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LONG-TERM CONDITION SURVEY BY ULTRASONIC VELOCITY TESTING OF OUTDOOR MARBLE SCULPTURES

Marisa Pamplona and Stefan Simon

Rathgen Research Laboratory – National Museums Berlin, Schloßstraße 1A, 14059 Berlin, Germany

Abstract

Conservation of outdoor stone sculptures requires planning, which besides visual inspection should be based on physic-mechanical parameters. Diagnosis with a non-destructive, cheap and reliable method can be performed with ultrasonic pulse velocity.

This paper presents the diagnostic study of several marble sculptures in Munich (Germany) by means of ultrasonic pulse velocity. The sculptures date from the middle of the 19th century to the beginning of the 20th century. The aim of this study is to assess the marble decay and in some specific cases the durability of consolidation treatments performed, e.g. by Acrylic Total Impregnation (ATI) more than 10 years ago.

In situ measurements took place in October 2011 and are compared to previous measurements dating back approx. 20 years. Measured values were converted into damage classes and represented in mappings.

The monument to Ernst von Grossi, exposed and consolidated by the ATI-process 12 years ago, has been checked in respect for the treatment durability. The monument to Lindpaintner, fire-damaged, sheltered and consolidated 3 years ago by ATI, improved in mechanical resistance in comparison with the data acquired 20 years ago. The Madonna relief, an exposed and restored sculpture is in danger of breakdown with areas of complete structural damage. The monument to Muschler, sheltered and locally consolidated 2 years ago, shows a gradient of ultrasonic values from the bottom to the top of the sculpture. Its condition varies from fresh areas to areas in risk of immediate breakdown. This gradient had already been observed 20 years ago. The monument to Greif is still, as 20 years ago characterised by progressive granular disintegration. Some areas are better preserved with increasing porosity, and some areas are in danger of breakdown. 1. Introduction

Cultural heritage made of marble displayed outside decays as a function of several factors. Different deterioration processes can act on the surface level (giving rise to the formation of crusts or scaling) or in association with material loss. Fuchs noticed already in 1829 the damage caused by temperature changes in marble grains (Gottgetreu 1869). Loosening grains result especially from the anisotropy of thermal dilatation from calcite. The thermal stress leads to higher porosity, which enhances the capillary transport of solutions (Sage 1988).

Diagnosis of marble with a non-destructive, cheap and reliable method can be performed with ultrasonic pulse velocity (UPV). This method has been widely used to evaluate the damage of stone (Ahmad, Pamplona and Simon 2011). Results depend on the stone’s density, porosity and water content, therefore, they can be related to the stones’ compactness and to its conservation conditions (Cardu, Gomez and Mancini

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1989, Kohler 1988). Measuring ultrasonic velocity before and after a conservation treatment allows to evaluate the efficacy of the applied product in improving substrate intergranular cohesion to be evaluated (Antonova, Sizov and Simon 1997, Boos 1996).

The acrylic total impregnation (ATI) was developed in Germany by Ibach and has been applied since the 1970’s in the conservation of stone cultural heritage in Europe (Sinner 1991). This method is often advised for open air marble artefacts that are endangered of total structural disintegration (Köhler 1988, Snethlage, Ettl and Sattler 1999), as it penetrates in depth. However, with the treatment being irreversible, it is usually not applied on objects of major value (Wihr 1986).

The aim of this study is to perform ultrasonic investigation of outdoor marble sculptures located in Munich, Germany, in order to assess their decay and the durability of past consolidation treatments. Most sculptures are located in the Old South Cemetery of Munich and one is located close to Sendlinger Tor. This study was conducted in the framework of a research project entitled 'Vom Gottesacker zum öffentlichen Ehrenplatze', Der Alte Südliche Friedhof in München, coordinated by Claudia Denk and John Ziesemer. Sculptures from the Old South Cemetery of Munich are identified with the inventory system used in this research project. 2. Ultrasonic investigation

Longitudinal ultrasonic waves in the direct transmission method were measured with the portable measurement device USG 20 (Geotron Elektronik, Germany), operated with a point-shaped 46 kHz-vibrator (UPG-T) and receiver (UPE-T). The accuracy of measurements is assumed to be ± 10%. During the measurement the coupled pressure was steadily increased until the measured transmission time was constant and therefore independent from the pressure applied (Köhler and Simon 1993; Simon and Snethlage 1993). Transducers were coupled to the surface with a suitable elastic material. Classification systems that categorize marble damage based on UPV values were developed and applied to quantify damage (Galan et al. 1991, Köhler 1991); these were further adapted by (Sizov et al. 1999) and are used in this study (Table 1).

Table 1. Classification of crystalline marble damage based on velocity-porosity correlation after (Sizov et al. 1999). Vl [km/s] Marble conservation state description Damage class vl > 5 Unweathered 0

3 < vl < 5 increasing porosity I 2 < vl < 3 Progressive granular disintegration II 1,5 < vl < 2 Danger of breakdown III

vl < 1,5 Complete structural damage IV 2.1 Ultrasonic single measurements

Ultrasonic pulse velocity (UPV) values were mapped on the front view of each sculpture. Values from 2011 are the average of ca. 3 determinations. The precise

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measured spot and the surrounding area are then colored in accordance to the corresponding class of damage (Table 1). 2.2 Ultrasonic tomography

Ultrasonic computer tomography is based on multiple sending and receiving of ultrasonic waves from positions of known coordinates on the stone. The aim is to determine, with high resolution, the numerical value of the longitudinal ultrasonic velocity in each area inaccessible to direct observation and measurement (Montoto et al. 1994, Simon and Lind 1999). For each selected plane, the travel times and the coordinates of the investigation points are entered into a tomographic software with an iterative algorithm (RAI-2D (Côte et al. 1992), which works out the most likely ultrasonic paths in the stone and calculates the velocity in each element of the investigated section. The tomogram is presented through contour plots with the velocity variations inside the stone (Côte et al. 1992). Ultrasonic tomography can be used to study and estimate the degree of weathering of the whole stone structure (Montoto et al. 1994; Simon 2001). In this study the following parameters were used: radius of zone of influence = 1,5 cm; grid step size = 1 cm; number of iterations: 10. Due to the small grid step size and radius of zone of influence the presented ultrasonic velocity is less certain but the location of weaker areas is improved. 3. Outdoor Marble Sculptures 3.1 Monument to Ernst von Grossi

The Carrara marble monument to Ernst von Grossi was created in 1831 and it was located in the gardens of the State Hospital of Munich. Since 1900, the bust is located at the Sendliger Torplatz (Munich). The monument to Ernst von Grossi, exposed and consolidated by ATI 12 years ago, has been checked in respect for the treatment durability. The bust was characterised by ultrasonic technique before, shortly after treatment, and last autumn, namely in 01/98, 10/99 and 10/11.

3.2 Monument to Lindpaintner

The Carrara marble monument to Lindpaintner [N-A-170] was produced by Julius Renn in ca. 1882. The sculpture was fire-damaged due to World War II. In 1944 the shelter of the sculpture was destroyed and in 1954 a new roof was built (Bock 1993). The marble sculpture was consolidated 3 years ago by ATI. It has been subject to several UVP measurements in 1992, 2006, 2008 and 2011.

3.3 Madonna Relief

The marble Madonna relief [23-13-617] was produced around 1858. This exposed sculpture is mentioned by Snethlage as a case of total structural disintegration (Snethlage 1995). Based on oral information, the sculpture was cleaned with micro particles and then consolidated with Paraloid® dissolved in acetone ca. 7 %. The date of treatment’s application is between 1990 und 1995, therefore this treatment is estimated in ca. 15-20 years old.

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3.4 Monument to Muschler The Carrara marble monument to Muschler [M-l-66/67] was produced by

Kosenbach probably around 1903 (Vergès Belmin and Simon 1992) and it is sheltered. The sculpture was cleaned in 2009 with laser and afterwards it was locally consolidated with an acrylic resin (Sattler 2010). It has been subject to several UVP measurements in 1992, 1994, 1999, 2009 and 2011. 3.5 Monument to Greif

The Carrara marble monument to Greif [M-l-68/69] was produced possibly from Schneider around 1886. The sculpture is exposed and no records of previous consolidation treatments were found. It has been subject to several UVP measurements in 1992, 1994, 1999 and 2011. 4. Results and Discussion 4.1 Monument to Ernst von Grossi

The monument to Ernest von Grossi presented in 1997 microbiologic colonisation and sugaring, as documented by (Simon 2001). Besides some lacunae in the nose and in the ears, vertical fissures were detected on the west side. The effectiveness of ATI was assessed with ultrasonic pulse velocity measurements carried out before (1998) and after treatment (1999). In weathered condition the bust showed ultrasonic velocities ranging within 3 and 5 km/s, which corresponds to the damage class I and II (Table 1, Table 2).

Localised areas of the bust, as the left ear and the right side of the breast, presented

risk of breakdown (Table 1). After consolidation with ATI a clear stabilisation was achieved, as proved by average ultrasonic velocity around 4,7 km/s. From 1999 to 2011 most tested areas show a decrease of the ultrasonic velocity values from 0,5 to 1 km/s. This result indicates that the product is still in good condition after 12 years of natural weathering (Figure 1).

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Table 2. Statistics of UPV measurements campaigns in six marble sculptures from Munich.

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Figure 1. Ultrasonic transmission measurements in the monument to Ernest von Grossi before, after one year and after 13 years of consolidation with ATI.

The tomographic investigation indicates similar results to the direct transmission

measurements. The vertical fissures identified macroscopically in 1998, could be characterised as crossing over the direction SW-NE. After treatment the bust ultrasonic velocity and mechanical strength improved considerably. The consolidation product reached the inner core, however, it is possible to recognise lower velocity values at the surface level, which indicate a lower concentration of consolidation product there (Figure 2). 4.2 Monument to Lindpaintner The monument to Lindpaintner sheltered and consolidated with ATI 3 years ago, shows a clear improvement of the mechanical resistance in comparison with ultrasonic data acquired before the treatment (18 years ago). The low UPV values from 1992 are typical of a fire damaged marble. This data (Köhler 1992) correlates well with data measured in 2006 before the treatment (Sattler 2008). In 2008 Sattler measured after treatment UPV average value 5,3 ± 0,5 km/s (Sattler 2008). Measurements in 2011 denote increasing porosity of the statue with an UPV average value of 4,4 ± 0,6 km/s (Table 2). Comparing data from 2008 and 2011, a loss in the products’ effectiveness associated with a decrease of ca. 1 km/s within 3 years can be suggested (Figure 3). UPV values from 2011were plotted as a function of the measuring distance, in order to have an idea if the weathering profile is homogeneous or not. The consolidation product seems to have reached the inner core (Figure 4).

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Figure 2. Ultrasonic tomogram made in a horizontal plane within the bust’s base of the monument to Ernest von Grossi before (1998) and after (1999 and 2011) treatment with ATI.

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0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

0 10 20 30 40 50

vl [km/s]

Distance [cm]

Monument to Lindpaintner [N-A-170]

Figure 3. Ultrasonic transmission measurements in the monument to Lindpaintner before and after 3 years of consolidation with ATI. Mapping from 1992 is adapted from Köhler (1992).

Figure 4. UPV values in the monument to Lindpaintner plotted as a function of the measuring distance.

4.3 Madonna Relief

The Madonna relief with average UPV value of 1,4 ± 0,4 km/s is in danger of breakdown with areas of complete structural damage (Table 2, Figure 5). This condition agrees with the description made by Snethlage (1995) based on visual inspection. The consolidation treatment made around 1995, if it had been effective initially at all by now has no more consolidation effect.

4.4 Monument to Muschler The monument to Muschler ultrasonic mappings’ from 1992 to 2011 show an heterogeneous conservation condition (Figure 6), with areas ranging from damage class 0 to IV (Table 1). A common tendency among all mappings seems to be a gradient of higher velocities at the base and lower velocities at the top of the sculpture. Literature from 1992 refers that the “marble grains are easily detached (sugaring)” (Wollenzien, Urzi and Krumbein 1995). In this year, Vergès Belmin and Simon (1992) mentioned whitened and eroded areas, as well as areas with black crusts. This suggests that the sculpture is not completely protected and explains the very low UPV values. From the six ultrasonic testing campaigns, maximum values range from 4 to 6 km/s and minimum values from 0,4 to 1,3 km/s (Table 2). Sattler characterized the UPV of the sculpture in 2009 before and after treatment. He mentions an average increase of UPV of ca. 0,6

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Figure 5. UPV values in the Madonna Relief measured in 2011. km/s after treatment (Sattler 2010, Table 2). Two years after treatment, the UPV average value agrees well with the data from Sattler (Figure 6, Table 2). In 2011 the gradient of UPV values is still recognizable, being the upper part of the sculpture in danger of breakdown on complete damage (Table 1). 4.5 Monument to Greif

Based on literature, in 1980 the monument to Greif showed no visible deterioration apart from superficial erosion and washed out areas (Vergès Belmin and Simon 1992). The ultrasonic mappings from the monument to Greif dating from 1992 to 2011 show an more homogeneous conservation condition (Figure 7), when compared with the mappings of Muschler (Figure 6). Nowadays as 20 years ago the sculpture is characterised by progressive granular disintegration. Based on ultrasonic data from previous investigations, the areas that are in danger of breakdown increased in 2011 (Table 2, Figure 7). This more severe decay in the last 10 years is not concentrated in the upper part of the sculpture, as in the case of the monument to Muschler, but in the external parts. 5. Conclusions

In this paper six marble sculptures from Munich were studied by ultrasonic velocity testing. With help of previous measurement data it was possible to monitor more closely the condition of sculptures in the last 10 to 20 years.

The monument to Ernst von Grossi is in good condition and the ATI consolidation performs well after 12 years of natural weathering. Based on tomography data the

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consolidation product reached the inner core. The monument to Lindpaintner, fire-damaged, sheltered and consolidated 3 years ago by ATI, improved in mechanical resistance in comparison with the data acquired 20 years ago. The consolidation product seems to have reached the inner core as well. The Madonna relief is in danger of breakdown to complete structural damage and any consolidation treatment applied 15-20 years ago is not effective anymore. The monument to Muschler was locally consolidated with acrylic resin 2 years ago. This treatment is stable when comparing ultrasonic data immediately after treatment. A gradient of UPV values is recognizable in data since 20 years. The monument to Greif is still, as 20 years ago characterised by progressive granular disintegration, with some exposed areas nowadays in danger of breakdown.

The non-destructive character of ultrasonic velocity measurements and its ability to quantify physico-mechanical conditions for natural stone, allows for a long-term, reliable monitoring of change, degradation or consolidation by conservation intervention.

Figure 6. Ultrasonic transmission measurements in the monument to Muschler before and after 2 years of consolidation with acrylic resin. Mapping from 1992 is adapted from Köhler (1992).

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Figure 7. Ultrasonic transmission measurements in the monument to Greif. Mapping from 1992 is adapted from Köhler (1992). Acknowledgments We thank Claudia Denk and John Ziesemer for historical information and working conditions provided in situ. We thank Katharina Meier, Franziska Kolba, Sina Setzer and Linyuan Zhao for participating in the 2011 ultrasonic campaign 2011; and Niklas Underwood for running ultrasonic data from Grossi into the tomographic software. References Ahmad, A., Pamplona M. and Simon S. (2009). ‘Ultrasonic testing for the investigation

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