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 measuraments using test pads and b) consolidation with a squirt bottle.
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
