Category 18 April 2019

FING-ART-PRINT-case studies

FING-ART-PRINT aimed to develop a non-contact method for unequivocally identifying objects, mainly artworks, through a deep 3D profilometric characterisation of small areas. By measuring the roughness and visible spectral reflectance of this area a unique “fingerprint” can be obtained. Then, the roughness profile is able to define submicrometric details of paintings and art pieces, and spectral diffuse reflectance measurements in the visible light range provides essential information on the pigments of the paint surfaces.

Along the development of this technique several case studies were carried out for validation of the technique in the field of cultural heritage conservation. So, thirty objects were provided by ten museums and two private galleries in order to obtain their roughness fingerprints. Through these studies the feasibility of taking fingerprints in short times was shown, and the uniqueness of fingerprint could be assessed. Moreover, these studies were applied to determine which kind of objects could be fingerprinted, and to demonstrate the relocation of fingerprints. Finally, a valuable feedback of users could be obtained from these sensing trials.

The first set of trials was carried with a table model, so the location of scanned areas had to be found by visual inspection and manual position. In the second set of studies photographs were used for relocating fingerprints, using a robot to assure that repeated fingerprints were made on the same area.

As an example from the first set, several measurements were made on the icon “St Georgios from Ioannina” during the construction of the prototype. The item is a greek 19th century art piece painted on wood with egg tempera paint. Roughness measurements were taken using a confocal white light profilometer, with a spatial resolution from 2 to 3 micrometers and a depth resolution of 1 micrometer. Lower depth resolutions, although possible, were discarded due to the risk of damaging valuable objects. Spectral fingerprints were taken with a portable spectrophotometer equipped with an integration sphere using a spot size of 4mm diameter and an angle of 2° observer. The specular component was excluded and the spectral resolution was set in 10 nm for a wavelength range between 400 and 700 nm.

Surface roughness was measured in several areas of the picture, locating their exact position to enable relocation of fingerprints in the future. The roughness profiles were displayed as false-colour microtopographic maps, where craquelure networks and zones with flaked varnish could clearly be identified. Moreover, since the spatial resolution was at the level of pigment particles, handmade copies of this picture could be unequivocally distinguished from the original art piece. Reflectance spectra were also considered as unique for a particular paint. In conclusion, just by combining roughness and reflectance patterns an irrefutable identification of artworks can be made, protecting them from falsification and theft. In general terms, fingerprints can be considered the “passport” of artworks.

In spite of these good results, additional measurements were necessary in order to assess some issues, such as the effect of aging or damages on the fingerprints. Furthermore, an additional issue was assuring the repeatability of measure positions for fingerprinting. In order to solve this problem, the profilometer was mounted on a robot arm in the second set of studies. Unfortunately, in spite of improving reproducibility, vibration of the robot arm led to a slight loss of resolution.

Within the second set of experiments, roughness profiles were found to be equal to the first set of measurements. However, it should be noted that the relative height difference between points, and not the absolute height values, must be considered for comparison purposes. For instance, a recognisable impasto structure could be identified in both sets when roughness profiling was applied to a painting reproducing Van Gogh’s painting technique on linen. On the other hand, matching profiles between table and robot models was complicated in other cases, for instance when roughness profiling was applied to an Egyptian faience vessel. Due to the vibration of the robot, a fine structure, attributed to microscopic bubbles in the glaze layer, could not be detected with the robot model. Other art pieces, like paintings applied on wood, exhibited the same issue, since the fine structure of the grain wood could not be discriminated with the robot model.

As a general conclusion of these tests, the developed technique can be used for discovering and tracking unique patterns on different artwork types, such as metals, plastics, printed paper, paintings, wood, and ceramics. In practice fingerprints can even differentiate metal objects obtained from a same mould, or pages taken from different copies of a same book. However, quality fingerprints cannot be obtained from textiles, unprinted paper and unstable objects (i.e. corroded ones), mainly due to legal aspects but also because textile and paper weaves can move slightly. In this case, the general orientation of the waves and the surface roughness of yarns and/or fibres could be used as a reference.

Related references:

W. Wei, P. Boher, M. Elias, J. Frohn, K. Martinez, and S. Sotiropoulou, ” A New Non-Contact Fingerprinting Method for the Identification and Protection of Objects of Art and Cultural Heritage Against Theft and Illegal Trafficking”, Proceedings of the 7th EC conference Safeguarded Cultural Heritage – Understanding & Viability for the Enlarged Europe – SAUVEUR, 31 May – 3 June 2006, Prague, Czech Republic (2006).Wei, W, Frohn, J, Sotiropolou, S and Weber, M. (2007). Experience with a New Non- Contact Fingerprinting Method for the Identification and Protection of Objects of Cultural Heritage Against Theft and Illegal Trafficking. In: Argyropoulos, V, et al (eds.) Proceedings of the International Conference on Strategies for Saving Indoor Metallic Collections. Cairo, Egypt, 25 February–1 March 2007. Athens: Technological Education Institute of Athens.Wei, W, Frohn, J and Weber, M. 2007. Characterisation of the varnish-object interface using white light confocal profilometry. In: Proceedings of SPIE Volume 6618: SPIE European Symposium on Optical Metrology, O3A: Optics for Arts, Architecture, and Archaeology. Costas Fotakis, C, Luca Pezzati, L and Renzo Salimbeni, R (eds.). Bellingham, WA: SPIE.

S. Sotiropoulou; S.and W. Wei, ” Non-contact method for the documentation, evaluation and monitoring of conservation treatments for icons”, International Meeting Icons: Approaches to research, conservation and ethical issues, Benaki Museum, Athens, 3-7 December (2006).

W Wei, V Bakker and A Lagana, “A semi-automatic system for the non-contact ‘fingerprinting’ of objects of art and cultural heritage”, Legal issues in conservation, ICOM COMMITTEE FOR CONSERVATION, 2008, Vol. I, pp. 1011-1016.