Monument Future. Siegfried Siegesmund

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

       The Bartholomew cemetery and its tombs

      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.

       Preservation and the causes for damage

      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 objects

      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.

       Tombstone material and weathering

      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.

       Methods of investigation: diagnosis and conservation

      Hydrostatic weighing on drilling cores was carried out to acquire the particle and bulk density as well 265as the porosity (DIN 52102). Hydric dilatation was measured on drill core samples taken perpenticular to the bedding using a dial gauge under conditions of complete immersion in demineralized water.

      The structural properties of the areas near the surface as well as the material cohesion of the tombstones were examined using two different methods: micro-hardness and ultrasonic velocity. Both measurements were carried out before the consolidation and after.

      An Equotip 3 device (proceq) was used for the surface hardness measurements. A rebound hardness impact device with hardness level D was used.

      For the measurements of the ultrasonic velocity, the tombstones were measured with a 52 kHz compression wave transducer. The pundipLab+ (procec) was used as the pulse generator. The measurements on the sandstone tombstones were carried out in a grid by transmission.

      To determine the causes of the observed deterioration, the electrical conductivity of the tombstone was investigated using two different methods.

      The electrical conductivity was measured using a portable measuring device (Protimeter Surveymaster, General Electric). This technique allows conclusions to be drawn about hygroscopic salts and moisture.

      In addition, the electrical conductivity was measured using a cotton test pad moistened with distilled water (Fig. 3a). This measurement method can also help to detect non-hygroscopic and less soluble salts like gypsum.

      The three sandstone tombstones were desalinated using a combined process of directional moisture flow and compresses (Domaslowski 2003, Wedekind 2016a). For this purpose, a permanently moist compress was applied to the west side of the tombstone, which was moistened with distilled water over a period of about three months by a system of drip devices (Fig. 2a). Around 50 l of destillated water was used for each tombstone during the desalination process. Drying could only take place on the damaged east sides of the tombstone, on which a poultice was also placed. The rest of the tombstone body was covered with a plastic film (Fig. 2a).

      The poultices were made from fine washed sand and cellolose in a volume fraction of 4 : 1 mixed with destillated water. After total drying the poultices were sampled gridwise and each sample diluted with a controlled amount of destillated water with respect to each sampling area, mixed and measured by electrical conductivity. The dried poultice has a porosity of 57 % (about twice as much as the sandstone of the tombstone) and a pore size distribution ranging in the size classes of 1 to 20 µm as measured by mercury intrusion porosimetry.

      After salt reduction the damaged areas of the three sandstone tombstones were consolidated with a silica acid esther (KSE 300, Remmers company) (Fig. 3b).

       Results

      The porosity of the three tombstones varies between 22 and 24 % (Tab. 1). The hydric dilatation for the sample from tombstone I was 0.7 mm/m, for tombstone II 0.1 mm/m and for tombstone III 0.9 mm/m.

      A significantly increased electrical conductivity could be measured in the areas with observable weathering (Fig. 4). The three tombstones show different forms and intensities of weathering. In the lower part of the eastside of tombstone I, a semi-circular weathering of the stone due to sanding has developed, whereas the opposite side (west) shows an alveolar-like pitting (Fig. 1a). On the lower part of tombstone II (eastside), a low alveolar weathering can be observed, the backside does not show any relevant deterioration (Figure 1b). Tombstone III shows intense flaking and sanding parallel to the layer over the entire engraved writing part, while the backside is nearly intact (Fig. 1c).

      The values for the porosity of the three different tombstones are also comparable (Tab. 1). However, the electrical conductivity values of the west sides of all tombstones are very low (green).

      Testing of salt reduction show different results for all tombstones. The highest electrical conductivity in the poulice samples could be measured in tombstone III followed by tombstone I and finally 266tombstone II (Fig. 5). The electrical conductivity of tombstone I ranges from 0.2 to 5.07 mS/cm, for tomstone II from 0.16 to 3.83 mS/cm and for tombstone III from 0.949 up to 12.0 mS/cm.

      Figure 3: a) Electrical conductivity