Siegfried Siegesmund - Monument Future

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

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.

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).

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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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Wanja Wedekind, Christoph Schmidt

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

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.

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 a combined infusion and poultice desalination method (Wedekind et. al 2008).

The historical Bartholomew Cemetery is closely connected to the growth of the Georgia-Augusta University of Goettingen. The cemetery is the last resting place for many distinguished German and European personalities involved in the humanities and scientific research.

The number of tombs preserved today comprises a total of 167. The types of graves found at the cemetery consist of simple enclosure graves, tomb slabs, steles, gravestones, stone pillars, gothic pinnacle-pillars, obelisks, cubic-shaped columns and two mausoleums.

The construction material predominately used for the tombs at the Bartholomew Cemetery is the highly porous Buntsandstein. Different types of damage and stress-strain phenomenon is evident 264on the stones (Fig. 1). They range from locally-formed holes to finely sanded surfaces, crack formation, flaking and crusts. The analyses show that in large part the material loss is due to the high salt accumulation resulting in salt crystallization (Kracke et al. 2007).

Figure 1: The east and west sides of the three tombstones at the Bartholomew Cemetery in Goettingen.

The treated and examined sandstone objects are three comparable classicist tombstones from the 19th century. The tombstones are of local sandstone and were made in the workshop of the sculptor and architect Andreas Rohns (1787–1853).

The grave stelae are approximately two meters high. The left tombstone (I, Fig. 1a), is a little wider and taller than the two on the right (II and III, Fig. 1a and c). All three sandstone objects show the same thickness (Fig. 1).

The middle tombstone (II) had obviously been badly damaged in the past and was restored with cement mortar. The tombstones are oriented to the east and the engraved writing is largely disfigured due to weathering to the point of illegibility.

In the Göttingen region, the local Buntsandstein unit was deposited during the Early Triassic. The rock fabric is strongly inhomogeneous, as the layering, changes in composition and partly show a high clay content (Kracke et al. 2008).

By comparing all the grave monuments in the historical cemetery, it became clear that, the east sides of the objects are especially affected by weathering. This orientation-dependent damage is due to the prevailing wind and rain direction from the west. While the west sides are regularly moistened and washed off by driving rain, the drying processes, preferably take place on the east sides where harmful salts accumulate (Fig. 2b). The salts are primarily nitrate and sulphate compounds (Wedekind et al. 2008).

Figure 2: a) The desalination of the three tombstones and b) weathering model.