Monument Future

Methods

The new ultrasonic system consists of the following components: up to 32 ultrasonic transducers with a variable-length holding system, single-channel electronics with 32x multiplexer, a magnetic field tracking system and software for system control and tomography calculation. Figure 1 shows the ultrasound system mounted on a marble colomn of the Marmorpalais in Potsdam. The support system with the ultrasonic transducers (green) is mounted on the marble column, the electronics on the table in the background and the magnetic field generator on the wooden stand.

Figure 1: the new system on a column of the Marmorpalais in Potsdam.

The ultrasonic sensors allow the system to determine times of flight in moderately weathered marble up to approx. 1 m thickness. The opening angle is approx. 60° and the aperture is 9 mm. In this way, the sound, which is coupled in at one point of the test object, can be received again on as many alternative positions on the object as possible. This ideally supports the planned tomographic measurement procedure. The transducer itself is as small and lightweight as possible, so that a high density of sensors can be achieved on the object to be measured without applying too much stress on the holding system or the object itself. Therefore and in order not to influence the magnetic field tracking, the housing of the sensors is made of polymer. In addition, the housing has a specially shaped end plate for safe contacting of a magnetic field sensor by a clamp. It is also important that the sensor can be coupled dry to the marble so that the object itself is not contaminated by a coupling medium. The transducer consists of a 200 kHz ceramic disc on a soft carrier. To optimize the energy transfer, an acoustic matching layer and a coupling layer made of soft polymer, which cannot be rubbed off by the marble, are applied.

The variable-length holding system consists of sensor plates that hold the sensors and flexible polymer connectors (figure 2). The sensor plates have the possibility to mount additional support screws. This prevents vertical tilting of sensor and belt at 241the measuring object. The belt system can be assembled individually for each measuring object. By using flexible connecting elements of different lengths, a more or less high sensor density can be selected if required, or the overall length can be adjusted to suit the object.

Figure 2: holding system with sensors on a marble cylinder.

The electronics consists of a single-channel transmitter and receiver unit that can generate transmission voltages of up to 1,000 Vpp and a multiplexer capable of switching 32 sensors to transmit or receive. The transmission voltage of up to 1,000 Vpp is provided by a transformer. Together wit the adjustable input gain of the electronic, the adjustable output power allows to adjust the received signal. The transformer has two primary windings and one secondary winding. The Amidon BN43-5170 suitable for the frequency range was selected as the core material. The two primary windings are connected in series. An adjustable DC voltage (0 to 48 V) is applied to the connection node. The other two ends of the primary winding are connected to a push-pull transistor circuit. Both transistors can independently connect the windings to ground. The transistors are controlled by a logic, that allows the frequency and signal coding to be adjusted. The maximum secondary output voltage can be varied by this principle adjustable over the primary DC voltage. The multiplexer has 32 channels to drive 32 ultrasound transducer. The multiplexer circuit consists of 32 switching elements, which can be switched on and off in transmit or receive regime independently. Since this application involves transmission voltages of up to 1000 Vpp, the multiplexer components of the HV series MicroChip used in standard systems could not be used. Therefore, discrete switching elements in the form of a “Solid State Power Relay” (CPC1988 of the manufacturer IXYS) were used. This is a MOSFET technology with an integrated opto-coupler. The 32 relay modules arranged in two banks are controlled by a multiplexer from Maxim. Their outputs are directly connected to the optically coupled input of the CPC1988. All outputs of the multiplexer can be activated by a microcontroller via its serial interface.

If a CPC1988 is in the “off” state, its optical control input is connected to ground by means of the Maxim multiplexer which prevents unintentional switch-on. To protect the CPC1988 switches from impermissibly high voltage peaks, a SMAJ440CA suppressor diode is connected in series.

The received signals of the 32 ultrasonic transducers can be connected to the input amplifier via four 8-channel Maxim multiswitches (MAX4598). Any switch positions are possible. To protect the multiswitches from high voltages (especially the transmit voltage), diode limiters are connected in series.

The individual transducer can be connected to the multiplexer via BNC connectors. On the back there is also a USB interface for connection to an external PC as well as a mains plug with switch. LEDs on the front of the housing indicate the current operating status of the system.

Position detection is carried out with the trakSTAR 2 system from Ascension Inc. The electromagnetic method offers a simple method of determining both the position and orientation of an object in space. In electromagnetic tracking, a magnetic field is generated within 1 m³. The objects to be measured are marked with tracking sensors. These sensors determine their position and position on the basis of the magnetic field lines in which they are located. Since up to 32 ultrasonic transducers cannot be equipped with sensors and the working range of the magnetic field generator is not sufficient to detect all positions from one location without errors, a new algorithm has been developed in which only 2 sensors are necessary, which are successively attached to two adjacent ultrasonic transducers and the position of the generator can be changed. The active surface of the first transducer defines the coordinate origin. All other positions are measured relative to this. However, the tomographic algorithms assume a two-dimensional problem, therefore the points are transformed into a best-fit plane. As an alternative to magnetic field tracking, optical position detection using Aruco markers was developed.

Due to the inhomogeneity and scattering events in marble, there are a large number of paths and therefore also times of flight from the transmit to the receive transducer. As a result, the detected 242sound wave is a superposition of all incoming sound waves and therefore very long in time. For tomographic reconstruction, however, only the linear propagation is of interest as an input parameter. In a first approximation it is assumed that this wave also arrives at the receiver first. Therefore it is necessary to detect the beginning of the recorded signals. One way of detection is the cross correlation method. The correlation result describes the similarity of two signals. A measured transmission signal through water is used as the reference signal (kernel). The spectrally filtered signal is correlated with the kernel, the envelope is calculated and its first peak above a threshold value is determined. The detection of the signal beginning now results from the detection of the peak under consideration of the size of the correlation kernel.

Figure 3: tomography of a water filled bucked with a POM cylinder.

Figure 4: tompgraphy of an artifically aged marble cylinder.

243The software of the system offers the possibility to calculate tomographies by algebraic reconstruction technique (ART), simultaneous iterative reconstruction technique (SIRT), Kaczmarz method, and inverse radon transformation of the velocities calculated by converter positions and times of flight.

Measurements

The new ultrasound system was first tested in the laboratory on so-called ultrasound phantoms. The first phantom was a cylindrical bucket filled with water in which a rod (Ø = 90 mm) made of polyoxymethylene (POM) was placed. Tomography was performed with 24 ultrasound transducers.

Figure 3 shows the result of the evaluation. The rod made of POM is clearly visible as a yellow area. The sound velocity in water was calculated with approx. 1.500 m/s and the sound velocity in POM with approx. 1.900 m/s was calculated correctly. The second phantom was a cylinder made of Carrara marble which was artificially aged. The cylinder was then heated to 150 °C and thrown into cold water. Thus, the average speed of sound could be reduced to approx. 3.200 m/s. Figure 4 shows the results of the evaluation. These correspond well with the expected values.

After successful measurements in the laboratory, the system was tested on the columns (Ø = 550 mm) of the Marmorpalais in Potsdam (see Figure 2). These are heavily weathered and show cracks, some of which have been filled.

Figure 5 shows the found structure of the marble. Although signals could be received at this object, they had large interference components. An automatic detection of the signal beginnings and thus an evaluation of tomography was not possible. The performance limit of the system was reached because of the strong fissuring of the stone.

Figure 5: typical structure of the marble columns of the Marmorpalais in Potsdam.

Discussion

The measurements on the ultrasound phantoms prove the principle functionality of the new ultrasound system in the creation of tomography on stone. The positioning of the ultrasonic transducers with the holding system takes approx. 30 min. and the recording of the positions approx. 10 min. The actual measurements and the calculation of the tomography take approx. 20 min. with max. 32 transducers. The determination of a tomography is thus easily possible within one hour. The transmission voltage of 1,000 Vpp at 9 mm aperture of the transducers is not sufficient to receive evaluable signals in case of strongly weathered and fissured marble. In addition in all cases, the high transmit voltage on the multiplexer generates a coupling to the receive channels. The resulting signals allows a reliable measurement only from 150 mm marble thickness.

Conclusion

The principle feasibility and the associated time saving could be demonstrated. However, a device that can be used practically requires the revision of the multiplexer and the electronics. As mentioned before, the high transmitt voltage generates a coupling to the receive channels and therefore the begin of the signal is difficult to define and structures thinner than 150 mm cannot be examined. This problem can be solved by a new layout which carefully separates the transmit and the receive parts of the electronic and their electrical grounding.

Furthermore, the holding system of the transducers 244should be optimized to allow a strong and secure mounting of the transducers on the marble surface. The existing system software has many useful features and is easyly to handle. Together with an optimized hardware, the system could accelerate the tomography of stone made cultural heritage.

Acknowledgements

The development was supplied by the Federal Ministry of Education and Research of Germany, VIP+00291.

References

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[3] Siegesmund, S. & Snethlage, R. (Eds.) 2014. Stone in Architecture. Springer. 5th Ed, DOI10.1007/978-3-642-14475-2, Sprinter-Verlag Berlin Heidelberg, 552pp.

[4] Ruedrich, J., Knell, Chr., Enseleit, J., Rieffel, Y., Siegesmund, S. 2013. Stability assessment of marble statuaries of the Schlossbrücke (Berlin, Germany) based on rock strength measurements and ultrasonic wave velocities. Environ Earth Sciences 69:1451–1470. DOI: 10.1007/s12665-013-2246-x.

[5] Köhler, W., and Simon, S., ‘The Monument to Gustav II Adolf in Göteborg – Ultrasonic investigations on the Carrara marble base’, in Eurocare-Euromarble EU 496: Proceedings of the 3rd Workshop, Göteborg, 30 September–3 October 1992, Bayerisches Landesamt für Denkmalpflege-Zentrallabor, Munich (1993) Forschungsbericht 11, 117–121.

[6] Făcăoaru, I., and Lugnani, C., ‘Contributions to the diagnosis of stone and concrete historical structures using non-destructive techniques’, in Conservation of Stone and Other Materials, Proceedings of the International RILEM/UNESCO Congress, Paris, 29 June–1 July 1993, ed. M. J. Thiel, E & FN Spon, London (1993) Vol. I, 238–251.

[7] Zezza, F., ‘Computerized analysis of stone decay in monuments’, in Proceedings of the 1st International Symposium on the Conservation of Monuments in the Mediterranean Basin, Bari, 7–10 June 1989, ed. F. Zezza, Grafo, Brescia (1990) 163–184.

[8] Siegesmund, S. & Dürrast, H. Mechanical and physical properties of rocks, 2011. In: S.Siegesmund & R.Snethlage. Stone in Architecture. 97–225. DOI: 10.1007/ß78-3-642-14475-2_3 Springer-Verlag Berlin Heidelberg.

[9] Côte, P., Gautier, V., Pérez A., and Van-Hoove J. P., ‘Mise en œuvre d’auscultations tomographiques sur Ouvrages d’Art’, Bulletin de Liaison des Laboratoire des Ponts et Chaussées 178 (1992) 47–54.

245

FUENTE DE CIBELES OF MADRID AND DECAY OF MONTESCLAROS MARBLE

David Martín Freire-Lista, Luís Sousa

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.

UTAD – Universidade de Trás-os-Montes e Alto Douro, Quinta de Prados, 5001-801 Vila Real, Portugal

CGeo – Centro de Geociências da Universidade de Coimbra, Rua Silvino Lima, Universidade de Coimbra, Polo II, 3030-790 Coimbra, Portugal

Abstract

Fuente de Cibeles of Madrid (Spain) was carved in Montesclaros marble between 1777 and 1782. In order to know its modifications and anthropogenic decay throughout its history, a documentary search was carried out. Montesclaros marble and its deterioration have been evaluated with optical microscopy, mercury intrusion porosimetry and colourimetry. In addition, the linear microcrack density from the surface to the interior of an altered sample was calculated. This dolomitic marble has coarse equigranular blasts and granoblastic texture. Dolomite crystals are surrounded by smaller calite crystals that dissolve more easily. This dissolution produces pores, a slight colour change and finally granular disintegration. Rhombohedral exfoliation microcracks are more numerous in the first superficial millimeters of altered stones.

Keywords: Fuente de Cibeles, marble decay, weathering processes, maintenance

Introduction

Traditional building stones are an important cultural and social focus; they are identifying and differentiating traits particular to each city. Their conservation is a challenge for the scientific community dedicated to the built heritage (Siegesmund and Török, 2011). The care and conservation of stones are essential to protect and to guarantee the successful future of monuments (De Wever et al., 2016). The historical quarries of traditional building stones should be recognized as valuable assets to provide restoration stones (Bednarik et al. 2014). Anisotropy, thermal changes and dissolution condition the deterioration of marbles (Ondrasina et al., 2002; Luque et al., 2010).

Construction of Fuente de Cibeles

It is written on the Fuente de Cibeles plan (1777) that the goddess, the carriage, the lions and the plant motifs of the rocky promontory where the fontain set is installed would be sculpted in Montesclaros marble, and that the rocky promontory and the base of the basin would be sculpted in Redueña dolostone (the latter one was crossed out).

Originallly, the fountain was at ground level, protected by 20 cylindrical bollards of granite with two smooth borders at their ends and topped with a rosette of twelve petals and a central button at the top (Figs. 1a, b, and c). A cobblestone flint pavement, measuring ten feet wide, was placed around it in the year the fountain was inaugurated, 1782.

246Two zoomorphic water-spouts, a dragon to the right and a bear to the left of the goddess Cybele began to operate in 1798. They supplied water for human consumption through bronze pipes inserted into their mouths. In addition, there were manual water pumps on the basin, and draft animals drank from the basin (Figs. 1b and c). The zoomorphic water-spouts and the bollards were removed in 1862, when the fountain was no longer used to supply drinking water. After this, a Figure 1 cast-iron fence was installed around the fountain (Fig. 1d).

In 1895, Fuente de Cibeles was transferred from its initial location to the centre of a circular island or sidewalk in the current Plaza de Cibeles (with the goddess facing west towards calle de Alcalá), and the fountain set was raised and repositioned (Fig. 2). The sculpture of goddess Cybele rose two meters and sixty centimetres from the pavement. The granite basin was placed on top of a circular platform of four granite steps on a circular sidewalk (Figs 2a and b).

Figure 1: Fuente de Cibeles in its original location. a: Engraving by Isidro Velázquez, 1788. Goddess Cybele is looking towards Salón del Prado and horses are drinking; b: Drawing by David Roberts and engraving by J. T. Willmore, 1835. Bear-shaped water-spout; c: Photograph, 1853. Bear- and dragon-shaped waterspouts, three manual water pumps and the peripheral bollards; d: Photograph, 1864. The original water-spout came out through a pipe in the mouth of the mask and was divided into four jets.

Eight equidistant rectangular cuboid blocks of limestone were inserted in the first three steps of the platform, on top of the sidewalk (Fig. 2a). The rocky surface promontory on which the carriage rests was also raised with a granite base covered with flint rocks (Figs. 2a and b), and the original base of the basin disappeared.

The rocky promontory was enlarged towards the back of the carriage to install two putti (Figs. 2a and b). The putto on the left pours water from an amphora; the one on the right stands and holds a conch shell. These putti were sculpted in Carrara marble (López de Azcona et al., 2002).

A snake and a felled tree trunk of the rocky promontory were moved backwards at that time to their current position under the putto that holds the amphora. Two groups of ornamental jets, with the highest vertical central jet surrounded by smaller parabolic ones were installed on the sides of the goddess (Fig. 2a). The water-spouts of these jets were also coated with flint rocks. A few years after the repositioning of the fountain, a new cast-iron perimeter fence was installed around the entire fountain. The new fence had more ornamentation than the previous one, was fixed to the lowest step 247and attached to eight bollards of granite installed on the eight rectangular cuboids of limestone, whose upper vertices were carved for such purpose (Figs. 2a and b).

Figure 2: Fuente de Cibeles elevated and enlarged after it was transferred to its current location. a: Photograph, c. 1895. Sidewalk and platform on which Fuente de Cibeles was placed composed by four steps of granite with eight limestone cuboids; b: Photograph, c. 1906. Sidewalk and the four steps on which the fountain was installed. Eight granite bollards with a base of limestone, cast-iron perimeter fence, and two putti on the extension of the rocky surface promontory to the back side.

A hand, the keys, the sceptre and the nose of goddess Cybele were damaged during the Second Republic in 1931 and were restored in the same year. The left lion was damaged during the beginning of the Spanish Civil War by the impact of projectiles (Fig. 3a). It lost the snout and suffered damage to the left front leg and tail, so the fountain was protected with sandbags and bricks between 1937 and 1939 (Fig. 3b).

Figure 3: Fuente de Cibeles during the Spanish Civil War. a: Photograph, 1936. Lion with damaged snout; b: Photograph, between 1937 and 1939. Fuente de Cibeles was covered under a pyramid of sandbags and bricks on July 3th, 1937.

After the Spanish Civil War, an interim restoration was made. The snout was reconstructed with plaster and fastened with metal staples and the cast-iron perimeter fence was removed. Ornamental jets were added around the rocky promontory and on the interior perimeter of the basin. The two groups of lateral jets were replaced by two new groups of higher jets whose spouts were no longer coated with flint rocks (Fig. 4). The external perimeter of the fountain basin was landscaped (Fig. 4b). In addition, a lighting system bordering the jets and behind the carriage was installed for night illumination of the fountain (Fig. 4c).

Fuente de Cibeles gained its current appearance in 1968, with the addition of two granite basins with cascading water from the upper basin to the new external basins. The flint rock covering the base of the rocky promontory was also removed (Fig. 5).

The most recent restoration works were performed in 2016. Biological crust and cracked mortars were removed, and unstable elements were secured. Water-repelling treatment was applied in the last phase of the restoration.

248

Figure 4: Fuente de Cibeles after the Spanish Civil War; a: Photograph, c. 1941. Ornamental jets. Vertical jet without flint rock covering the water-spouts; b: Photograph, c. 1941. Landscaped area around the fountain; c: Photograph, c. 1960. Night lighting.

Figure 5: Current appearance of Fuente de Cibeles.