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Article Application of ReflectanceReflectance TransformationTransformation Imaging Imaging to Experimental Archaeology Studies to Experimental Archaeology Studies 1, 1, 2, 3, , Silvia Florindi †, Anna Revedin † , Biancamaria Aranguren † and Vincenzo Palleschi * † Silvia Florindi 1,†, Anna Revedin 1,†, Biancamaria Aranguren 2,† and Vincenzo Palleschi 3,*,† 1 Istituto Italiano di Preistoria e Protostoria, 50121 Firenze, Italy; silvia.fl[email protected] (S.F.); 1 [email protected] Italiano di Preistoria (A.R.) e Protostoria, 50121 Firenze, Italy; [email protected] (S.F.); 2 [email protected] Archeologia (A.R.) Belle Arti e Paesaggio di Siena, Grosseto e Arezzo, (retired), 53100 Siena, Italy; 2 [email protected] Archeologia Belle Arti e Paesaggio di Siena, Grosseto e Arezzo, (retired), 3 53100Institute Siena, of Italy; Chemistry [email protected] of Organometallic Compounds, CNR, 56124 Pisa, Italy *3 InstituteCorrespondence: of Chemistry [email protected]; of Organometallic Compounds, Tel.: +39-050-315-2224 CNR, 56124 Pisa, Italy These authors contributed equally to this work. *† Correspondence: [email protected]; Tel.: +39-050-315-2224 † These authors contributed equally to this work.


Received: 5 October 2020; Accepted: 30 October 2020; Published: 31 October 2020
 Received: 05 October 2020; Accepted: 30 October 2020; Published: 31 October 2020 Abstract: In this work, we present a study on experimental archaeology replicas of 170,000-year-old Abstract:digging sticks In this excavated work, we in present 2012 in a the study archaeological on experime sitental of archaeology Poggetti Vecchi replicas (Grosseto, of 170,000-year-old Italy). One of diggingthe techniques sticks excavated used for documentingin 2012 in the andarchaeological studying the site sticks of Poggetti was the Vecchi reflectance (Grosseto, transformation Italy). One ofimaging the techniques (RTI) technique, used for which documenting allows the and creation studying of the an interactivesticks was the image reflectance by varying transformation the angle of imagingillumination. (RTI) A technique, reconstruction which of theallows 3D profilethe creation of the surfaceof an interactive was also made image by by applying varying the the technique angle of illumination.of photometric A stereo reconstruction imaging to of the the RTI 3D images. profile of the surface was also made by applying the technique of photometric stereo imaging to the RTI images. Keywords: experimental archaeology; digging sticks; Middle Paleolithic; Neanderthal; reflectance Keywords:transformation experimental imaging; photometricarchaeology; stereo digging imaging sticks; Middle Paleolithic; Neanderthal; reflectance transformation imaging; photometric stereo imaging

1. Introduction Introduction The recoveryrecovery of of perishable perishable material material from from Paleolithic Paleolithic contexts contexts is particularly is particularly rare in rare Western in Western Europe Europebecause because of the easy of the degradation easy degradation in the relatively in the relatively temperate temperate climate climate that characterizes that characterizes this region. this Therefore,region. Therefore, the discovery the discovery of several of woodenseveral wooden tools, found tools, in found the Middle in the PaleolithicMiddle Paleolithic site of Poggetti site of VecchiPoggetti (Grosseto, Vecchi (Grosseto, Italy) [1], hasItaly) spurred [1], has a greatspurred interest. a great In 2012,interest. during In 2012, the works during for the the works realization for the of realizationa thermal pool, of a thermal several archaeologicalpool, several archaeological findings came findings to the light came (see to Figurethe light1). (see Figure 1).

FigureFigure 1. 1. TheThe site site of of Poggetti Poggetti Vecchi, Vecchi, Grosseto, Italy.

Heritage 2020, 3, 1279–1286;1279–1286; doi:doi:10.3390/heritage304007010.3390/heritage3040070 www.mdpi.com/journal/heritagewww.mdpi.com/journal/heritage Heritage 2020, 3 1280

The prompt intervention of the archaeologists of the Soprintendenza Archeologia Belle Arti e Paesaggio di Siena, Grosseto e Arezzo, led by Gabriella Poggesi and Biancamaria Aranguren, evidenced a complex stratigraphic sequence; in the oldest stratigraphic unit (U 2), the archaeologists found the remains of Palaeoloxodon antiquus (an extinct elephant species), as well as several objects in stone, bone and wood [2] (see Figure2).

Figure 2. The digging sticks (pointed by the red arrows) and the remains of Palaeoloxodon antiquus.

In the overlying stratigraphic unit (U 4), geological and organic remains were dated to around 170 ky from present, giving a limit ante quem for the chronology of the underlying findings. Among them, more than 50 wooden fragments, of size ranging from 10 cm to 1 m, were recovered. The recovery of such kind of organic material (which, because of its intrinsic perishability, is said to constitute the “missing majority” of the archaeological findings), including several almost intact “digging sticks”, was exceptionally important. Manufacts of comparable importance are very rare, including the wooden spears discovered in the archaeological site of Schöningen (Germany), dating about 400 ky from present, and the ones found in the site of Lehringen (Germany, 110–130 ky) and Clacton-on-Sea (England, abt. 350 ky). Part of what is interpreted as a digging stick was found in the site of Aranbaltza (Spain) [3]. This latter manufact shows an evident deformation, due to the conservation processes applied, so that the only way of possibly inferring the original shape of the manufact is from the photographs acquired at the time of the recovery. The unfortunate fate of the Aranbaltza digging stick fragment evidences the need for developing instruments and methodologies for an accurate documentation of the artefacts which must have the capability of operating quickly in situ, minimizing any contact with the object. In that respect, photogrammetric techniques seem to be the favorite tools for this task [4]. However, it should be noted that the flexibility of conventional photogrammetry techniques, allowing to acquire the 3D model of the object from photographs taken at arbitrary distances and angles with respect to the object, is paid by the need for reconstruction algorithms to infer both the 3D model of the surface and the position and angle of the camera at the moment of the shot. The unavoidable indeterminations resulting from the application of these algorithms might hinder1 some fine details of the object’s surface, which are critical for studies aimed to find tool marks and signs deriving from use. In that respect, a fixed geometry acquisition, as used in several approaches based on the application of reflectance transformation imaging (RTI) [5], might be preferrable to other more conventional photogrammetric techniques. RTI is a technique widely used for documentation of archaeological Heritage 2020, 3 1281 and cultural heritage objects [6,7]; contrarily to conventional photogrammetry, the acquisition of the set of images needed for the object’s surface reconstruction is acquired keeping the camera fixed and varying the direction of the illumination. The RTI technique was developed about 20 years ago by Malzbender et al. [8] for the reconstruction of polynomial texture maps (PTM) of a sample’s surface. The RTI technique is usually applied following two possible strategies. In both variations of the technique, the position of the camera is maintained fixed, and the angle of illumination is varied, to obtain different photographs that are then elaborated to obtain the surface profile of the object under study. The differences between the two methods rely on the illumination strategy. In one case, the position of the lights is fixed in a definite pre-determined scheme, while in the second—as in standard photogrammetry for the position of the camera—the lighting is varied with some degree of freedom, and the illumination geometry is determined at the time of acquisition (highlights method). This latter approach has been discussed in detail in a recent paper by Antonio Cosentino [9]. Due to its intrinsic simplicity and portability, the RTI technique has been widely used in cultural heritage and archaeology applications. An extensive review on archaeological applications has been recently published by Mytum and Peterson [10]. One of the main disadvantages of the RTI technique is the fact that, by this technique alone, the surface reconstruction is only qualitative, and no metric information can be recovered. However, the set of images acquired for PTM reconstruction by RTI can be further elaborated, using photometric stereo imaging (PSI) algorithms [11,12]. The application of the PTM/RTI technique to organic material has allowed for recovering tiny marks and butchery traces in bone and antler objects [7] and to study wooden statuettes and icons [13,14]. Earl et al. [5] successfully applied RTI to the reconstruction of wooden legal tablets burned in the Mount Vesuvius eruption that destroyed Pompei in 79 A.D. Finally, in a work particularly relevant to the issues studied in this paper, Karsten and Earl [15] demonstrated the possibility of comparing metrically the surface maps of previously waterlogged timber, acquired at different stages of the conservation process. The authors used a fixed illumination scheme, for improving the resolution of the surface reconstruction.

2. Materials and Methods Due to the intrinsic fragility of the wooden findings from the Poggetti Vecchi site, the feasibility study on the use of RTI for identifying the tool mark and use wear was performed on faithful replicas of the digging sticks obtained from experimental archaeology (see Figure3). So far, six experimental sessions were performed, using a specific protocol starting from branches of common boxwood (Buxus sempervirens), the same timber used for the original findings, using tools compatible with the Paleolithic technology (stone and animal bone tools and firings) [6]. The sticks found in Poggetti Vecchi, in fact, showed evident traces of intentional firing [16], in order to obtain artifacts with a higher level of finishing, with a completely smooth shaft surface, without raised knots and with rounded handles. The experimental working process was carefully documented, to correlate the working signs remaining on the sticks with the procedures and the tools used for the manufacturing [3,16].

Figure 3. One of the experimental archaeology sticks, with the tools used for manufacturing it.

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The experimental archaeology work evidenced that the realization of the sticks required physical strength, time and specific skills; the use of fire for removing the bark and refining the sticks implied having control of the fire, which was used not only for providing light and heat, but also exploited as Heritage 2020, 3 1281 a tool for manufacturing objects. The digging digging sticks sticks replicas replicas were were analyzed analyzed using using conventional conventional photogrammetric photogrammetric techniques techniques and reflectanceand reflectance transformation transformation imaging imaging (RTI), (RTI), as well as wellas digital as digital microscopy. microscopy. RTI was was recently recently used used for for investigation investigation of ofwooden wooden artifacts artifacts by byAbdrabou Abdrabou et al. et [13] al. and [13] Munteanuand Munteanu et al. et[14] al., using [14], using the highlight the highlight method. method. In our Incase, our as case, in [15], as inthe [15 reconstruction], the reconstruction of fine detailsof fine detailsof the surface of the surface was of wasparamount of paramount importance. importance. Therefore, Therefore, the first the approach first approach was preferred. was preferred. The lightingThe lighting was provided was provided by 46 by white 46 white LEDs, LEDs, equally equally spaced spaced at fixed at positions fixed positions on a 3D-printed on a 3D-printed semi- sphericalsemi-spherical dome dome of 20 of cm 20 in cm diameter in diameter (see (see Figure Figure 4).4 ).The The photographs photographs were were taken taken with with a compactcompact Canon camera camera (14.1 (14.1 megapixe megapixels)ls) synchronized synchronized with with the the lights lights thro throughugh a simple a simple electronic electronic circuit. circuit. The acquisitionThe acquisition of the of whole the whole set of set ph ofotographs photographs takes takes about about one oneminute. minute.

FigureFigure 4. 4. TheThe Reflectance Reflectance Transformation Transformation Imaging (RTI) instrument used in this work.

The RTI interactive images were obtained in a couple of minutes using open source software (RTI Processor).Processor). TheThe surface surface profile profile was was then then metrically metrically reconstructed reconstructed by applying by applying a photometric a photometric stereo stereoimaging imaging (PSI) algorithm (PSI) algorithm [11] to the[11] RTI to imagesthe RTI previouslyimages previously acquired. acquired. This method This is method very powerful, is very powerful,but it involves but it ainvolves complex a elaborationcomplex elaboration of the images of the which images can which take can several take hours, several if hours, the original if the originalresolution resolution of the images of the images should should be maintained. be maintained. For this For reason,this reason, it is it advisable is advisable to use to use a subset a subset of ofthe the original original RTI RTI photoset photoset for for minimizing minimizing the the processing processing time time andand thethe numericalnumerical errors associated with it. Masking Masking of of the the object object surface surface against a background is also needed. We We wrote wrote a simple Matlab®Matlab® script, implementingimplementing thethe depth depth gradient gradient fusion fusion photometric photometric stereo stereo method method [17 [17]] developed developed by bythe the VORTEX VORTEX Team Team of the of IRIT the LabIRIT of Lab the Universityof the Univ ofersity Toulouse, of Toulouse, France [18 France], to apply [18], these to apply algorithms these algorithmsto the reconstruction to the reconstruction of the experimental of the experime archaeologyntal replicas archaeology of the Poggettireplicas of Vecchi the Poggetti digging sticks.Vecchi diggingUsing sticks. a subset of 12 images of the 46 acquired for the RTI reconstruction, the time needed for obtainingUsing the a subset height mapof 12 of images a surface of ofthe about 46 acquir 5 cm2edis aroundfor the thirtyRTI reconstruction, minutes on an average-performancethe time needed for obtaininglaptop, working the height at the map maximum of a surface resolution. of about Medium-resolution 5 cm2 is around images, thirty minutes useful for on a an preliminary average- performanceassessment of laptop, the surface working profile, at the can maximum be obtained resolution. in less than Medium-resolution 5 min. images, useful for a preliminaryThe results assessment were also of the compared surface profile, with the can images be obtained taken in using less athan digital 5 min. microscope (DinoLite, 5 Megapixel).The results were also compared with the images taken using a digital microscope (DinoLite, 5 Megapixel). 3. Results 3. ResultsIn Figure 5, we show the comparison of the digital microscope image and a screenshot of the RTI interactiveIn Figure image, 5, we evidencing show the comparison a tiny scratch of onthe thedigital stick microscope surface. The image RTI and image a screenshot covers a lengthof the RTI interactive image, evidencing a tiny scratch on the stick surface. The RTI image covers a length of several cm of the stick; however, the resolution of the image, which is the combination of all 46 images acquired by the instrument, is high enough to allow a magnification comparable to the one of the digital microscope. However, the possibility of interactively varying the illumination angle makes RTI advantageous with respect to digital microscopy, for evidencing details in grazing light that can be hard to detect in the portable microscope image.

Heritage 2020, 3 1283 of severalHeritage 2020 cm, 3of the stick; however, the resolution of the image, which is the combination of1281 all 46 images acquired by the instrument, is high enough to allow a magnification comparable to the one of The RTI image, although very powerful for documenting at high resolution a relatively large the digital microscope. However, the possibility of interactively varying the illumination angle makes portion of the sample, does not allow for measuring the depth of the surface features, produced by RTIHeritagethe advantageous tool 2020 used, 3 for with manufacturing respect to digitalthe stick microscopy, or by its use. for To evidencing this purpose, details we inapplied grazing the light photometric that can1281 be hardstereo to detect imaging in thealgorithm portable to microscopea subset of the image. RTI images to metrically reconstruct the surface profile. The RTI image, although very powerful for documenting at high resolution a relatively large portion of the sample, does not allow for measuring the depth of the surface features, produced by the tool used for manufacturing the stick or by its use. To this purpose, we applied the photometric stereo imaging algorithm to a subset of the RTI images to metrically reconstruct the surface profile.

(a) (b)

FigureFigure 5. 5.(a )(a Digital) Digital microscope microscope detail detail ofof oneone ofof the digging stick stick replicas; replicas; (b ()b screenshot) screenshot of ofthe the RTI RTI interactiveinteractive image. image.

TheTo RTI this image, purpose, although we( firsta) masked very powerful the zone for of interest documenting in the images at high (Figure resolution 6).(b ) a relatively large portion of the sample, does not allow for measuring the depth of the surface features, produced by Figure 5. (a) Digital microscope detail of one of the digging stick replicas; (b) screenshot of the RTI the tool used for manufacturing the stick or by its use. To this purpose, we applied the photometric interactive image. stereo imaging algorithm to a subset of the RTI images to metrically reconstruct the surface profile. ToTo this this purpose, purpose, we we first first masked masked thethe zonezone ofof interestinterest inin thethe imagesimages (Figure6 6).).

Figure 6. Masking of the region of interest for the application of the Photometric Stereo Imaging (PSI) technique.

Then, the PSI algorithm evaluated the normals of the surface (Figure 7) and, from the integration of the normals, obtained the 3D profile of the surface (Figure 8). FigureFigure6. 6. Masking of of the the region region of ofinterest interest for forthe theappl applicationication of the of Photometric the Photometric Stereo Stereo Imaging Imaging (PSI) (PSI)technique. technique.

Then,Then, the the PSI PSI algorithmalgorithm evaluatedevaluated thethe normals of th thee surface (Figure (Figure 77)) and, from the integration integration ofof the the normals, normals, obtained obtained thethe 3D3D profileprofile ofof thethe surfacesurface (Figure(Figure8 8).).

Figure 7. Normals of the surface, obtained from the RTI images. The red arrow identifies the point where the surface scratch depth profile was reconstructed.

Figure 7. Normals of the surface, obtained from the RTI images. The red arrow identifies the point where the surface scratch depth profile was reconstructed.

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The RTI image, although very powerful for documenting at high resolution a relatively large portion of the sample, does not allow for measuring the depth of the surface features, produced by the tool used for manufacturing the stick or by its use. To this purpose, we applied the photometric stereo imaging algorithm to a subset of the RTI images to metrically reconstruct the surface profile.

(a) (b)

Figure 5. (a) Digital microscope detail of one of the digging stick replicas; (b) screenshot of the RTI interactive image.

To this purpose, we first masked the zone of interest in the images (Figure 6).

Figure 6. Masking of the region of interest for the application of the Photometric Stereo Imaging (PSI) technique.

HeritageThen,2020, 3the PSI algorithm evaluated the normals of the surface (Figure 7) and, from the integration1284 of the normals, obtained the 3D profile of the surface (Figure 8).

FigureFigure 7.7. NormalsNormals ofof thethe surface,surface, obtained from the RT RTII images. The The red red arrow arrow identifies identifies the the point point Heritage 2020, 3 1281 Heritagewherewhere 2020 the, the3 surface surface scratchscratch depthdepth profileprofile waswas reconstructed.reconstructed. 1281

FigureFigure 8. 8. 3D 3D reconstruction reconstruction of the woodwood surface. surface. The The re redd arrow arrow identifies identifies the the point point where where the the surface surface Figure 8. 3D reconstruction of the wood surface. The red arrow identifies the point where the surface scratchscratch depth depth profileprofile waswas reconstructed. scratch depth profile was reconstructed.

FromFrom the thethe 3D3D 3D modelmodel model shown shown in inFigureFigure Figure 8,8, we 8we, wewere were were able able ableto to measure measure to measure the the depth depth the depthof ofthe the scratch of scratch the at scratch theat the positionatposition the position marked marked marked inin FiguresFigures in Figures 7 and 78 and withwith8 awitha rreded aarrow.arrow. red arrow. The The results results The results are are shown shown are shownin in Figure Figure in Figure9. 9. 9.

Figure 9. Profilometry of the scratch on the surface of the digging stick replica, obtained from the 3D Figure 9. Profilometry of the scratch on the surface of the digging stick replica, obtained from the 3D Figuremodel 9.of Profilometry the surface shown of the in scrat Figurech on 8. the surface of the digging stick replica, obtained from the 3D model of the surface shown in Figure8. model of the surface shown in Figure 8. 4. Discussion 4. Discussion The analysis of the RTI image and the reconstruction of the surface profile obtained from PSI allowsThe for analysis making of some the RTIconsiderations image and on the the reconstruction suitability of thisof the approach surface for profile the in obtained situ analysis from of PSI allowsfragile for archaeological making some items. considerations The RTI instrument on the suitability realized ofat thisthe approachApplied and for Laserthe in Spectroscopysitu analysis of fragileLaboratory archaeological in Pisa operates items. withoutThe RTI contact instrument on relatively realized large at thesurfaces, Applied allowing and Laser to quickly Spectroscopy locate Laboratoryzones of interest in Pisa that operates can be visualizedwithout contact with a onresolution relatively comparable large surfaces, to that allowing of a digital to microscope,quickly locate zonesat a lower of interest cost. Moreover,that can be the visualized possibility with of metrica resolutionally reconstructing comparable theto that surface of a profiledigital exploitingmicroscope, atthe a lowerphotometric cost. Moreover, stereo imaging the possibility technique, ofalthou metricgh allyquite reconstructing complex and time-consuming, the surface profile gives exploiting well- theresolved photometric depth stereo profiles imaging with technique,a micrometric althou resolution,gh quite complex which andis bettertime-consuming, than conventional gives well- photogrammetry and comparable to much more complex techniques which are not suitable, for cost resolved depth profiles with a micrometric resolution, which is better than conventional and dimension of the instrumentation, to be used in the field. photogrammetry and comparable to much more complex techniques which are not suitable, for cost and dimension of the instrumentatio n, to be used in the field.

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4. Discussion The analysis of the RTI image and the reconstruction of the surface profile obtained from PSI allows for making some considerations on the suitability of this approach for the in situ analysis of fragile archaeological items. The RTI instrument realized at the Applied and Laser Spectroscopy Laboratory in Pisa operates without contact on relatively large surfaces, allowing to quickly locate zones of interest that can be visualized with a resolution comparable to that of a digital microscope, at a lower cost. Moreover, the possibility of metrically reconstructing the surface profile exploiting the photometric stereo imaging technique, although quite complex and time-consuming, gives well-resolved depth profiles with a micrometric resolution, which is better than conventional photogrammetry and comparable to much more complex techniques which are not suitable, for cost and dimension of the instrumentation, to be used in the field.

5. Conclusions The application of RTI to fragile archaeological objects offers a cheap but powerful tool for documentation and study of objects that cannot be easily removed or manipulated. The non-contact nature of the method and the robustness of the instrument, without mobile parts, envisage the possibility of using this system, minimizing the manipulation of the objects, for a quick in situ documentation and assessment, to be eventually studied in greater detail at later times, if needed, using the PSI approach.

Author Contributions: Conceptualization, V.P.; methodology, V.P., S.F., A.R. and B.A.; software, V.P.; validation, V.P., S.F., A.R. and B.A.; formal analysis, V.P., S.F., A.R. and B.A.; investigation, V.P., S.F., A.R. and B.A.; data curation, V.P., S.F.; writing—original draft preparation, V.P.; writing—review and editing, V.P., S.F., A.R. and B.A.; supervision, V.P., A.R. and B.A. All authors have read and agreed to the published version of the manuscript. Funding: This research is funded by the Italian Institute of Prehistory and Protohistory (IIPP) and is part of the IIPP project “VIWA_P–Visual Imaging for Wooden Artefacts in the Palaeolithic”. Conflicts of Interest: The authors declare no conflict of interest.

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