|
Abstract
Laser techniques are being successfully employed in the conservation of historical buildings in many European countries and not only, proving the advantage in preserving historical layers otherwise impossible, especially for very degraded stone or metal. Today, optoelectronical systems make products out of the laser cleaning research projects, for example, and laser investigation-diagnosis systems are more often desired. An increasing number of professional conservator-restorers are also being acquainted with these new instruments and methods. The future perspective of lasers in conservation will deal more and more with current applications. This paper describes how a good practice demonstration based on an advanced laser technique generated useful results in the frame of the “STONE HOUSE” Project - CLT2006/A1/RO-80 - under the Culture 2000 Program. Long-term microclimate monitoring and air quality periodical evaluation were associated with temperature distribution on exposed outdoor walls to identify the deterioration stress. Thus, a 3D model with the thermal representation of the walls was generated, which is much more useful than a traditional thermal representation. Measurements were made during the cold season up to July 2007. Also, repeated laser scanning allowed to complete successive three-dimensional records of the surface at different moments that could evaluate erosion speed and to check preliminary theoretical (analytical) stress distribution on the surface. Introduction Historical buildings and cultural heritage are affected not only by natural ageing but also by natural calamities, inappropriate interventions and modifications, and even vandalism. That is the reason why documentation of works of art is so important, mainly in what concerns fragile artifacts, monuments to be restored, or archaeological sites under excavation works. Moreover, the documentation of works of art, especially high accuracy documents, allows the construction of real and virtual models, providing basis for restoration studies and knowledge dissemination. The general objectives of the project have been focused on: shafting and highlighting the common cultural heritage of European significance; disseminating know-how and promoting good practices concerning conservation and safeguarding the community heritage; fostering an intercultural dialogue and know-how exchange between Romania, Bulgaria, France and other countries from the Balkan area (Serbia, Greece, Macedonia, Croatia and Turkey). Particularly, the challenge of the project consists in the revitalization of a multi-ethnic element by placing it in the chore of a regional anthropological research net. The historical site, located at 35 km from Bucharest and 15 km North from the Danube River, includes a piece of land of 6 ha, a 17th century Church, and two country castles. One of those, known as the Stone House, is an architectural monument built in 1646 by the Chancellor of the King Matei Basarab, following an Italian Renaissance pattern. The import of this European model was perceived at the time by the contemporaries as a sign of modernity. This particular building, made of an unusual material for the village, has become in time the mark of a local identity. The project aimed to place this memory object in a larger values map (Balkans region). An on-site demonstration of the results of this multidisciplinary applied research implementation for historical monuments investigation was organized, as good practice example, as one of the planned activities of the project. Although the practice of scientific conservation is more and more applied by universities and important museums it is less known and respected when the monument is outside of urban areas. The trained group included students, young architects and conservator-restorers, but local people also assisted. This demonstration certainly showed the polyvalent result of the project. Being characterized by shape, the building was recorded and documented in 3D (three-dimensional or volumetric) images in order to contain more complete geometrical information and to make possible volumetric reproductions, like material replicas or virtual images. 3D Laser Scanning is a technique that takes advantage of the coherence properties of laser radiation, which consists in a very pure color and highly directional light beam. This technique is able to acquire, store and process 3D computer images and information of the objects using a low power laser beam as the light source and detecting the light reflected from the object surface on very sensitive sensors. As high quality solution, comprehensive and supported by expert professionals, 3D scanning is a proper solution for a fast, complex and accurate recording of historical building digital model. Because of the unfriendly environment conditions’ dynamics, with high temperature gradients and direct exposure to seasonal strong wind, the evaluation of differential erosion risk was only possible by corroborated investigations and measurements. 3D Shape recording The scanning measurements were structured, from the resolution point of view, in both medium and high resolutions scans. The 3D recordings were made inside the house, namely in one of the basements, shown in figure 2, at a resolution less than 2 mm, and two communicating rooms, and outside the house. For 3D data acquisition a phase shifting scanner was used, with a range from 1.5 up to 22 m. Angular resolution may vary from 17 up to 180 lines per degree. This factor may give the value of the spatial resolution, the highest value being less than 200 µm when the distance between the object and the scanner is the minimum (1.5 m) and the angular resolution is the highest (180 lines/ degree). The laser power is 15 mW with a wavelength of 690 nm. The scanner is connected to a laptop which controls all the acquisition parameters (angular resolution, angular coordinates, estimative distance or object) and all the data are downloaded on the laptop’s hard disk. The format of exported data may be *.stl or *.obj mesh format or 4 columns ASCII file (the first 3 columns the xyz coordinates while the 4th represents the value of the red intensity reflected and recorded by the scanner) for the points format. The highest resolution scans where made on the inscriptions: three on the entrance of the basements and another one in the right side of the right entrance (reading “29 Juin 1883 / 29 Juin 1888 rendez-vous", see figure 4). One scan was also made on the main facade, with a planar resolution less than 3 mm.
Figure 1. View of the Stone House in Heresti; Figure 2. General view of the basement in 3D;Figure 3. High resolution scan of the façade inscriptions; Figure 4. High resolution representations of the right entrance inscription; Figure 5. 3D representation of the main facade;
Figure 13. 3D view of the first floor chambers. As many protocols and procedures request, microclimate monitoring is a compulsory activity for the conservator-restorer from the very first stages till the end of the intervention, and a permanent responsibility of the monument keepers. The importance of the microclimate monitoring will not be stress here as it is beyond the objective of the present paper. A simple network of 4 dataloggers was implemented in both levels (figures 6 and 7) for 1 year long monitoring of the air temperature and relative humidity. Obviously, all data collected during the training and dissemination of scientific conservation practice have its own value, useful to determine microclimate dynamics and could be immediately used by the conservators and administrative team. Due to a relative mild winter, neither the temperature nor the relative humidity gradients reach alarming values. However, the parameter dynamics has some high risk: possible biological contamination and organic matter degradation. Detailed measurement plots and values are available on the project’s website at
Additional measurements of humidity in the walls were made inside and outside of the main facade (figure 9). An interesting fact is the similarity of the corresponding values at 0.5 and 2 meters height. Furthermore it can also be observed that it converges to the same values as the ones at 0.5 meters once the measured points get farther than North East wall.
Figure 6. Plan of the ground floor with the localization of sensors S7 and S15.
Figure 7. Plan of the first floor with the localization of sensors S31 and S32. Figure 8. Plots of temperature and relative humidity variations recorded by the 15th sensor. Figure 9. Humidity walls measurements (investigated points are highlighted in red). Figure 10. Relative humidity variation on the facade wall in March 2007.
Monitoring of historical buildings walls represents one of the possible thermovision applications in order to rebuild or conserve cultural goods. The results of this technique are given with thermal images, displayed in pseudocolors. The advantage of this display method is that it offers better intensity variations which are easier to identify. Qualitative studies in thermography consist in the study of the thermal images in order to identify the presence of anomalies and their localization. Using 3D matching algorithms, a 3D thermal representation was obtained, the thermal image being projected on the 3D surface. As a consequence of the building’s age and its location on a higher river bank with no tall vegetation, the windy winters strongly washed out the old stone surfaces. The original allure of the building was kept due to some new aesthetically well integrated stone pieces. A perfect “fingerprint” of the wind still exists on many of the original stones and proves an unsuspected environmental stress over the building. The most interesting observation after preliminary investigation can be extracted from the thermovision inspection. All thermal images, collected in different seasons and at different moments of the day, indicate a permanent distinct distribution of the temperature on the new and old stone pieces. On these, the maximum gradient of temperature did not exceed 3° C. From previous experience, the surface decay in similar cases but with high temperature gradient on surface could also be explained by the significant temperature stress. We will start a long-term study on the possible tension induced by similar categories of material but with not enough appropriate values of optical absorbance, thermal emissivity and other parameters.
Figure 11. 3D thermal representation of the facade (December 2006).
Figure 12. 3D thermal representation of the façade (January 2007).
Three dimensional representations of different sets of measurements may represent a new and better way to inspect the object characteristics. This representation of the thermal distribution on a 3D surface offers a better visualisation of the thermal distribution, identifying the existing problems and stressful areas of the walls in a different way and offering, finally, an improved method for diagnosis. Similarly, a three-dimensional representation of imaging techniques may be obtained adding results of laser induced fluorescence (LIF), which may be visualised in a 3D environment, offering detailed 3D mapping of selected spectra. The visualisation can also be improved using Doppler vibrometry techniques. Vibration is induced to the surface of the object by loudspeakers or piezoelectric sensors. Then, the laser is scattered and the Doppler shift (between an original beam and the shifted beam) is recorded. Measuring the frequency response, and combining it with a technique that acquires the response in time (laser scanning), a 3D visualisation of the vibration’s amplitude can be achieved. In this particular case, the vibration may also be correlated with the actual shape of the object. Acknowledgments The activities at Heresti Sone House were developed under the Culture 2000 Program Project - CLT2006/A1/RO-80. The authors are also thankful to PNCDI II program 91-009-Imagist project and Nucleu Program 09-27.01.01 for financial support.
About the authors
Dragos Ene
Dragos Ene graduated in 2007 from the Faculty of Electronics, Telecommunications and Information Technology at the Polytechnic University of Bucharest, with specialization in Applied Physics. Since the same year he is a PhD student at the Faculty of Applied Science with the main focus on optical methods used to study conservation status of the monuments. t the present he is part of the research team from the Department of Advanced Methods and Techniques for Artwork Restoration Conservation of the National Institute of Research & Development for Optoelectronics (INOE) 2000 in Bucharest.
Roxana Radvan received the BS degree in Applied Optics and Fine Mechanics in 1990 from Polytechnic University of Bucharest and her doctoral degree on non-conventional optics in 1996 from Technical Army Academy of Bucharest. She is a researcher at the National Institute of Research & Development for Optoelectronics (INOE) 2000 and is working on optoelectronics application on cultural heritage protection and conservation-restoration. She coordinates a thematic national network in this field – PRO RESTAURO, is COST G7 vice-president and member of LACONA Permanent Scientific Committee.
▲TOP
This html version may not include all features of the article.
To read this article with images at full quality please Download this article
Permanent link to this resource: http://www.e-conservationline.com/content/view/864 |
||||||||||||
