6th World Conference on NDT and Microanalysis in Diagnostics and Conservation of Cultural and Environmental Heritage, Rome, 1999 May. Published by AIPnD, email: aipnd@numerica.it

TABLE OF CONTENTS

ABSTRACT INTRODUCTION EXPERIMENTAL SETUP ON-SITE MEASUREMENTS IN MEDIEVAL FUNERAL CHAPEL CONCLUSIONS ACKNOWLEDGEMENT REFERENCES

ABSTRACT

Partial detachment of plaster or paint layers is a severe problem in the preservation of historical murals. For a remote localization of such defects a combined acoustical-optical method can be used: the loose areas can be excited to tiny vibrations by sound irradiation and are then detected optically. For this purpose, a special TV-holography system was designed, which features enhanced sensitivity obtained by reference wave modulation. Since there is no need for a direct access to the surface, detached plaster areas can thus be monitored even in case of long distances between mural and measuring system. Experimental results obtained at a historical site are presented. The comparison with results received with traditional percussion method show very good agreement.

1. INTRODUCTION

The deterioration of historical works of art as for example ancient frescoes is of major concern as these objects represent part of our cultural heritage. The causes of detorioration are consequences of century-long climatic impacts accelerated by man-made pollution of recent decades. Fluctuations of humidity and temperature, especially crystallization and dissolution cycles of salt in the wall cause partial detachment of painted plaster layers from the supporting wall [1]. In the preservation of such delicate artwork it is important to identify the debonded regions and to assess the stability of the mechanical bond between the plaster and the wall. A commonly used technique is the so-called percussion method: the restorer knocks slightly with his finger against the test section and estimates the quality of the bond from the acoustical response. Obviously, the method has a couple of disadvantages: the knocking procedure is very cumbersome and time consuming, especially if repeated exploration of large wall sections is required. Furthermore, the method is intrusive because the finger contact may affect the delicate layer of paint and expensive scaffolding is usually necessary. Finally, the results may differ depending on the individual experience of the restorer.

To overcome these drawbacks, an optical based method is proposed. TV-holography or electronic speckle pattern interferometry (ESPI) has been introduced successfully for the investigation of historical buildings and monuments [2]. Portable equipment was developed that operates directly at the monument and allows long-term measurements. In murals, for example, micrometer-size displacements in plaster sections due to diurnal variations of temperature, humidity and solar radiation have been observed. Detection of volume changes due to crystallization or dissolution of salt that is present in the wall and plaster material is another example. Such studies have contributed much to the understanding of the deterioration processes. In our contribution during ART'96 we basically showed, that TV-holography combined with acoustical equipment can be used as a new powerful tool for remote monitoring of detached plaster areas, too [3]. In this paper we will focus on the development of the system and on further applications at a historical site.

2. EXPERIMENTAL SETUP

The fundamental idea was already pointed out previously [3]. It is based on the fact, that a detached plaster part in front of a rigid wall forms an oscillatory system. It can thus be remotely excited to tiny vibrations by sound irradiation. Since excitation is most effective at resonance, the acoustical driving frequency is swept through the resonance which varies depending on the condition of the particular wall-plaster system. The protection of the delicate artwork as well as an acceptance of the method in practice calls for minimum sound pressure requiring very small vibration amplitudes even in the resonant case. To detect these tiny vibrations of only some tens nanometers a sensitized analog TV-holography system is designed, adapted to the often rough conditions at the historical monuments.

In Fig. 1 the basic configuration of the method is outlined. Vibration measurement by TV-holography is reprorted in detail in literature [4] and thus, only a brief description of the method follows. The object under investigation, in these applications a section of a plastered wall, is excited by sound waves generated by a loudspeaker and simultaneously illuminated by the light of a laser. The backscattered light is collected by the objective lens of CCD-camera. A smooth reference beam guided by a single-mode fiber is superimposed onto the object beam by a beam splitter cube placed in front of the camera target thus forming an image plane hologramm.

Fig 1: Experimental setup for vibration monitoring. (MO: microscope objective, PZT: piezoelectric cylinder, L: lens, BS: beam splitter)

Very similar to time averaging holography, image plane holograms are time averaged for each video cycle. The camera signal then is rectified and high-pass filtered in analog-electronic circuits and finally displayed on a monitor. Optionally, the signal can be fed to a computer with image processing capabilities.

Standard time-averaging vibration measurements yield a resolution in the order of l /5, where l represents the wavelength of the laser light, presently about 800 nm. It has been demonstrated in literature, that high sensitivity of some tenth of a nm can be achieved by periodically changing the path length of the reference beam at an appropriate frequency [5]. For this purpose, the reference beam in Fig. 1 is guided by a mono-mode optical fiber, which is partly wrapped around a piezoelectric cylinder PZT, a device that changes its diameter when applying a voltage. The necessary path length modulation of the reference beam is achieved by stretching the fiber appropriately.

In order to keep the hole system small and compact, miniaturized components were used as for example a laser diode and a CCD-camera. The laser diode has an output power of 150 mW, which is enough to measure areas of up to about two sqare meters. The optical head of the instrument weightsless then two kilogramms and is thus very handy and mobile: it can operate by mounting it on a stable tripod. A zoom objective lens is used for easily changing the field of view. Even big plastered areas can thus be investigated quite quickly by scanning it field by field.

3. ON-SITE MEASUREMENTS IN MEDIEVAL FUNERAL CHAPEL

Fig 2: View of the inner church St. Just in Kamenz, eastern Germany. The apse is covered with a fresco from 14th century. In the left part some components of the measuring equipment can be seen.

Performance and features of the new system were originally investigated at a test wall with artificially produced detachments. In this paper, we demonstrate results that were obtained on medieval murals in the small funeral chapel St. Just at Kamenz in Saxony. Here, the apse is covered with a fresco originating from the 14th century. In Fig. 2 a photograph of the inner church is shown taken during the measurements. In the left part of Fig. 2 some components of the equipment can be recognized, like e.g. the monitor and the loudspeaker mounted on tripod. Probably due to high amounts of salt and humidity inside the walls, parts of the mural show pronounced dilapidation. Especially in the bottom near sections humidity and water-soluble salts infiltrate from the ground leads and lead to severe damages. Some conservatory remedies already have been accomplished. A main thoroughfare directly adjoining the church was assumed to be a further source for deterioration.

In 1996 the adherence of the plaster to the wall was examined in the whole apse by the percussion method. Thus, a complete map of percussion exploration of the fresco is available. On this basis, a comparision could be carried out between the results obtained by the new acoustical-optical method and those from the traditional method.

Fig. 3: First test section in vaulted ceiling of apsis. Investigated area has a horizontal extension of about 4 m. Fig. 4: Combined exploration map of investigated area in vaulted ceiling. Detached areas are hatched.

Fig. 5: Second test section at northern wall of apsis. Horizontal extension of investigated area about 4 m (Photo by R. Berg, HFBK Dresden). Fig. 6: Combined exploration map of investigated area at northern wall. Detached areas are hatched.

For this purpose, two test sections at different parts of the apse were selected for the optical investigation. The first area is located about 8 meters high at a clearly vaulted part of the ceiling of the chorus. A photograph of the area is shown in Fig. 3. The horizontal extension of the test section is about 4 meters. The second test area is shown in Fig. 5. It is located at the northern wall of the apse. Its horizontal dimension also amounts about 4 meters. The optical setup and the loudspeaker for the acoustical excitation were mounted on two stable tripods and placed on the bottom of the apse. With the maximum laser output power of about 150 mW, an area of about one to two square meters, depending on its amount of reflectivity, could be observed simultaneously. Thus, the whole investigated areas for both the test sections were divided in several subareas and the measured results were finally patched up to reveal a complete exploration map for each area. The frequencies of the acoustical excitation and of the reference beam modulation were swept from 30 Hz to 500 Hz to find the resonances of the debonded areas. The sound pressure was kept constant to about 85 dB, the induced vibration amplitudes were about 20 nm. The resulting electronically filtered TV frames and one unprocessed original image of the test area were recorded on a video tape for later evaluation.

In Fig. 4 the resulting map of loose areas for the first test section in the ceiling as detected by the new and by the traditional method are shown. The regions hatched vertically represent detached plaster areas measured by the new method, diagonal hatching (from bottom left to top right) marks the detached areas found with the percussion method. In addition, the whole area in Fig. 4 is marked with different orientated diagonal hatching (from top left to bottom right). This indicates the judgement of the restorer, that all the area is not perfectly fixed to the supporting wall. Fig. 6 represents the situation at the second test section, a part of the northern wall. Again, the measured detached areas obtained by the two different methods are overlayed and hatched in a similar manner as in Fig. 4.

Basically, the results obtained by the two different methods as presented in Figs. 4 and 6 show a very good agreement Almost all the loose areas found by percussion method could also be identified with the new acoutical-optical method. Only very few discrepancies remained. In case of the first test section, the vibration measurements show that the complete test area could be excited to vibrations at a whole at low frequencies of about 30 Hz. This is not marked in Fig. 4. It could not be prooved whether the whole plaster carrying brickwork or solely the whole plaster itself was excited to vibrations. In any case, however, the results support the restorer's estimation of missing perfect attachment.

In case of the second test section located at the wall, repeated explorations with both methods were performed in order to clarify the situation at discrepant areas. It was interesting to see, that some of the inital percussion obtained results had to be corrected. However, in very few areas complete conformance of the results was not obtained. In future investigations a careful dissection of such areas is intended where allowed, e.g. at test and demolition objects.

5. CONCLUSIONS

Vibrational TV-holography in combination with acoustical excitation of walls has prooved to be a well suited new method for the monitoring of detachments in historical murals. Since the method can inspect the quality of plaster bonding remotely, repeated applications also at ceilings and vaults can easily be performed without expensive scaffolding. Further greatest advantages are the non-interfering application and its full-field and real-time capabilities. Because of the very small vibration amplitudes it should not affect the delicate layer of paint. The method may become a good complement for the traditional percussion method. Moreover, analysis of the resonance frequencies of the cavities and comparison to acoustic theories should yield further relevant information on the plaster-wall system. In future developments the setup will further be miniaturized to become as compact as a common hand-held TV-camera. In addition, a fully computer controlled system based on a laptop is outlined and will be realized soon. This will provide the possibility to automatically evaluate the results immediately after finishing the measurement. Visualization tools will be applyed in order to present exploration maps well adapted to the needs of conservation and restoration.

ACKNOWLEDGEMENT

The investigations were done in close co-operation with R. Berg and R. Möller from the 'Hochschule für bildende Künste' Dresden (HBFK). The present work is funded by the German Federal Foundation for the Environment (Deutsche Bundesstiftung Umwelt).

REFERENCES

1. P. Mora, L. Mora, P. Phillipot: "Conservation of Wall Paintings", Butterworths, London, 1984
2. Gülker, G.; Helmers, H.; Hinsch, K.D.; Meinlschmidt, P.; Wolff, K.: "Deformation mapping and surface inspection of historical monuments", J. Opt. Las. Eng, Vol. 24, 183-213 (1996).
3. Fricke-Begemann, T.; Joost, H.; Gülker, G.; Hinsch, K.D.; Wolff, K.: "Detection of plaster detachments in historical murals by TV-holography of wall vibrations", Proc. '5th International Conference on Non-Destructive Testing, Microanalytical Methods and Environmental Evaluation for Study and Conservation of Works of Art' (1996) 287-300, Budapest.
4. O.J. Løkberg: "ESPI - The ultimate holographic tool for vibration analysis", J. Acoust. Soc. Am., 75 (1984), 1783-1791
5. Høgmoen, K., Pedersen, H.M., "Measurement of small vibrations using electronic speckle pattern interferometry: Theory", J. Opt. Soc. Am. 67(11), 1578-1583 (1977).