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      Figure 4: Values of electrical conductivity for a) tombstone I, b) tombstone II and c) tombstone III. Values of electrical conductivity using test pads for d) tombstone I, e) tombstone II and f) tombstone III.

      The ultrasonic velocity of the three sandstone tombstones show different values. At tombstone I, the ultrasonic velocity ranges between 2.011 km/s and 2.309 km/s and reaches an average value of 2.154 km/s. Areas with a comparatively weak cohesion could be detected in the middle area of the tombstone (Fig. 6a). After consolidation, an increase in the ultrasonic velocity could only be found in the lower, heavily weathered zone (Fig. 7a).

      Tombstone II showed, overall, higher values. These lie between 1.994 km/s and 2.621 km/s and reach an average of 2.25 km/s (Fig. 6b). After consolidation, hardly any changes in the ultrasonic velocity could be detected (Fig. 7b).

      Tombstone III showed the lowest ultrasonic velocity values. They range from 1.34 km/s to 2.234 km/s and are only around 1.94 km/s on average. The lowest values are concentrated in the middle and in the lower, middle area of the stone (Fig. 6c). Due to consolidation, cohesion was strengthened in almost all areas that were severely weathered. However, only a few areas reach values of 2 km/s.

      Before consolidation, the surface hardness at tombstone I reached a maximum of 444 HLD and a minimum of 303 HLD. The average is 398 HLD. A slight softening can be seen in the upper center of the tombstone (Fig. 6d). After consolidation, the surface strength could only be slightly increased in some parts (Fig. 7d). However, the values reach around 400 HLD, which speaks for a certain strengthening affect.

      Comparably higher values between 316 HLD and 499 HLD are achieved with tombstone II (Fig. 6e). On average, however, a slightly lower value of 392 HLD was measured as compared to tombstone I. This also corresponds to the observations and the overall measurements, which illustrate that in particular whose central area of the tombstone shows strong softening (Fig. 6e) The consolidation showed clear successes here.

      Almost all areas with comparably low values could be adjusted to the surface strength of apparently intact areas (Fig. 7e). Some of these areas will return to values above 400 HLD after consolidation.

      The most pronounced weathering and the lowest surface hardness values were observed and measured in tombstone III (Fig. 6d). They ranged from 487 HLD to just 272 HLD. Only an average of 319 HLD could be given. Due to the consolidation, the values were partially increased. However, 267they do not achieve a satisfactory result everywhere (Fig. 7d). Values of over 400 HLD could not be achieved in any of the measured areas after consolidation.

Table 1: Porosity and density of the investigated tombstones.

      Figure 5: Results of salt reduction testing using the single poultice samples for a) tombstone I, b) tomstone II and c) tombstone III.

      Figure 6: Values of ultrasonic velocity for a) tombstone I, b) tombstone II and c) tombstone III and values of surface hardness for d) tombstone I, e) tombstone II and f) tombstone III before consolidation.

      Figure 7: Values of ultrasonic velocity for a) tombstone I, b) tombstone II and c) tombstone III and values of surface hardness for d) tombstone I, e) tombstone II and f) tombstone III after conservation.

      Restoration of the tombstones were done using a hot lime reaction mortar (Fig. 8). Similar mortars were already sucessfully prepared for tuffs and sandstones (Teipel et al. 2020, Wedekind et al. 2016b).

       Conclusions

      The results of the investigations on the objects have shown that, at least in some areas, there is a correlation between the surface hardness measurements and the ultrasonic velocity. In these areas a structural damage can be assumed with low values.

      Both values can be positively influenced by consolidation. The intensity of weathering apparently also plays a role here. With heavily softened objects such as tombstone III, the results can be interpreted to mean that the consolidation was only 268partially successful in some areas. If necessary, the amount of consolidant used was not sufficient.

      Figure 8: The three tombstones after restoration with the hot lime reaction mortar.

      Figure 9: The ultrasonic velocity correlated with the surface hardness of the three tombstones before conservation.

      Practical consolidation measures on stone objects can be checked with the presented examination methods. They can provide valuable information on the success or failure of the consolidation method used and are therefore suitable as a control or quality inspection procedure.

      The results of the salt reduction showed that there is a clear correlation between the level of electrical conductivity and the intensity of the weathering. This leads to the assumption that salts are primarily responsible for the weathering observed. The intense weathering on tombstone III is probably also due to the high hydric dilatation of the rock material. Clay minerals are probably responsible for the latter, as further investigations should clarify.

       Acknowledgements

      We would like to thank Ms. Birgit Busse from the Green Spaces Department of the city of Göttingen for trusting our expertise and for her engaged commitment to the maintenance of the Bartholomew cemetery.

       References

      Domaslowski, W. (2003) Preventive conservation of stone historical objects. Torun.

      Wedekind, W. (2016 a) Verwitterung und Salzreduzierung von monolithischen Baukörpern aus Sandstein. Vortrag auf der Tagung „Aktuelles aus Forschung und Praxis zum Thema Salz“, Dresden 1.4.2016, doi: 10.5165/hawk-hhg/291.

      Wedekind, W., Lopéz-Doncel, R., Ruedrich, J., Siegesmund, S. (2016b) Evaluation of innovative treatments and materials for the conservation of the strongly salt-contaminated Michaelis church in Zeitz, Germany. in: Hughes, J. J., Howind, T. (Eds.) Science and Art: A Future for Stone. Proc. of the 13th Internat. Congress on the Deterioration and Conservation of Stone. Paisley 2016, Volume II, p. 981–990.

      Kracke T., Müller C., Krinninger S., Wedekind W., Ruedrich J., Siegesmund

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