAE PROJECT

      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.

      Applied Conservation Science (ACS), Leinestrasse 24, 37073 Göttingen, Germany

       Abstract

      Diagnostic investigations, desalination, consolidation and restoration have been applied and carried out on three historical tombstones at the Bartholomew Cemetery in Goettingen (Germany).

      The tombstones were investigated by electrical capacity, electrical conductivity with and without testpads, ultrasonic velocity and surface hardness. The method for desalination applied in this study is desalination by capillary flow combined with poultices. The salt content within the poultices was measured by electrical conductivity on poultice material taken in the grid from the stone surface. Furthermore, the stone material of the three tombstones was investigated by porosity, density and hydric dilatation.

      Follow-up examinations made it clear that a proof of in situ consolidation is possible. A clear relation to the weatherability due to salts and the degree of alteration as well as the possible development of clay minerals could be determined.

       Introduction

      Damages on structures built from natural building stones are in many cases due to the impact of salt crystallization. The fundamental prerequisite is the reduction of the stresses created by salinization.

      The transport of salts largely occurs by solution in the pore spaces of natural building stones. This can also be utilized by the measures designed to reduce the presence of salts. By far the most practical method for the preservation of historical tomb monuments is

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