Antique Nazca Sculpture

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The utility of industrial computed tomography for the examination and authentication of ancient ceramics.

As an example serves the examination of a vessel from the Peruvian Nazca culture

Peter Grassmann, art historian, Villingen-Schwenningen / XRAY-LAB

For private collectors and museums alike, the high number of forged antique forgeries is a major challenge for private collectors and museums alike. Forgery has been going on since there has been a market: Michelangelo was already proven to have Michelangelo was proven to have produced a Cupid that was declared to be antique. Since the 20.

Since the 20th century, however, the number and quality of falsifications have and with the emergence of internet trade, the problem has taken another the problem has taken on a new dimension in the last 30 years. In addition to the classical tools of art history such as stylistic-iconographic analyses, the use of scientific methods for forgery scientific methods for the detection of forgeries. In the following overview of the possibilities and opportunities for the use of industrial computed industrial computed tomography in ceramics research. will be presented. As a practical application example, a vessel from the Peruvian Peruvian Nazca culture was examined

 

Forgeries and over Restorations

According to an estimate by the Swiss art dealer Christoph Leon, up to 50% of the antiquities on the market are fakes.[1] Although this figure cannot be verified, it must be assumed that the proportion of fake pieces is enormous and affects not only all areas of the art trade, but also museum collections. This is made clear not least by spectacular and widely publicized cases of the past.[2] Although complete forgeries are the most common, even among ceramics, there is also a large number of other problematic cases, some of which are in the gray area between original and plagiarism. This is especially true for objects with extensive restorations/additions, which are not always based on an intention to forge, but are sometimes the product of improper or historical (and thus often more “addition-friendly”) restorations.

Since the 18th and 19th centuries, antique collection pieces have served to satisfy their owners’ need for representation and to demonstrate their claim to education, which is why the vessels had to be as complete and undamaged as possible. This has not fundamentally changed to this day. This was and still is often contrasted by the real conditions of preservation, which were not infrequently “helped along” by extensive restorations. In some cases, it is the modern filler alone that holds together a patchwork quilt of disparate original fragments (“pastiche” or “pastiche”). Although such fabrications are a widespread phenomenon, especially among Chinese and West African antiquities, they are also known to have occurred in Europe and South America.[3] Peruvian forger Zenón Gallegos Ramirez, for example, is quoted as saying that often most of the vessels he “restored” were modern.[4]

The additions are typically modeled from plaster, which is painted over and varnished. Common authentication methods may then fail: Paintings applied after firing can actually be removed by a cotton swab soaked in solvents (e.g. acetone), but since the paint is often resistant to conventional solvents, the method is not meaningful here.[5] The result of the actually extremely reliable thermoluminescence dating must also be questioned in this case.

Problems of thermoluminescence Dating

Thermoluminescence dating measures the energy stored in the quartz components of the clay from natural radioactive decay processes, which is emitted in the form of light emissions when heated. Since the energy is “zeroed out” during firing and then the process of exposure and absorption begins anew, conclusions about the age of the sample can be drawn based on the amount of energy measured, taking into account environmental conditions. The method is well researched and very reliable, but since only the sample is dated and not the entire object, it has little validity with regard to pastiches / assemblages of disparate individual fragments. Restorer Mark Rasmussen points out other potential problems, including the practice of sculpting objects from ancient material (e.g., bricks), which has been documented in individual cases.[6] The Oxford Authentications laboratory has collected interesting case studies that make it clear that TL dating should be supplemented by further investigation in individual and suspected cases. [7] These include, for example, Chinese forgeries that have ancient fragments inserted at the base where TL samples are typically taken (as this is often the only unglazed area).

CT for the examination of ancient Ceramics

Industrial computed tomography is a non-invasive and non-destructive method of studying the structure of a ceramic and thus also detecting additions with almost 100 percent certainty.[8] While medical CT systems are optimized for the human body and are therefore limited in their imaging accuracy and penetration capability, industrial CT can be used to detect inhomogeneities with high precision, penetrate all layers and reliably distinguish between different materials on the basis of their different absorption values. Despite these advantages, the use of CT has been slow to catch on in archaeological, museum, and art history research, due in part to the availability of appropriate equipment. A pioneering project was carried out in the 1990s at the Allard Pierson Museum in Amsterdam, where the structure of Greek vases was investigated using CT.[9] One of the main focuses was to depict vessel profiles as accurately as possible, and in contrast to conventional drawing, it was also possible to precisely record the inner contour of the vessel wall. A remarkable result was the differentiation of various types of pottery on the basis of their respective typical gray values, which corresponded with the stylistic assignment to individual artists.[10]

Since these first experiments, the use of CT has proven itself many times, among other things, for the archaeometric recording of vessel profiles and wall thicknesses. Other groundbreaking research projects have taken place in Graz and Vienna,[11] most notably the documentation of historical restorations on two Greek vessels from the Universalmuseum Joanneum. One vessel was identified as an assemblage and the restorer’s procedure was reconstructed: Individual fragments of different vessels had been selected from a larger find, which were combined with a supplementary mass to form an overall shape. The different origins of the sherds could be clearly identified by their wall thicknesses and degrees of brightness in the CT.[12] A thin varnish had led to a dark surface coloration and an optical leveling of the sherds and supplementary mass, so that the manipulations were difficult to recognize from the outside.[13] On another vessel, it was possible to document, among other things, modern overpaintings on the basis of the higher density of the paint used.[14] In the case of the other vessel, it was possible to identify the sherds and the supplementary mass as a whole.

Several papers dealt with the radiographic analysis of manufacturing techniques, showing, among other things, that the arrangement and orientation of air bubbles allowed conclusions to be drawn about the particular method of construction used (e.g., manufacture on the potter’s wheel, bead technique, etc.).[15]

The non-invasive nature of CT, of course, helps in a special way with questions that would otherwise require irreversible damage to the objects. At the Hebrew University in Jerusalem, for example, clay containers for cuneiform tablets were examined using medical CT, which until then had to be broken after prior documentation in order to study their contents.[16] CT can also provide important information for assessing the stability and damage pattern of objects, and thus for conservation work.[17]

Restrictions

Since TL dating is based on the energy stored in the ceramic, it is possible (and expected) that a previous CT examination will falsify the result. Estimates of the extent of the changes vary widely and depend, among other things, on the equipment used.[18] In principle, it can be assumed that a distinction between ancient and modern should still be possible after X-ray and CT examinations, but such potentially irreversible influences must of course always be taken into account in the choice of examination method and the sequence in which it is applied. Since physical changes to the object are to be expected in principle with all human interventions, albeit in some cases to an extent that cannot be measured, the term “non-destructive” for CT examinations is not without controversy.[19]

Study example: Nazca ceramics

As a practical example, an anthropomorphic vessel of the early Nazca culture from Peru (style phase 3, ca. 200-300 AD) was examined by XRAY-LAB GmbH in spring 2021. A previous thermoluminescence dating determined an age of 1200 – 1800 years, which agrees with the stylistic classification. The aim of the investigation was to gain further information about the structure of the vessel body and possible modern additions. The material showed a uniform appearance of a single, rather fine clay type, which is to be expected for culture and epoch. The average wall thickness was about 4-5 mm and was greatest at the head as well as in the middle of the belly; in contrast, the base was comparatively thin-walled. The clay showed numerous smaller air inclusions, which were evenly distributed over the entire body. A uniform orientation could not be recognized, but single elongated, horizontal inclusions indicated a construction by means of the bead technique,[20] as it is also frequently documented for the Nazca culture.[21] The “ears” and the “nose” of the vessel imitating a human figure seemed to have been formed and attached by separate pieces of clay, which was indicated by elongated air inclusions at the presumed attachment points. Overall, no inhomogeneities could be detected in the body of the vessel or on the surface that would have suggested modern restorations or significant alterations (see figures in the appendix).

Zusammenfassung

Industrial CT offers a wide range of applications in archaeological and art historical research, for restoration and for the authentication of antique objects. In the present example of an antique Nazca vessel, the combined informative value of provenance research, iconographic and stylistic analysis, thermoluminescence dating and CT largely ruled out the possibility of a forgery. Such a multi-stage approach is generally recommended to minimize risks when dealing with antiquities whose provenance cannot be fully traced. Although the use of CT is efficient, non-invasive and basically non-destructive, it should be carefully weighed up in individual cases, both in terms of the cost-benefit ratio and the possible influence of the energy-intensive radiation on the results of thermoluminescence dating. It is recommended to perform the latter in advance, if necessary.

References

[1] Vgl.: https://www.dw.com/de/der-spanische-meister-das-schmutzige-gesch%C3%A4ft-mit-gef%C3%A4lschten-antiken/a-18993860, abgerufen am 30.5.2021.

[2] Als spektakulärster Fall gilt das Goldmuseum in Lima, von dem im Jahr 2001 bekannt wurde, dass 85% der ausgestellten Exponate gefälscht waren. Vgl.: https://www.forbes.com/2002/01/09/0109connguide.html?sh=59487c5a285f, abgerufen am 30.5.2021.

[3] Bruhns/Kelker weisen auf ein Moche-Porträtgefäß aus der Arthur M. Sackler-Sammlung hin, das sich als Kombination aus einer authentischen Basis und umfangreichen modernen Ergänzungen entpuppte, vgl. Bruhns/Kelker, S. 21. Sawyer beschäftigte sich mit Beispielen der Nazca-Kultur, darunter einer aus Fragmenten von zwei verschiedenen Gefäßen zusammengesetzten Neuschöpfung, vgl. Sawyer S. 21.
[4] Vgl. Bruhns/Kelker, S. 21.
[5] Vgl. ebd.
[6] Vgl. Rasmussen/Amble, S. 54f, abgerufen am 30.5.2021.
[7] Vgl. https://www.oxfordauthentication.com/case-studies/, abgerufen am 30.5.2021.
[8] Bruhns/Kelker weisen unter Bezug auf Mark Rasmussen auf die Bedeutung von CT und Röntgen in Ergänzung zur TL-Datierung hin, vgl. Bruhns/Kelker, S. 31.
[9] Vgl. Karl/Rosc, S. 74f.
[10] Vgl. Ebd., S. 76.
[11] Vgl. Ebd., S. 77.
[12] Vgl. Fürhacker/Karl, S. 137 ff.
[13] Vgl. ebd., S. 140.
[14] Vgl. ebd., S. 153.
[15] Für eine Übersicht über den Forschungsstand siehe: St. John, S. 54ff, abgerufen am 30.5.2021.
[16] Vgl. Applbaum, S. 231-245.
[17] Vgl. McKenzie-Klar/Magnussen, S. 208-221.
[18] Vgl. Karl/Rosc, S. 88ff.
[19] Vgl. ebd., S. 90.
[20] Für ein sehr ähnliches Erscheinungsbild vgl.: Takenouchi/Yamahana, S. 9.
[21] Vgl. Carmichael, S. 31-48.

Literatur

Applbaum, Nachum: The Use of Medical Computed Tomography (CT) Imaging in the Study of Ceramic and Clay Archaeological Artifacts from the Ancient Near East, in: Uda, M. et al. (Hg.): X-rays for Archaeology, Dordrecht 2005.

Carmichael, Patrick: Nasca Pottery Construction, in: Ñawpa Pacha. Journal of Andean Archaeology, Nr. 24, London 1986.

Fürhacker, Robert; Karl, Stephan: Die Dokumentation historischer Restaurierungen antiker Keramik am Beispiel zweier Gefäße aus dem Universalmuseum Joanneum unter besonderer Berücksichtigung der industriellen Computertomografie, in: Trinkl, Elisabeth (Hg.): Corpus Vasorum Antiquorum Österreich, Beiheft 1, Wien 2013.

Karl, Stephan; Rosc, Joerdis: Berührungsfreie und nicht invasive Untersuchung antiker Keramik mittels industrieller Röntgen-Computertomografie, in: Trinkl, Elisabeth (Hg.): Corpus Vasorum Antiquorum, Österreich, Beiheft 1, Wien 2013.

McKenzie-Klar, Jaye; Magnussen, John: Real and virtual: The role of computed tomography and 3D imaging in museum practice, in: A Cultural Cocphony. Museum Perspectives and Projects.

Rasmussen, Mark; Amble, Thomas: Scientific Techniques in the Authentication Process, http://www.rare-collections.com/PrehistoricAmerican2006-4.pdf

Sawyer, Allen: The Falsification of Ancient Peruvian Slip-decorated Ceramics, in: Falsifications and Misreconstruction of Pre-Columbian Art, Washington 1982.

St. John, Amy: Inside Perspectives on Ceramic Manufacturing. Visualizing Ancient Potting Practices through Micro-CT Scanning, Electronic Thesis and Dissertation Repository, 7502, https://ir.lib.uwo.ca/etd/7502

Takenouchi, Keita; Yamahana, Kyoko: Fine pottery shaping techniques in Predynastic Egypt: A pilot study on non-destructive analysis using an X-Ray CT scanning system, in: Journal of Archaeological Science, Reports 37, 2021.

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