The Eye of the Medusa: XRF Imaging Reveals Unknown Traces of

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The Eye of the Medusa – XRF Imaging Reveals Unknown Traces of Antique Polychromy Matthias Alfeld, Maud Mulliez, Philippe Martinez, Kevin Cain, Philippe Jockey, and Philippe Walter Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b03179 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 20, 2016

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Analytical Chemistry

The Eye of the Medusa – XRF Imaging Reveals Unknown Traces of Antique Polychromy Matthias Alfeld1*, Maud Mulliez1, Philippe Martinez1, Kevin Cain2, Philippe Jockey1,3, Philippe Walter1 1 Sorbonne Universités, UPMC Univ Paris 06, CNRS, UMR 8220, Laboratoire d’archéologie moléculaire et structurale (LAMS), 4 place Jussieu 75005 Paris, France 2 Institute for Study and Integration of Heritage Techniques (insightdigital.org), P.O. Box 1166, Berkeley CA 94701-2166, USA 3 Université Paris Ouest Nanterre La Défense, UMR 7041 - Archéologies et sciences de l'Antiquité (ArScAn), Maison René Ginouvès, 21 allée de l'université, 92023 NANTERRE Cedex, France The colorful decoration of statues and buildings in antique times is commonly described by the term antique polychromy. It is well known among scholars but less so to the general public and its exact form is the subject of research. In this paper we discuss results obtained from the Frieze of the Siphnian Treasury in the Sanctuary of Delphi (Greece). We will present the first application of a mobile instrument for macro-XRF imaging for the in-situ investigation of antique polychromy and show that it allows one to identify significant traces not visible to the naked eye and not detectable by XRF spot measurements or any other mobile, non-invasive method. These findings allow for a partial reconstruction of the polychromy. Furthermore, we present a novel approach enabling the correct interpretation of artifacts resulting from changes of the detection geometry in the investigation of complexly shaped samples by XRF imaging. This approach is based on the 3D surface model acquired by photogrammetry and fundamental parameter calculations.

INTRODUCTION The colorful decoration of statues and buildings in Antiquity is well known among scholars, but less so among the general public. While there is no doubt as to the existence of this antique polychromy, the exact degree of decoration and the techniques used are the subject of research. This gap in knowledge is caused by the fact that, with the exception of a few well-preserved pieces, in general only microscopic remnants of paint remain on the surface of marble art works. These remnants are commonly studied by microscopic techniques under different illuminations and observation in the near infrared.1 Spectroscopic techniques, such as X-ray fluorescence analysis (XRF), are well suited for the identification of pigments without the need of taking samples and thus damaging the object.2,3 These investigations have until now been limited to spot analyses, restricting them to traces of pigments visible on the surface. Recent years have seen a number of originally spot analysis techniques gaining imaging capabilities, which allows to automatically survey large areas and thus yield more representative results.4 To study the distribution of remnants of pigments on the surface of stationary artifacts one needs a method that works in reflection geometry, as the back of the object might not be accessible; allows for a lateral resolution better than a few millimeters to identify the location of minor remnants of pigments; and provides some form of chemical or elemental contrast to distinguish between remnants of pigments from the marble and surface contaminations. If one wants to also detect remnants of pigments below soil depositions, penetrative

primary and secondary radiation is also needed. Finally, all this needs to be realized in the form of a mobile instrument. These demands are only met by XRF imaging. The possibility of obtaining elemental distribution images by scanning the surface of an object with a focused X-ray beam, was established over two decades ago. Recent progress in X-ray tube and detection technology has made it practicable to investigate large areas (several hundred to thousands of square centimeters) on stationary objects by mobile instruments.5 XRF imaging has mostly been applied to historical paintings, given both their high artistic value and the comparable ease of investigating a flat surface. However, also other objects have been recently investigated by XRF imaging, such as stained-glass windows6 or illuminated manuscripts.7 Until now XRF imaging has found only limited application in the investigation of antique polychromy. Powers et al. employed XRF imaging to visualize the distribution of elements in marble inscriptions of the Roman age.8 However, they utilized a synchrotron light source, which required the transport of the object to the synchrotron for investigation, thus limiting the size and number of the investigated objects. A general problem of spectroscopic imaging is that it produces flat projections of the investigated objects. For understanding the investigated object and the acquired spectroscopic data a (preferably complete) 3D model is needed. As this is a common problem of various scientific fields, several methods have been developed and are applied for the accurate acquisition of 3D models.9 In our case the specifications of the model’s acquisition were problematic, mostly because of the short period spent on site and the exhibition of the investigated

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Figure 1 a) Achilles on the eastern pediment of the Siphnian frieze and b) the magnified Gorgoneion decorating his shield. Achilles is approx. 50 cm high, the Gorgoneion is approx. 10 by 10 cm. c) Reconstruction of the Siphnian Treasury, seen from the northwest. Drawing by Theophil Hansen. d) Gorgoneion, painted pottery, 5th century BC.

object in a museum hosting the daily presence of hundreds of visitors. It thus seemed important to chose an approach that would be robust but rather simple and economically sound, while remaining contactless and as non-invasive as possible. These considerations led us to opt for the use of photogrammetry, i.e. the calculation an object’s 3D shape based on series of photographs. We made use of these 3D models to simulate the investigation of the object by XRF imaging with the help of Fundamental Parameter (FP) calculations. This allowed us to distinguish from changes in recorded fluorescence, line intensities resulting from changes in detection geometry and changes in local composition and stratigraphy. FP based approaches have previously been used to correct for shape variations of archeological objects10 and absorption artifacts in elemental distribution images of historical paintings,11,12 but none of them used 3D data acquired by another technique. Below, we will present results obtained on the Frieze of the Siphnian Treasury in the Delphi Archaeological Museum (Greece) in the summer and fall of 2015. We investigated the frieze to gain additional insight in its polychromy and how this changed the perception of it. In the framework of these investigations several unknown remnants of pigments were found, which will be the subject of forthcoming publications. In this publication we present the first in-situ application of XRF imaging for the study of antique polychromy and demonstrate its advantages over spot analysis. This is done with one selected example, which also allows us to show the value of the FP and photogrammetry based calculations.

THE SIPHNIAN TREASURY Delphi was one of the most important sanctuaries of the ancient Greek world. Wealthy cities showed their devotion to the gods by making donations and bringing sacrifices. Several cities donated richly decorated votive buildings, so-called Treasuries, among them the city of Siphnos. The building13 covered a base area of 6.1 x 8.5 square meters with a richly decorated Ionic polychrome frieze of approx. 65 cm height around it, featuring scenes from Greek mythology. The scene on the East Frieze discussed in this paper is commonly identified as part of the Trojan wars (see Figure 1c).1,14

The Frieze of the Siphnian treasury, an Archaic high relief masterpiece, is also an important source of information on art in the Archaic period of Ancient Greece for at least three reasons: a) The beginning of its construction is precisely dated by written sources to the end of the archaic age in 525 BC. b) The date of the destruction of the treasury is not known, but it was most likely abandoned before Christian times and the reign of Constantine the First (306-337). The fact that Pausanias, the author of the Description of Greece (2nd century AD) when visiting Delphi noted that the Sycionian Treasury was empty of any kind of offerings as its neighbors (among them the Siphnian Treasury) were,15 suggests that it was abandoned at this time. Anyway, the last devastation of the sanctuary of Apollo in 620 by Slavs gives us the terminus ante quem of its final destruction.16 Its debris was covered by the ground for more than a millennium, probably after several landslides and earthquakes. During its burial, the debris was not exposed to weather and human activity, until its excavation by the French School at Athens (École française d'Athènes, ÉfA) under its director Théophile Homolle in 1893.13,14,17,18 And c): Only limited cleaning of the surface was carried out after the excavation, so that in many places the marble frieze is still covered by the sediments it rested in. This also preserved remnants of pigments, which may appear to a casual observer as sediments or “dirt”, which should be removed for an impression of the pure, “original”, white marble. About three quarters of the total length of the frieze was recovered from the ground and is exhibited today in the Delphi Archaeological Museum, near the site of the ancient city. The polychromy of the frieze was first described by Charles Picard and Pierre de la Coste Messelière in their first publication of the treasury and its high relief panels 17. There, it was studied in detail by Vinzenz Brinkmann in the 1980s, who provided a catalogue of the conservation and polychromy of the 115 preserved figures and animals.1 Brinkmann used for his studies microscopes with visible and UV-illumination and IR imaging. He described remnants of polychromy at several spots on the surface of the Frieze, and assumed the presence of blue Azurite (Cu3(CO3)2(OH)2), green Malachite (Cu2CO3(OH)2) and Egyptian blue (CaCuSi4O10) being left in small quantities in several locations. Yet,


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Figure 2 Mobile XRF scanner of LAMS during the investigation of the Siphnian Frieze. Not in picture: Dino-lite camera for sample observation.

the most striking areas are those colored red by iron oxides (probably Hematite, Fe2O3), which still covers large surfaces, such as in the inside of several shields, as seen in the photographs provided as digital supporting information (DSI, Figure S1). The area in focus for this paper is the Gorgon head modeled as shield decoration of the 19th figure on the eastern frieze. The person is identified as Achilles. The shield, created on a separate piece of marble and attached to the frieze, is decorated with a Gorgoneion, a depiction of the head of Medusa, who was one of the three Gorgons, feared monsters of Antique mythology. In many modern depictions of Medusa, the snakes on her head are dominant and clearly modeled, but in the Archaic period other monstrous aspects of Medusa were focused on, and the snakes at times only indicated, as can be seen in another Gorgoneion in Figure 1d. While Brinkmann described several traces of polychromy on the personal armor of Achilles, he did not mention any traces of polychromy on the Gorgon head.1 No cleaning of the Gorgon head is documented and it is covered on many places with a thick blackish crust, possibly from the degradation of marble in the ground and/or soil stuck to its surface. The white marble is visible only in a few places on the forehead and the cheeks of the visage. The area was selected for investigation by XRF imaging, as it was expected that such an expressive face, positioned several meters above a human observer would be highlighted with suitable polychromy.

EXPERIMENTAL SCANNING MACRO-XRF For the investigation of the Siphnian Frieze an in-housebuilt instrument was used, featuring a Pd anode end window X-ray tube (Moxtek MAGNUM, Orem, UT, US) operated at 30 kV and 100 µA and a Silicon Drift Detector (X-123FAST SDD, Amptek, Bedford, MA, USA) with an active area of 25 mm2 collimated to 17 mm2 and a nominal thickness of 500 µm. The X-ray tube was connected to the detector via a holder produced by 3D printing, fixing the angle between both to 45 degrees. As primary optic a Pd tube of 800 µm inner diameter was used, yielding a beam size of approx. 0.851.2 mm, depending on distance between measurement head and sample. The typical working distance is 1-3 cm.

A digital microscope (Dino-lite, AnMo Electronics Corporation, New Taipei City, Taiwan) and a laser distance measurement device (OADM20, Baumer, Frauenfeld, Switzerland) allow for measuring the position of the primary beam on the surface of the object. The measurement head was mounted on an XY motorized stage of 20 cm travel range (M-403.8PD, Physik Instrumente (PI) GmbH & Co. KG, Karlsruhe, Germany) with an additional manual Z translation. The motorized stage was fixed to a 3axis manual rotation stage mounted on a photography tripod. In this configuration the primary beam impinged in normal angle on the surface sample with the detection angle being 45 degrees. Photographs of the instrument are shown in Figure 2. The low power X-ray tube and the (comparably) small active area of the detector render the instrument rather insensitive compared to commercial instruments designed for the same use, e.g. the Bruker Artax.19 However, the main design goal of the instrument was to be flexible and lightweight, yielding an instrument that can be transported in normal check-in luggage on an airplane. For the acquisition of elemental distribution images the measurement head was moved in a plane parallel to the surface of the marble, recording the fluorescence radiation emitted from the illuminated area on the marble, acquiring pixel after pixel and line after line. The horizontal, inner loop motor was moved continuously and the spectra were read out “on the fly”. As the frieze is not flat, this resulted in variations in the detection geometry, but it is assumed safer to not automatically adjust the distance in order to prevent collisions and damage to the object. To avoid an irradiation of humans by scattered primary and fluorescence radiation the area around the instrument was fenced off during measurements. The Gorgon head was scanned in the exhibition hall of the Delphi Archaeological Museum. The detector was positioned to the left of the X-ray tube (as shown in Figure 3) and the spectra were recorded with a dwell time of 250 ms per pixel and a horizontal step size of 0.5 mm and a vertical one of 1 mm to scan an area of 110 x 100 mm2. After acquisition the image was re-binned to obtain a uniform pixel size of 0.5 x 0.5 mm2. To verify a correct interpretation of the detection geometry, a second scan of the area was carried out with a step size of 1.0 x 1.0 mm2 (results not shown).



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Elemental distribution images were obtained from the raw spectral data by the use of PyMCA20 and datamuncher.21 The grey-scale in all images shown was adjusted by gamma correction for enhanced readability. Unless otherwise noted the K-shell fluorescence signal is displayed.

PHOTOGRAMMETRY For the acquisition of the several thousand photos of the Siphnian Frieze for photogrammetry a 24 MPixel Sony NEX-7 camera was used. The diffuse, homogeneous illumination in the museum allowed for the acquisition of the photographs without additional light sources. Only in a few shadowed corners artificial illumination had to be used. Photogrammetry belongs to a very active field of research in computer graphics and vision, known as Image Based Modeling and Rendering, that is giving birth to tens of more or less developed software packages. We decided to use Plexus developed at insightdigital.org (Institute for Study and Integration of Heritage Techniques). Plexus makes use of the latest state-of-the-art algorithms (like MVE for Multi-View Environment, a structure-from-motion and multi-view stereo pipeline),22 which allows for the automatic reconstruction of 3D shapes with an absolute minimum of user intervention. The open source release of Plexus is planned for the near future. Photogrammetry allows for the virtual rotation of the object and the viewing of its surface from different angles, however in this study we only used the 3D surface model as base for FP calculations of the interaction with X-rays. The measurement head to surface distance map obtained by photogrammetry is shown in Figure 4. The proper left of the face is not correctly modeled, as on the broken edge no original pigments are expected to be present so that only few photographs of this area were taken. The lateral resolution of the 3D model is estimated to be better than 200 µm, considerably smaller than the pixel size of the elemental distribution images obtained by XRF (500 µm).

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measurements. The characteristics of X-ray tube and detector are known from the measurement of reference materials. The geometry parameters resulting from the sample (γ and D) can be obtained from the photogrammetry data. From these the detection geometry can be approximated for each point of the image and the recorded intensity estimated from the formulas given as DSI. Fundamental Parameters were obtained from xraylib.24,25

Fundamental Parameter Simulation

Figure 4 Measurement head to marble surface distance map used for FP calculations. FP simulated elemental distribution images of Ca and Fe in case of a clean marble face and a marble face painted with a Fe containing pigment. Color scale of simulated elemental distribution images is normalized to the most intense pixel of the individual image.

Figure 3 Detection geometry dependent on surface shape with geometric parameters. Not to scale.

The calculation of the expected elemental signals of a sample with a well characterized instrument under a known geometry is straightforward by using FP equations.23 In case of complex sample shapes the detection geometry changes from point to point (see Figure 3). The instrument’s geometry parameters (o1 and o2) are constant and known from direct

The marble itself was assumed to consist of pure Calcium carbonate (CaCO3) with a thickness of 3.0 cm and a density of 2.7 g/cm3. Two cases were calculated: In the first a clean marble surface was assumed, while in the second the presence of a uniform 10 µm thick Fe2O3 layer with a density of 4.0 g/cm3 was simulated, describing the situation in that the face is painted with a Fe containing pigment. As these calculations aim at a comparison of signals and not at absolute quantification the exact composition of the layer is unimportant and may be simplified as done here. The results of both cases are


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shown in Figure 4. When comparing the Ca intensity images yielded by both models it can be seen that the presence of an additional absorber on the surface of the marble enhances the degree to that changes in the detection geometry influence the intensity of the recorded Ca signal. Previously, Geil et al.10 described an approach to correct for variations of γ, especially in case of trace elements on the surface of archeological artifacts. However, their approach was not suitable for us, as it assumes a homogeneous sample composition without covering layers and a constant detector sample distance and thus solid angle. A scheme of the entire data evaluation workflow is provided in Figure S3 of the DSI.

RESULTS AND DISCUSSION: In Figure 5 distribution images of the elements Ca, Fe, Cu and Pb are shown. The Ca distribution image resembles well the simulated data, which confirms the validity of the model. However, gaining additional information from this image, e.g. on damaged areas on the surface is difficult as changes in the intensity of the recorded signal might be due to changes in local composition and stratigraphy or changes in detection geometry. The ratio between the measured and simulated Ca signal, normalized to the signal ratios on the exposed marble area on the forehead, in Figure 6 simplifies this. Areas in which the marble is exposed due to a damaged surface feature a value of 1.0 or higher and areas in which some form of absorber is present at the surface show a value below 1.0. A good agreement between visually observed spots with a thinned or missing covering layer and a relatively high signal are observed. The only clearly visible artifact is the intense line on the viewer’s right edge of the face, which is the result of a slight misalignment between the elemental distribution image and the projected photogrammetry data. Cu and Pb are present only locally, so that their relative signals are less affected by changes of detection geometry and their interpretation is straightforward. Pb is distributed homogeneously over the surface of the Gorgon’s face at a low abundance level, barely above the limit of detection. While the signal is very weak, it is notably enhanced in the left eye, taking the shape of a crescent. In the right eye also a spot of slight enhanced Pb-L signal is observed. The enhanced signal in the left eye was already observed in a preliminary investigation of several spots by XRF, but without the elemental distribution images it was not possible to distinct between a random enhancement of Pb and a remnant of antique polychromy. It is difficult to explain the presence of Pb all over the surface of the investigated area (and several other areas of the Frieze). Powers et al.8 made similar observations during synchrotron based XRF imaging of worn and weathered inscribed roman marble blocks. There, the Pb was found following the shape of the letters and being present several hundred micrometers below the original marble surface. They did not provide a final explanation for this but discussed the possibilities of this being due to the use of Pb containing tools or pigments. In case of the Siphnian Frieze the presence of Pb (at a low concentration level) in a preparatory paint layer on large areas of the marble cannot be excluded. Moreover, tools containing

Figure 5 Experimentally acquired elemental distribution images (110 x 100 mm2) of the Gorgon head with a brighter hue indicating a stronger signal. Lower right: Overlay of Ca (brown), log(Cu) (green) and Pb-L (pink). Only Pb-L signals above an empirical threshold are used for the overlay.

Figure 6 Left: ratio of experimentally recorded elemental distribution image of Ca and the one calculated without a covering layer. A value of less than 1.0 indicates the presence of an absorbing layer on the surface, while a value above 1.0 indicates exposed marble. Right: ratio image of measured and calculated Fe intensity the color scale is normalized to the area to the right of the mouth.

significant amounts of Pb would be too soft to allow for easy working of the marble, so we do not expect this to be the source of Pb. Yet, one has to be aware that Pb pieces were used to connect marble blocks in antique buildings. It is possi-



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ble that these pieces were oxidized and dissolved in the ground with a small amount of the Pb being absorbed by the marble surface. It is not possible to determine whether the presence of Pb in the eye is the result of the use of a lead based pigment (lead white (cerusite PbCO3 and hydrocerussite 2PbCO3 Pb(OH)2) or minium (Pb2O3)) or if Pb was a minor component in another pigment, as the crescent is not visible by bare eye or microscopy. However, we tend towards the use of lead white, as it is most suitable from an iconographic point of view (see below). On the Gorgon’s head strong Cu signals were found in the depressions between the braids of hair, less strong signals were recorded from the braids of hair and below the preserved ear, while much lower signals were detected over the entire surface of the face. Given its high intensity between the braids, it is save to assume that the presence of Cu is due the remnants of a Cu based pigments. Under the microscope small green crystals were observable on a few spots, but no traces of blue, so that we assume that the pigment used to be green. The degradation of Azurite to green compounds has been reported,26 but given the preservation of blue Azurite on other places of the Frieze we do not believe that a complete transformation occurred in the area under discussion. On the surface of the marble of the Siphnian Frieze in several spots of the figures Cu containing pigments have been preserved and are visible today, but typically only a few square millimeters or less. Thus, the areas on the Gorgoneion constitute the largest area of original Cu containing pigment on the frieze. On the braids of hair also an enhanced Cu signal can be detected. It has less than 5% of the intensity of the Cu in the depressions, but it is significantly higher than the Cu signal on the face, as can be seen in the logarithmic Cu image in Figure 5. This might be interpreted as small remnants of an original paint layer, preserved in the roughness of the marble surface. Between the braids of hair also enhanced Si signals were recorded. We do not believe this to be due to the use of Egyptian blue, but in agreement with the visual inspection to be from silicates of the soil, which are present on top of the Cu containing layer and preserved it until today. The low Cu signal over the face might be explained by the same reasoning as for the low Pb signals: Remnants of pigments or deposition during burial. The interpretation of the Fe distribution image is more challenging than that of other elements. Next to the same geometric effects as for the Ca image, there are four sources of Fe in the investigated area: a) marble contains a small amount of Fe which - together with possibly a thin dirt and dust layer constitutes a background signal. Minor amounts of Fe might be added by the same mechanism that saw Cu and Pb deposited onto the surface of the marble. These signals are of negligible intensity compared to other sources. b) Fe is present in the soil, so that encrustations with soil result in an enhanced Fe signal. Furthermore, Fe can be used in c) brown and d) red pigments (Fe oxides) used to decorate the surface. To compensate for the geometric effects the ratio of measured and simulated Fe data is calculated and shown in Figure 6. The Fe signal between the braids of hair is assumed to be due to the presence of soil encrustations, as discussed for Cu. The Fe signal has only 20% of the intensity of the Cu signal

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here, so that if present in a pigment it would also be just a minor component next to the Cu containing pigment. An enhanced Fe signal can also be observed in the edges of eyes and mouth. These are protected places that might preserve original paint but also soil during a coarse cleaning. As we lacked during the experiments in the Museum a method for chemical speciation we cannot identify the reason for the presence of Fe, but a red highlighting of the eyes would be aesthetically reasonable. The visually warm brown colored area to the viewer’s right edge of the mouth, features a stronger Fe signal that the broken area to the upper right of it. This indicates that the Fe containing layer is part of the original artwork and not the result of later depositions. Whether this layer was an independent part of the polychromy or a preparation layer for a pigment today lost cannot be determined. The enhanced Fe signal at the left edge of the nose is considered genuine and not an artifact due to changes of the detection geometry. It is attributed to a well-preserved part of the warm brown preparatory layer.

ORIGINAL APPEARANCE: When discussing the polychromy of the Siphnian Frieze one has to be aware that while the building is beyond doubt from the Archaic period, this is not necessarily the case for the polychromy. During the excavations no evidence of the construction of later buildings on the site of the Siphnian Treasury was found.13 It was mentioned by Pausanias in the 2nd century AD,15 about 600 years after its construction, and it can be assumed that the building existed for several hundreds years beyond this. While marble buildings can last millennia, polychromy fades much more rapidly and repair or the re-painting of ancient marble statues has been described.27,28 So, our reconstruction is a description of the last version of the polychromy before the Siphnian Treasury’s destruction, but not necessarily the polychromy of the Archaic period. Furthermore, we can only reconstruct pigments of which traces have been found. No scientific method allows one to reconstruct the distribution of pigments of which no traces are left. An overlay of three elements (Ca, Cu and Pb) - that gives an impression of the original appearance - is shown in Figure 5. Some pictorial elements worth discussing are: - Braids of hair/snakes: The braids on the Gorgon’s head were originally colored by a green Cu based pigment. The lower abundance of Cu on the surface of the braids, compared to the depressions is the probably the result of erosion of the paint layer over time and not of the result of deliberate accentuation of color. - Shield: A few areas containing remnants of Cu based pigments were found on the ground of the shield. They do not allow a reconstruction of the original polychromy of the shield, but indicate that it was not homogeneously painted. It is possible that Medusa’s snakes were not only modeled in stone but also painted on the shield, or that a geometrical pattern was applied. - Iris of the eye: As illustrated in Figure 5, the Pb containing crescent is present in the center of the eye. It cannot be said if the current shape was a highlight on the iris of a lifelike eye, or if it originally constituted a full ring and featured staring, stylized eyes (similar to Figure 1d).


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- Eye lines: As discussed above, we cannot identify the species of Fe present in the edges of the eyes, but it would make aesthetical sense to highlight these lines in red. - Skin of the Gorgon: Given the presence of the warm brown layer to the viewer’s right of the Gorgon’s mouth, we assume this to be part of the original polychromy with a claybased pigment as the main colorant. If this layer was used to create color by itself, or was a preparation layer for a pigment that is no longer present, is impossible to say. A brown skin color would not be expected for a living human, but it might be used for a monster or a dead object. While many reports of preserved pigments on archeological objects exist, the chemical mechanism of preservation and degradation is seldom studied. A notable exception is the polychromy of the Chinese Terracotta Army.29 Here it was observed that many pigments remained on the surface of the Terracotta soldiers albeit the binders having decayed during burial. flaking off, if the surface dries after excavation. This, in combination with the finding of pigment remnants under the soil encrustations of the Gorgon head, confirms that soil encrustations on the surface of a marble artifact protect pigments. Yet, only under very few soil encrustations on the surface of the Siphnian Frieze signals of pigments were detected. We are not aware of a convincing explanation for the decay and dissolution of several different pigments on the Frieze, all originally insoluble in water, during burial. Instead, we believe that the Siphnian Treasury collapsed and was buried after a long period of neglect, so that most pigments faded during exposure to the environment, and that the soil encrustations did indeed preserve pigments, but only those that were still present at the moment of burial.

The complex shape of antique marble objects creates strong artifacts in the acquired elemental distribution images that complicate the interpretation. Our approach of comparing the measured elemental distribution images with FP simulated ones, based on 3D surface models obtained by photogrammetry, allows for a significantly easier interpretation of the data and communication of the results. We expect strong improvements in all technical aspects of this study in the near future. In the current instrument, the elemental distribution images and 3D information are acquired separately. This might be joined in the same instrument, employing either photogrammetry or fringe projection, for which a scanning instrument was recently presented.30 This would also simplify the alignment of elemental distribution images and 3D information. In any case, it is intended to install a second detector to gain sensitivity for the angle of the surface, comparable to Smilgies et al..31 Algorithms for photogrammetric reconstruction are an active research field and together with ongoing improvement of computing hardware and cameras, we expect an improvement of photogrammetry in terms of speed and ease of operation. The algorithm used for the FP simulation is not perfect (see DSI for details) and will be improved alongside further studies.

ASSOCIATED CONTENT Supporting Information Supporting Information Available: High quality photographs of the Siphnian Frieze, Formulas of the FP calculation and a scheme of the data evaluation (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION CONCLUSIONS Visual inspection using different illuminations and possibly microscopes and spectroscopic spot analysis techniques, allows us to identify many traces of pigments on the surface of antique marble objects. However, it fails to identify remnants of pigments hidden under encrustations (e.g. the Cu between the braids of hair) and it does not allow to distinguish between random surface contaminations and faint traces of pigments (such as the Pb in the iris of the eye). We have shown that XRF imaging is capable of doing this by acquiring representative overviews of small areas in an automatic fashion. It is to be expected that XRF imaging and similar spectroscopic imaging techniques will allow in the next few years new and deeper insights into the polychromy of Antiquity. This is highlighted by the fact that the Siphnian Frieze is a well-known and well-investigated object on which we could find the largest area of preserved Cu containing pigments, which was previously completely unknown. The elemental distribution images allow for a partial reconstruction of the polychromy of the investigated area and constitute a warning not to remove soil encrustations on the surface of artworks with levity. Together with other areas of the Frieze, these findings suggest that the building collapsed after a period of neglect with most of the polychromy already degraded. However, these findings also highlight the lack of knowledge and research in the preservation of pigments on antique objects during burial and after.

Corresponding Author * Sorbonne Universités, UPMC Univ Paris 06, CNRS, UMR 8220, Laboratoire d’archéologie moléculaire et structurale (LAMS), 4 place Jussieu 75005 Paris, France. +33 (0)1 44 27 60 83, [email protected]

Author Contributions MA wrote the manuscript, evaluated the XRF data and wrote the FP calculations routines. MA and PW recorded the XRF data. MM and PJ surveyed the frieze for remnants of polychromy. MA, MM, PJ and PW interpreted the results together. PM took the photographs for photogrammetry and calculated the 3D model with KC. All authors read and agreed on the manuscript. PJ coordinated the investigation of the frieze.

Acknowledgments We thank the support from the Ile-de-France region (DIM Analytics, project IMAPAT) to build new instruments for a mobile laboratory for art studies and the French State managed by the National Research Agency under the program Future Investments bearing the reference ANR-11-IDEX-0004-02 (program POLYRE of Sorbonne Universités). Further, we thank the French School in Athens (École française d'Athènes, ÉfA, dir. Alexandre Farnoux) and the Ephorate of Antiquities in Phocis (dir. Athanasia Psalti), for their support. We thank Helen Glanville (LAMS) for proof reading the manuscript and Dominique Thirion (www.thirionancient-art.com) for allowing us to reproduce the painted pottery Gorgoneion in Figure 1. Many thanks also to Jonathan Devogelaere (Aix-Marseille Université, Centre Camille Jullian, 5, rue du Château de l’Horloge, BP 647, 13094 Aix-en-Provence Cedex



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2, France) for his help during the campaigns of June 2015 and 2016.

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