Authentication of Ancient Ceramics
The benefits of industrial computed tomography for the study and authentication of ancient ceramics
Peter Graßmann, art historian, Villingen-Schwenningen / XRAY-LAB
For private collectors as well as for museums, the high number of counterfeits of antique objects is a great challenge. Counterfeiting has been going on since there was a market: Michelangelo has already been able to prove the production of a Cupidos declared as antique. Since the 20th century, however, the number and quality of falsifications have increased considerably, and with the rise of Internet trade, 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 counterfeitingrecognition is also increasinglyoccurring. In the following, an overview of possibilities and opportunities for the use of industrial computed tomography in ceramic research will be given. As a practicalapplication example, a vessel of the Peruvian Nazca culture was investigated.
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 counterfeit. Although this figure cannot be verified, it must be assumed that the proportion of false 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. Although complete forgeries are the most common, even among ceramics, there are also a large number of other problematic cases, some of which are in the grey area between the original and plagiarism. This is particularly true of ob ings with extensive restorations/additions, which are not always based on an intention to counterfeit, but which are sometimes also the product of improper or historical (and thus often more “complementary”) restorations.
Since the 18th and 19th centuries, antique collector’s items have served to satisfy the need for representation of their owners and to demonstrate their educational claim, 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 is often contrasted with the real conservation conditions, whereby not infrequently by extensive restorations was “helped”. In some cases, it is only the modern filling mass that holds together a patchwork of different original fragments (“pastiche” or “pasticcio”). Although such fabrications are a widespread phenomenon, especially among Chinese and West African antiquities, they are also known in Europe and South America. The Peruvian forger Zenón Gallegos Ramirez is quoted as saying that often the majority of the vessels he “restored” were modern. The additions are typically modeled from plaster, which is painted over and painted over. Common authentication methods can then fail: paints applied after the fire can actually be removed by a cotton swab soaked with solvents (e.B. acetone), but since the paint is often resistant to conventional solvents, the method here is not meaningful. The result of the actual extremely reliable thermoluminedating must also be questioned in this case.
Problems of thermoluminescence dating
In thermoluminedating, the energy stored in the quartz components of the clays is measured from natural radioactive decay processes, which are emitted during heating in the form of light emissions. Since the energy is “set to zero” during the fire and then the process of exposure and absorption begins anew, conclusions can be drawn about the age of the sample based on the measured amount of energy, 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 significance with regard to pastes/assemblages from disparate individual fragments. The restorer Mark Rasmussen points out other potential problems, including the practice documented in individual cases of sculpting objects from ancient material (e.B. bricks). The Oxford Authentications laboratory has collected interesting case studies that make it clear that dating in individual and suspected cases should be supplemented by further investigations. These include, for example, Chinese counterfeits, which are used at ancient questions at the base, where TL samples are typically taken (as this is often the only unglazed spot).
CT for the Examination of Ancient Ceramics
As a non-invasive and non-destructive method of studying the structure of a ceramic and thus also recognizing additions with almost one hundred percent certainty, industrial computed tomography is a good way. While industrial CT systems are optimized for the human body and therefore limited in their accuracy and penetrating ability, industrial CT can be used to detect inhomogeneities with high precision, penetrate all layers and reliably distinguish different materials at hand their different absorption values.
Despite these advantages, the use of CT is only slowly gaining ground in archaeological, museum, and art historical research, which is due, among other things, to the availability of appropriate facilities. A pioneering project was carried out in the 90s at the Allard Pierson Museum in Amsterdam, where the construction of Greek vases was examined using CT. One focus was on the most accurate representation of vessel profiles, whereby, in contrast to conventional drawing, the inner structure of the vessel wall could also be precisely recorded. A remarkable result was the distinction between different types of ceramics on the basis of their typical gray values, which corresponded with the stylistic assignment to individual artists. Since these first experiments, the use of CT has proven itself many times over, among other things for the archaeometry recording of vessel profiles and wall thicknesses. Further pioneering research projects took place in Graz and Vienna. Particularly noteworthy is the documentation of historical restorations on two Greek vessels from the Universalmuseum Joanneum. A vessel could be identified
as an assemblage and the procedure of the restaurateur reconstructed: Aus a larger fundus, individual fragments of different vessels had been selected, which were combined with a supplementary mass to form a total form. The different origins of the shards could be clearly seen on the basis of their wall thicknesses and degrees of brightness in the CT. A thin liquid varnish had led to a dark surface coloration and an optical leveling of shards and complementary mass so that the manipulations were difficult to detect from the outside. On another vessel, modern overpainting could be documented on the basis of the higher density of the paint used.
Several works dealt with the radiographic analysis of production techniques, whereby it could be shown, among other things, that the arrangement and orientation of air bubbles conclusions on the respective construction method (e.B. production on the potter’s wheel, bead technology, etc.) allowed. Of course, the non-invasive nature of CT 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 medicalCT, which until then had to be broken according to previous documentation in order to study their contents. Also for the assessment of the stability and the damage picture of objects and thus for the conservation work CT can provide important information.
Since the TL dating is based on the energy stored in the ceramic, it is possible (and expected) that a previous CT examination falsifies the result. The assessments of the extent of the changes differ widely and depend, among other things, on the system used. In principle, it can be assumed that a distinction between antique and modern should still be possible even after X-ray and CT examinations, but such potentially irreversible influences must of course always be taken into account when choosing the method of investigation and the order in which it is applied. Since in all human interventions basically with physical changes to the object is to be expected, albeit z. T. to an unmeasurable extent, the term “non-destructive” is not without controversy for CT examinations. 
Example of Investigation: Nazca ceramics
As a practical example, in spring 2021 XRAY-LAB GmbH investigated an anthropomorphic vessel of the early Nazca culture from Peru (style phase 3, ca. 200-300 AD) . A previous thermoluminescence dating determined an age of 1200 – 1800 years, ascoincides with the stylistic classification. The aim of the study was to obtain further information about the structure of the vascular body and possible modern additions. The materialshows asingle, for culture and epoch expected appearance of a single,quite fine key. The average wall thickness was about 4-5 mm and was greatest at the head and in the middle of the abdomen, the floor was comparatively thin-walled. The sound showed numerous smaller air
pockets, which were evenly spread over the entire body. A uniform orientation could not be discerned, but individual elongated, horizontal inclusions indicated a structure by means of the bead technique,  as it is also often documented for the Nazca culture. The “ears” and the “nose” of the vessel replicating a human figure seem to have been shaped and attached by separate pieces of clay, which is indicated by elongated air pockets at the presumed attachment points. Overall, neither in the vessel body nor on the surface could inhomogeneities be detected which could have been included by modern restorations or significant changes (see figures in the Annex).
Industrial CT offers a wide range of applications in archaeological and art historical research, for the restoration and authentication of ancient objects. In the present example of an ancient Nazca vessel, a forgery could be largely ruled out by the combined significance of provenance research, iconographic-stylistic analysis, thermoluminedating and CT. Such a multi-level approach isgenerally recommended for minimizing risk in dealing withantiquity, the provenance of which cannot be fully traced. Although the use of CT is efficient, non-invasive and basically non-destructive, it should be carefully considered in individual cases,both in terms of thecost-benefit ratio and the possible influence of energy-intensive radiation on the result of thermoluminedating. It is recommended to carry out the latter in advance if necessary.
 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.
 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.
 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.
 Vgl. Bruhns/Kelker, S. 21.
 Vgl. ebd.
 Vgl. Rasmussen/Amble, S. 54f, abgerufen am 30.5.2021.
 Vgl. https://www.oxfordauthentication.com/case-studies/, abgerufen am 30.5.2021.
 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.
 Vgl. Karl/Rosc, S. 74f.
 Vgl. Ebd., S. 76.
 Vgl. Ebd., S. 77.
 Vgl. Fürhacker/Karl, S. 137 ff.
 Vgl. ebd., S. 140.
 Vgl. ebd., S. 153.
 Für eine Übersicht über den Forschungsstand siehe: St. John, S. 54ff, abgerufen am 30.5.2021.
 Vgl. Applbaum, S. 231-245.
 Vgl. McKenzie-Klar/Magnussen, S. 208-221.
 Vgl. Karl/Rosc, S. 88ff.
 Vgl. ebd., S. 90.
 Für ein sehr ähnliches Erscheinungsbild vgl.: Takenouchi/Yamahana, S. 9.
 Vgl. Carmichael, S. 31-48.
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 besondererBerücksichtigung 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.