Multi-Scale Measurements of Neolithic Ceramics—A Methodological Comparison of Portable Energy-Dispersive XRF, Wavelength-Dispersive XRF, and Microcomputer Tomography

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Article Multi-Scale Measurements of Neolithic Ceramics—A Methodological Comparison of Portable Energy-Dispersive XRF, Wavelength-Dispersive XRF, and Microcomputer Tomography

Julia Menne 1,* , Astrid Holzheid 2 and Christopher Heilmann 2 1 Institute of Pre- and Protohistoric Archaeology, Kiel University, 24118 Kiel, Germany 2 Institute of Geosciences, Kiel University, 24118 Kiel, Germany; [email protected] (A.H.); [email protected] (C.H.) * Correspondence: [email protected]

Received: 9 September 2020; Accepted: 16 October 2020; Published: 21 October 2020

Abstract: Archaeometric investigation of ancient pottery with regard to their material composites allows insights into the material structures, production techniques and manufacturing processes. The applied methods depend on the classification of the pottery: some finds should remain unchanged for conservation reasons, other finds are less valuable or more common. While the first group cannot be destroyed for material analyses and the choice of analytical methods is limited, the latter can be investigated using destructive methods and thus can widen the spectrum of possible devices. Multi-element analyses of portable energy-dispersive X-ray fluorescence spectrometry (portable XRF) have become important for archaeological research, as portable XRF provides a quick overview about the chemical composition of potteries and can be used in non-destructive as well as destructive ways in addition to conventional microscopic examination and petrographic thin sections. While most portable XRF analyses of solely fracture surfaces do not provide satisfying results, portable XRF analyses on pulverized samples are a cost-efficient and fast alternative to wavelength-dispersive XRF (WD-XRF). In comparison to WD-XRF, portable XRF on pulverized samples provides reliable concentration data (K, Fe, Rb, Ti, V, Y, Zn, Zr), but other elements need to be corrected. X-ray microtomography (µCT) has proven to be a non-destructive technique to derive not only the porosity of ancient pottery but also to characterize temper components and non-plastic inclusions. Hence, the µCT technique has the potential to extract valuable information needed by archaeologists, for example, to deduce details about manufacturing.

Keywords: X-ray microtomography; portable XRF; WD-XRF; archaeometry; ceramic; Neolithic

1. Introduction Multi-element analyses of recent decades have gained importance in archaeological and geoarchaeological research. Those analyses enable a fast acquisition of large data sets beyond complex laboratory analyses. Thus, it is possible to explore many different questions such as the provenance and use of raw materials, production techniques, interaction models of people and their objects, as well as settlement structures. Research commonly agrees that portable energy-dispersive X-ray fluorescence spectrometry (portable XRF) provides sufficient and accurate results of chemical compositions needed for linking different assemblages of archaeological finds together. As the increasing number of ceramic studies using portable XRF shows, the potential and practical application of this method has expanded enormously in recent years [1–11]. However, the reliability of the chemical compositions solely gained

Minerals 2020, 10, 931; doi:10.3390/min10100931 www.mdpi.com/journal/minerals Minerals 2020, 10, 931 2 of 22 from portable XRF is still debatable. Numerous independent studies have shown that the quality of the portable XRF results depends on the heterogeneity of the sample(s). The more homogeneous the matrix, the more reliable the measurements [1,12–16]. As a case study aiming to clarify the reliability of portable XRF analyses even further, we measured the elemental contents of the ceramics of megalithic tombs, belonging to the Funnelbeaker Westgroup in north-western Germany (dating ca. 3500–2750 BC). These graves have a high amount of ceramic in them and were used as graves for a long period. They also were frequented in a post-use phase of the Single grave culture. Since investigations based on optical parameters (optical microscopy and thin-section petrography) were already conducted [17], quantitative measurements of a portable energy-dispersive (ed) XRF device (Niton XL3t900; Graduate School “Human Development in Landscapes” at Kiel University, Kiel, Germany) were compared to a conventional wavelength-dispersive XRF device (WD-XRF; Panalytical MagixPRO with sample changer PW 2540 VCR; Institute of Mineralogy, University Hamburg, Kiel, Germany) to determine the limits of applicability. In addition, the same fragments were examined with X-ray microtomography (µCT, SkyScan1172; Institute of Geosciences, Kiel University, Kiel, Germany). Two-dimensional (2D) and 3D images were constructed based on µCT scans to shed light on the material structure, e.g., to extract temper components as well as non-plastic inclusions and their related porosity non-destructively. To conclude, the aim of this study is to evaluate the quality and limits of those measurement techniques and to put constraints on their reliability and applicability for archaeological interpretations of ceramic studies in order to draw, for example, conclusions about the origin and production technique of the ﬁnds.

1.1. Main Aim and Question The following questions emerge from the interrelation of the analysis methods: As mentioned before, the percentage of portable XRF research on ceramics in archaeology has increased significantly since the early 2010s. The first application studies point out their possibilities and limitations [4,18–21]. Our aim is to investigate the practical applications regarding the position of the measurements on the sample, the measurement of powder and fractures in comparison, as well as the quality of the measurements after heating of the samples [22]. Falsification of the results by the ageing of the X-ray tube is a factor that should not be underestimated. Thus, we also studied how the maintenance of the software (and hardware) affects the results. Material studies in archaeology often use the apparent cost-effective portable XRF measurement with lack of verification of measured values compared to WD-XRF. Which differences, regarding the element compositions, can be recognized between the conventional WD-XRF and the portable XRF? In order to answer this question, the same samples were measured as fresh fractions and as powder using both X-ray fluorescence methods. A further aspect focuses on petrographic thin sections and non-destructive µCT. Can the identified components of the temper in the thin sections be verified by µCT? The surfaces of the fractures were reconstructed in order to define porosities and cavities. The question is whether in future the application of µCT can supplement or even replace thin sections.

1.2. Archaeological Context of the Used Ceramics All samples of our study belong to megalithic graves of the administrative district of Emsland, Lower Saxony, Germany. The megalithic tombs in Emsland are part of the north-central European Funnelbeaker culture covering an absolute chronological period from about 3500 to 2750 BC [23,24]. The Emsland is characterized by the flat mountain range of Hümmling and is well known for its many megalithic tombs. Palynological studies on the local wet soils reveal the first traces of human settlement activities and impacts of agriculture, livestock breeding and thus an opening of the landscape from about 3500 BC onwards [25–29]. The first urbanization of the landscape was Minerals 2020, 10, 931 3 of 22 Minerals 2020, 10, 931 3 of 25 majorityfollowed of shortly grave afterwards goods are bymostly the erectionpottery ofin thevarious first megalithicshapes and tombs. sizes. Over A large a period number of of several grave hundredgoods exist years, in the hundreds megalithic of vessels tombs ofwere the placed Funnelbeaker in the graves. Westgroup, Thus, i.e.,the theFunnelbeaker region between pottery the offers river aIjssel solid and basis the for river interdisciplinary Weser. In addition research to thedue deposit to its comparatively of flint and rock good artefacts preservation as well and as amber large numberobjects, theof finds majority as well of graveas its stylistic goods arediversity. mostly In pottery total, 134 in various tombs are shapes known and in sizes. the Emsland, Over a periodpartly onlyof several from travel hundred descriptions years, hundreds of the 19th of vessels century. were Of these, placed 66 in have the graves.been preserved Thus, the completely Funnelbeaker or as potteryremains o[17,30,31].ffers a solid Figure basis 1 for summarizes interdisciplinary the distribution research due of megalithic to its comparatively graves in goodthe Emsland. preservation The systematicand large number investigation of finds of as wellthis grave as its stylistic pottery diversity. began at In the total, beginning 134 tombs ofare the known 20th century in the Emsland, with a partlynumber only of excavations. from travel descriptionsWithin the framework of the 19th of century. the “Emsland Of these, Plan” 66 have in the been 1950s, preserved more excavations completely tookor as place remains e.g., [ 17in, 30Groß,31]. Berßen Figure 31 tosummarizes 5, 7, Emmeln the 1 distributionand 2, Lahn of4 [32–38] megalithic and gravesOstenwalde in the 1 Emsland. [39,40]. The systematicThe well-established investigation typological of this grave studies pottery mainly began of atthe the 2nd beginning part of the of the 20th 20th century century are with now a supplementednumber of excavations. by scientific Within analysis the framework used by archaeologists of the “Emsland starting Plan” primarily in the 1950s, in the more 2010s, excavations allowing insightstook place into e.g., social in Groß networks, Berßen landscape 3 to 5, 7, Emmelndevelopm 1ent and and 2, Lahn material 4 [32 studies–38] and of Ostenwalde grave goods 1 [41–47]. [39,40].

Figure 1.1. DistributionDistribution of of megalithic megalithic graves graves in the in studythe study area, districtarea, district Emsland, Emsland, Lower Saxony, Lower Germany.Saxony, Germany.Grey dots Grey= megalithic dots = megalithic grave, black grave, dots black= examined dots = examined sites (1: Emmelnsites (1: Emmeln 2, E2; 2: 2, Groß E2; 2: Berßen Groß Berßen 7, GB7; 7,3: GB7; Werpeloh 3: Werpeloh 43, WP43). 43, WP43). See Table See1 for Table more 1 for details. more details.

Minerals 2020, 10, 931 4 of 22

The well-established typological studies mainly of the 2nd part of the 20th century are now supplemented by scientiﬁc analysis used by archaeologists starting primarily in the 2010s, allowing insights into social networks, landscape development and material studies of grave goods [41–47].

2. Materials and Methods In total, 19 samples from three sites of megalithic graves in northwest Germany [17] were selected for a comparison of portable XRF, WD-XRF measurements, X-ray microtomography scans and thin sections (Table1). Additionally, two reference samples named Standard I and Standard II were generated to verify the results. All measurements were conducted on the exact same samples to gain an exact comparison between the methods used.

Table 1. Ceramic samples of this study and allocation to the selected megalithic tombs Emmeln 2 (E2), Groß Berßen 7 (GB7) and Werpeloh 43 (WP43) of the district of Emsland (Lower Saxony, Germany), including information about the samples and employed analytical methods.

Thin Portable Portable X-ray No. Sample Site Cate-Gory 1 Section XRF XRF WD-XRF Micro-Tomography No. 2 Fracture Powder 1 20080021200009_2253 WP43 4/b XXXX 2 20080015500007_2281 WP43 5/b X 360 XXX 3 20080027800005_0 WP43 s X 362 XXX 4 20080024000005_125 WP43 f XXXX 5 20080031400002_2325 WP43 f X 358 XXX 6 20080118100021_603 E2 3/b X 369 XXX 7 20080020100025_1112 E2 4/b XXXX 8 20080020100013_796 E2 4/b X 370 XXX 9 20080107300006_1310 E2 f XXXX 10 20080107300009_1138 E2 f X 367 XXX 11 20080029600055_2221 GB7 3/b XXXX 12 20080029600009_2008 GB7 5/b XXXX 13 20080022500017_2106 GB7 c X 364 XXX 14 20080011500010_2077 GB7 f XXXX 15 20080022500016_2103 GB7 f XXXX 16 20080020100013_769_ht E2 4/b 370 X 17 20080015500007_2281_ht WP43 5/b 360 X 18 20080027800005_0_ht WP43 s 362 X 19 20080012300012_2314 WP 43 s X 361 20 Standard I X 21 Standard II X 1 The categories describe the typochronological classification of the pottery according to the dating horizons 3, 4, 5 (according to A.L. Brindley (1986)), and types (b = bowl, c = collar flask, f = funnel beaker, s = single grave culture vessel), see Menne (2018) for more details. Categories with only letters are stratigraphically unclassifiable. 2 The numbers refer to the numbering of the thin sections made by Niedersächsisches Institut für Historische Küstenforschung (NIhK), Wilhelmshaven [17].

2.1. Optical Description: Macroscopic and Thin-Section Microscopy In general, ceramics consist of fired clay, temper and non-plastic material. Temper material can be sand, rock fragments, shell or bone remains or broken pottery (also called grog) to name a few. Non-plastic material is organic matter, e.g., plant fibers. Depending on the used temper and non-plastic materials and their quantities, the firing quality of the ceramic can be adjusted and shrinkage or cracking of the pottery during drying and firing can be prevented. The study of temper and non-plastic materials provides constraints on the manufacturing technique and might even allow identification of the place of manufacture. Two options exist for optical description: macroscopic descriptions and thin-section microscopy. Macroscopic descriptions are non-destructive. Hand lenses used for field geology (low power (10 ) magnification) or a high-resolution binocular loupe allow easy identification of phases up to × ~2 mm in size, but limitations exist for artifacts with weathered rinds. Thin section microscopy is an abrasive method. Thin sections of the ceramics are prepared by cutting, and polishing a slice of ceramic mounted on a glass slide to a thickness of 0.03 mm. As fresh cut faces are produced and Minerals 2020, 10, 931 5 of 22 polished, weathered rinds are less common. Not only is identification of minerals that are only 0.1 mm in size easily possible, crushed rocks (rock fragments) can be classified and textural relationships of the present temper materials can be derived. Thin sections are by far the most common use of optical mineralogy in geoscience, but less frequently used in archaeology.

2.2. Analytical Methods

2.2.1. Portable Energy-Dispersive X-Ray Fluorescence Spectrometry (Portable XRF) The portable XRF (portable energy dispersive X-ray fluorescence spectrometry) as an energy-dispersive handheld X-ray fluorescence technology is often used in archaeology [48–51]. It is non-destructive and reliable, requires no, or very little, sample preparation and is suitable for solid and powdered samples. The advantage of this method regarding the usage in archaeology is that either no sample or only a small part of the sample has to be destroyed (for producing powder) to determine chemical compositions. As results are provided within a few minutes, analyses of exhibits in museums or in the field are easy to obtain. Even if samples are allowed to be pulverized, measurements in archaeology are often carried out only on fresh fracture surfaces. Handheld XRF beam spot size usually is 10 mm in diameter and can be collimated down to 3 mm X-ray beam. Measurements on fresh fracture surfaces of pottery sherds with temper phases of 5 mm and larger will thus not provide reliable average chemical composition as the heterogeneity is larger than the area exposed to the X-ray beam [12,20]. Some of the sherds in the present study are heterogeneous and contain coarse inclusions (mineral grains, fragments of rocks, bones and/or even pottery sherds) with sizes of up to 2 mm. To judge if measurements on fresh fracture surfaces might still provide suitable average chemical compositions of the entire sherds, not only were those fracture surfaces measured, aliquots of the sherds were also ground to powder. Aliquots of those powders (1) were transferred into plastic containers (outer diameter: 29 mm; inner diameter: 18 mm; height: 25 mm), sealed with a special foil (polypropylene XRF foil TF-240-255 (4.0 µm) by FLUXANA) and directly analyzed by portable XRF, (2) were at first heated to 1050 ◦C for 3 h to transfer all elements in their highest oxidation state [8,22], transferred into containers, sealed and subsequent analyzed by portable XRF, or (3) were processed into glass disks by fusion and analyzed by wavelength dispersive X-ray fluorescence spectrometry (WD-XRF; see below for more details). This even allowed comparison of portable XRF and WD-XRF. In the present study, the NITON XL3t instrument of the 900 GOLDD series (ThermoFisher Scientific, Waltham, MA, USA) was used. The instrument was operated in the company’s pre-set mining mode, which measures 35 main and trace elements using 4 filters. Total duration of measurement was 300 s (main filter: 40 kV, 50 mA, 60 s; high filter: 50 kV, 40 mA, 60 s; low filter: 20 kV, 100 mA, 60 s; light filter: 6 kV, 100 mA, 120 s). The analytical range was from magnesium to uranium with detection limits of main elements of ~0.0025 wt.% and trace elements of 3–600 ppm (detection limits provided by the manufacturer for a SiO2 matrix in mining mode with a duration of measurements of 60 s/filter). All measurements were performed under constant helium flow in a laboratory with standard state conditions and a relative humidity of 40–50%. The waiting time between individual measurements was 60 s to ensure restoration of helium atmosphere surrounding the sample. Accuracy of measurements was regularly tested by the double measuring loess standard GBW07411 and metal standard IARM 35HN as unknown samples. On each fracture surface three optical homogeneous areas were measured. Powders were measured 4 to 9 times to check for precision (as also recommended by [3,12,52]). After each of those measurements, the containers were rotated to remix the powder and to ensure a homogenous mixture of the powder. Average values were calculated based on the individual analyses. Our study revealed that the measuring accuracy depends not only on the calibration of the device but also on the age of the X-ray tube. After about 4 to 5 years the measuring quality decreases as the age of the X-ray tube causes less precise detection of some elements. Standard GBW04711 (soil standard–loess, Institute of Geophysical and Geochemical Exploration, Chinese Academy of Minerals 2020, 10, 931 6 of 22

Geological Sciences) corresponds approximately to the chemical composition of the tomb ceramics. The standard was always measured as an unknown sample before and after each set of analyses as well as during one set of analyses. In addition, we compiled former and recent analyses of GBW04711 as unknown samples to determine any changes caused by the instrument and how those aﬀect the results. The instrument of the Graduate School at Kiel University, Germany, was bought in 2010, and underwent maintenance only in November 2014 and January 2019. Table2 lists literature values of GBW04711 and element concentrations of GBW04711 measured as unknown samples. Four measurement series are compared: (I) measurement series after maintenance in 2014; (II) measurement series before maintenance in 2019; (III) measurement series after maintenance in 2019 with old X-ray tube; (IV) measurement series with new X-ray tube in 2019.

Table 2. Element concentrations of GBW04711 (literature values) and measured as unknown sample.

Standard GBW07411 I II III IV After Maintenance Before Maintenance After Maintenance With New X-ray Literature Element 2014 2019 2019 Tube in 2019 1 [53] Mean Standard Mean Standard Mean Standard Mean Standard Deviation± Deviation± Deviation± Deviation± Ce [ppm] 66.3 68 5 91 8 90 4 88 5 ± ± ± ± Ba [ppm] 550 668 12 692 24 766 21 757 27 ± ± ± ± Sn [ppm] 64.3 31 2 35 2 70 2 70 3 ± ± ± ± Cd [ppm] 28.2 28 3 28 2 34 3 33 3 ± ± ± ± Nb [ppm] 15.1 11 1 18 2 13 1 13 1 ± ± ± ± Zr [ppm] 192 184 2 182 2 191 3 190 3 ± ± ± ± Y [ppm] 24.2 25 2 25 2 24 1 24 1 ± ± ± ± Sr [ppm] 130 129 1 86 1 128 1 128 1 ± ± ± ± Rb [ppm] 111 114 2 115 1 118 2 118 1 ± ± ± ± As [ppm] 205 169 8 162 10 150 9 159 12 ± ± ± ± Pb [ppm] 2700 2865 29 2677 16 2707 10 2708 10 ± ± ± ± Zn [ppm] 3800 3826 3 3857 10 3812 19 3810 17 ± ± ± ± Cu [ppm] 65.4 50 6 42 3 62 4 60 6 ± ± ± ± Fe [wt.%] 5.60 5.70 0.04 5.54 0.01 5.57 0.01 5.57 0.01 ± ± ± ± Mn [wt.%] 0.97 0.96 0.01 0.92 0.01 0.98 0.01 0.98 0.01 ± ± ± ± Cr [ppm] 59.6 113 20 117 10 147 13 146 12 ± ± ± ± V [ppm] 88.5 94.2 12 90 12 93 10 95 25 ± ± ± ± Ti [ppm] 4100 3855 167 3624 185 4474 155 4483 170 ± ± ± ± Ca [wt.%] 3.1 3.1 0.04 3.1 0.02 3.1 0.01 3.1 0.02 ± ± ± ± K [wt.%] 1.7 1.7 0.02 1.7 0.03 1.7 0.01 1.7 0.01 ± ± ± ± Al [wt.%] 6.4 3.2 0.38 5.9 0.51 6.1 0.16 5.9 0.41 ± ± ± ± P [ppm] 1400 3227 134 1155 163 1179 49.3 1139 77.4 ± ± ± ± Si [wt.%] 22 15 0.95 20 1.44 23 0.47 22 0.94 ± ± ± ± Mg [wt.%] 2.20 0.61 0.14 1.86 0.16 1.88 0.06 1.88 0.16 ± ± ± ± bal [ppm] n/a 2 680,827 599,993 563,506 572,249 Detection limits as provided by the manufacturer are ~0.0025 wt.% for major elements and 3–600 ppm for minor and trace elements. In this study measured mean values for major elements and standard deviations are with precisions of 0.01 wt.% and less. For minor and trace elements both mean values and standard deviations are provided with maximum precisions of 1 ppm; the portable XRF device additionally calculates the amount of unmeasured residues (listed as balance “bal” in the table); 1: kindly provided by M. Talma, Institute of Pre- and Protohistoric Archaeology, Kiel University; 2: not applicable.

Table2 clearly demonstrates the dependence of the accuracy of some elements on the age of the X-ray tube (e.g., Ce, Ba, Mn, Ti), while others are independent (e.g., Pb, Zn, Fe, Ca, K) or can be restored by the maintenance (e.g., Zr, Sr, Si). Caution has to be taken for elements close to the detection limits, such as Nb, As, or Cr. Elements’ accuracy that can be restored by the maintenance might be corrected prior to maintenance using correction factors. Minerals 2020, 10, 931 7 of 22

2.2.2. Wavelength Dispersive X-ray Fluorescence Spectrometry (WD-XRF) Wavelength-dispersive X-ray fluorescence spectrometry (WD-XRF) is a destructive and standard technique to determine bulk analyses. The higher resolution of WD-XRF compared to portable XRF provides advantages in terms of reduced spectral overlap. Complex samples can be more accurately characterized. In addition, with high-resolution backgrounds reduced, WD-XRF provides improved detection limits and sensitivity. Although samples can be measured as powder pellets as well as solid glass disks, usually glass disks are used in geoscientific applications. The glass disks are prepared using a fusion technique as this technique comprises of the reaction at a high temperature between a molten lithium borate flux and the specimen in order to develop a homogeneous molten glass that is finally cooled without crystallizing to produce an amorphous homogeneous solid glass disk. The sherds were coarsely crushed after completion of X-ray microtomography (see below for more details) and ground to fine powder in an automatic powder mill. Further processing of the fused glass disks and subsequent WD-XRF measurements were conducted at the Institute of Mineralogy and Petrography, University Hamburg. After determination of loss of ignition at 1050 ◦C, 600 mg of the remaining sample powder was thoroughly mixed with 3600 mg lithium tetraborate (Li2B4O7) and glass disks were prepared using the fusion technique. The WD-XRF measurements were performed using the Panalytical MagixPRO WD-XRF device with sample changer PW 2540 VCR. The analytical range was from sodium to uranium with detection limits of main elements Na and Mg of 0.01 wt.%, others less than 0.01 wt.% and trace elements Ce of 20 ppm, Ba and Nd 10 ppm, and others less than 5 ppm.

2.2.3. X-ray Microtomography (µCT) X-ray microtomography is a non-destructive technique that provides information of realistic 2D and 3D geometries of samples. Continuous image volume inspections can be undertaken by computer-aided morphometry and quantitative image analysis. It has been deployed for the characterization of true 3D microfabrics in respect to volume fractions, geometric surface, and size distribution of the fabric elements, including pores and porosity determination [54]. Because X-ray microtomography is a non-destructive technique it has a decisive advantage over XRF and thin-section petrography when investigating ancient pottery. Volume fractions of temper phases and non-plastic inclusions can be characterized and 3D visible orientation of pores can be detected and classified. This allows, for example, conclusions in respect to the manufacturing procedure of the pottery. The X-ray microtomography scans were undertaken using the SkyScan1172 system (Bruker-SkyScan, Billerica, MA, USA). The SkyScan1172 is a tabletop unit that allows high-resolution scans. It is suitable for samples with diameters of up to 35 mm and a height of 18 mm. Oversize scans, corresponding to three individual measurements, allow scanning of samples of up to 55 mm height. All samples were scanned with a beam energy of 100 kV, a current (flux) of 100 µA and, depending on the sample, aluminum or aluminum-copper filters. A 360◦ rotation with a step size of 0.5◦ and medium resolution was employed in the detector, resulting in a resolution of 17.3 µm per pixel. The SkyScan software NRecon (version 1.7.3.2, Bruker-SkyScan, Billerica, MA, USA) was used for reconstruction of the spatial information of the scanned ceramics. For each sample, approximately 1000 stacked layer imageswere created, summing up to 3000 stacked layered images in oversized scan mode. Each layer image consists of isometric voxels (volumetric pixels) whose sizes depend on the detector resolution. Thus, the stacked images have volumetric information. Information about various phases that result in a different degree of absorption of X-rays is stored in grey values and decoded into black and white images during evaluation. Segmentation and binarization, i.e., the assignment of materials and gray values, image analysis and visualization, was undertaken with the SkyScan software CTAn and CTVol. Proper characterization of pore volumes (porosities) and temper materials could be used to put constraints on the manufacturing procedure of the objects. This means that the porosities or air inclusions have to be quantified in order to define the ratio of matrix, temper particles plus non-plastic inclusions and cavities. Minerals 2020, 10, 931 8 of 25 assignment of materials and gray values, image analysis and visualization, was undertaken with the SkyScan software CTAn and CTVol. Proper characterization of pore volumes (porosities) and temper materials could be used to put constraints on the manufacturing procedure of the objects. This means that the porosities or air Mineralsinclusions2020 , have10, 931 to be quantified in order to define the ratio of matrix, temper particles plus8 non- of 22 plastic inclusions and cavities. For this purpose, a quantitative test was conducted on ink filler balls. These have a standardized For this purpose, a quantitative test was conducted on ink filler balls. These have a standardized diameter (2.5 ± 0.1 mm glass balls) and were filled into a plastic container of known volume. This diameter (2.5 0.1 mm glass balls) and were filled into a plastic container of known volume. This allows allows precise± calculation of the spaces or cavities as artificial porosities. An X-ray microtomography precise calculation of the spaces or cavities as artificial porosities. An X-ray microtomography scan scan was performed on the ink filler balls while placed in the container. Figure 2a depicts the was performed on the ink filler balls while placed in the container. Figure2a depicts the absorbed absorbed X-rays and Figure 2b is the reconstructed 3D view. All ink filler balls were made of glass, X-rays and Figure2b is the reconstructed 3D view. All ink filler balls were made of glass, except one except one ball that was made of plastic and was easily recognizable in Figure 2b as a darker sphere ball that was made of plastic and was easily recognizable in Figure2b as a darker sphere due to the due to the smaller density of plastic compared to glass. smaller density of plastic compared to glass.

Figure 2. (a) X-ray-microtomography scan of ink filler pens; (b) 3D view: the dark sphere is made of Figure 2. (a) X-ray-microtomography scan of ink filler pens; (b) 3D view: the dark sphere is made of plastic, all the others of glass. plastic, all the others of glass. After the scan a so-called region of interest (ROI) was generated around the ink filler balls and a After the scan a so-called region of interest (ROI) was generated around the ink filler balls and a 3D analysis was performed using the software CTAn. For manual calculation, the total volume of the 3D analysis was performed using the software CTAn. For manual calculation, the total volume of the balls was calculated by their number and diameter. The total volume of all balls ranged between 702 balls was calculated by their number and diameter. The total volume of all balls ranged between 702 and 893 mm3 by taking into account the diameter’s tolerance of 0.1 mm. Subtracting the total volume and 893 mm³ by taking into account the diameter’s tolerance of 0.1 mm. Subtracting the total volume of all balls from the ROI volume gave the pore space (see Table3). The software-based calculation of of all balls from the ROI volume gave the pore space (see Table 3). The software-based calculation of pore space (61%) was bracketed by the manually calculated values taking into account the varying pore space (61%) was bracketed by the manually calculated values taking into account the varying ball diameters (64 4%). Thus, porosities of the studied ceramics could be derived by the SkyScan ball diameters (64 ±± 4%). Thus, porosities of the studied ceramics could be derived by the SkyScan software CTAn. software CTAn. Table 3. Range of pore space in container with 97 ink filler balls considering the tolerance of the diameterTable 3. (2.5Range0.1 of mm).pore space in container with 97 ink filler balls considering the tolerance of the ± diameter (2.5 ± 0.1 mm). Ink Filler Ball Diameters Volume of One Ball Volume of All Balls Volume of Pore Space 1 Pore Space [%] 3 3 Volume 3 2.4 mm (minimum) 7.2 mm 702 mmVolume 1513 mm Pore 68 2.5 mm (average) 8.3 mm3 Volume794 of mm 3 1421of mm3 64 Ink Filler Ball Diameters of Space 2.6 mm (maximum) 9.2 mm3 One Ball893 mm3 Pore1322 mm3 60 All Balls [%] 1 Volume of pore space in container with 2215 mm3 (see text forSpace more 1 details). 2.4 mm (minimum) 7.2 mm3 702 mm3 1513 mm3 68 In order to not only determine the porosity in a ceramic sample but also to determine possible 2.5 mm (average) 8.3 mm3 794 mm3 1421 mm3 64 components of the temper and non-plastic inclusions, two reference standards were manufactured and 2.6 mm (maximum) 9.2 mm3 893 mm3 1322 mm3 60 analysed by X-ray microtomography. Both reference standards consisted of a clay matrix with either ¹ Volume of pore space in container with 2215 mm3 (see text for more details). temper material or temper material and non-plastic inclusions, which are also present in neolithic samples in approximately equal quantities. Standard I (Figure3) included the following components: clay 10.012 g, sand 1.003 g, feldspar 0.507 g, magnetite 0.028 g, biotite 0.101 g, hornblende 0.104 g, calcite 0.197 g, bone 0.088 g, tooth fragment 0.086 g, neolithic ceramic 0.069 g. The total volume of Standard I was 5561 mm3. A volume of 199 mm3 was accounted for by the temper components Minerals 2020, 10, 931 9 of 25

In order to not only determine the porosity in a ceramic sample but also to determine possible components of the temper and non-plastic inclusions, two reference standards were manufactured and analysed by X-ray microtomography. Both reference standards consisted of a clay matrix with either temper material or temper material and non-plastic inclusions, which are also present in neolithic samples in approximately equal quantities. Standard I (Figure 3) included the following components: clay 10.012 g, sand 1.003 g, feldspar 0.507 g, magnetite 0.028 g, biotite 0.101 g, hornblende 0.104 g, calcite 0.197 g, bone 0.088 g, tooth fragment 0.086 g, neolithic ceramic 0.069 g. The total volume of Standard I was 5561 mm³. A volume of 199 mm³ was accounted for by the temper components (biotite, hornblende, bone, calcite, tooth). In Standard II (Figure 4), mainly organic material was added: straw, culm/husks, barley grains, but also pottery fragments, tooth fragments. The aim was to identify the components in a reference sample, which could also be found in the Minerals 2020 10 original samples, , 931 either as impressions or fragments. 9 of 22 Pores as well as temper components or non-plastic inclusions could be easily identified in both (biotite,standards. hornblende, Even in Standard bone, calcite, II with tooth). its organic In Standard components, II (Figure the4 individual), mainly organic objects material were clearly was added:visible (Figurestraw, culm 4b,c)./husks, Based barley on grains,the experience but also pottery with fragments,the standards tooth and fragments. phases The therein, aim was comparable to identify componentsthe components of temper in a reference and non-plastic sample, whichinclusions could in also real be neolithic found in samples the original could samples be satisfactorily either as identifiedimpressions by or X-ra fragments.y microtomography.

Figure 3. StandardStandard I ( Ia ()a impregnated) impregnated with with epoxy epoxy and and polished; polished; (b) X-ray (b) X-ray microtomography microtomography scan scan(top (top view), visualized are only pores (green), heavy minerals, mainly magnetite (red), and some other Mineralsview), 2020 ,visualized 10, 931 are only pores (green), heavy minerals, mainly magnetite (red), and some other10 of 25 minerals, i.e.,i.e., feldsparfeldspar/biotite/calcite/biotite/calcite (yellow). (yellow). Please Please note note the visualizedthe visualized components, components, including including pores, pores,appear appear inside theinside standard. the standard. The scan The does scan not does depict not the depict components the components present at present the outer at surfacethe outer of surfacethe standard. of the standard.

Figure 4. Standard II with organic components (a) impregnated with epoxy; (b) X-ray microtomography Figure 4. Standard II with organic components (a) impregnated with epoxy; (b) X-ray scan (top view); (c) detailed x-ray microtomography scan; visible phases: straw (yellow), heavy minerals, microtomography scan (top view); (c) detailed x-ray microtomography scan; visible phases: straw mainly magnetite (red), culm (green), hasks (turquoise), barley grains (beige), tooth fragment (yellow), heavy minerals, mainly magnetite (red), culm (green), hasks (turquoise), barley grains (dark brown). (beige), tooth fragment (dark brown). Pores as well as temper components or non-plastic inclusions could be easily identiﬁed in both standards.3. Comparison Even of in Results Standard Derived II with by its organicVarious components, Methods the individual objects were clearly visible (Figure4b,c). Based on the experience with the standards and phases therein, comparable components 3.1. Macroscopic and Microscopic Observation of temper and non-plastic inclusions in real neolithic samples could be satisfactorily identiﬁed by X-rayThe microtomography. optical observations are summarized in Table 4. Descriptions can be either macroscopic, i.e., inspection of the sherds’ surface and/or fractured surface, or by thin-section microscopy [17]. 3. ComparisonThe limitations of Results of macroscopic Derived byobservation Various Methods of the surface are weathered rinds and adhered dirt. Robust statements regarding clay and temper material are barely possible; but ornament and 3.1. Macroscopic and Microscopic Observation decoration features can be identified. TheFractured optical surfaces observations provided are summarizedat first glance in impressions Table4. Descriptions of used clay can and be eithertemper macroscopic, of the ceramics, i.e., inspectionvisible as matrix of the sherds’and fragments surface inside and/or the fractured matrix. surface, In addition, or by different thin-section mineral microscopy and rock [17 fragments]. could be seen, although minerals like e.g., quartz and feldspar were not easy to distinguish and the portion of feldspar might be overestimated. Further identification of rock fragments was limited by magnification of hand lenses and binocular loupe. More detailed observation was possible by thin section microscopy albeit this being an abrasive method. Not only inorganic temper phases could be identified (i.e., mineral phases and rock fragments, including rock determination), also organic temper components could be detected. However, some experience was needed to distinguish various mineral phases, as more than solely quartz was most likely present as temper in addition to rock fragments and organic components.

Table 4. Optical description of the ceramic samples from the study from the selected megalithic tombs.

Sample Description No. Sample Macroscopic Description Thin Section Overview Thin Section ¹ (1) (2) (3)

qtz, 20080021 (altered) 1 200009_2 ++++ + fsp, mafic 253 minerals (?)

Minerals 2020, 10, 931 Minerals 2020, 10, 931 11 of 37 11 of 37

Minerals 2020, 10, 931 12 of 37

Table 4. Optical descriptionTable 4. Optical of the ceramicdescription samples of the fr omceramic the studysamples from from the the selected study megalithicfrom the selected megalithic Descri tombs. Mineralstombs. 2020, 10, 931 3 of 9 ption Macroscopic No. Sample Sample Overview Sample Thin Section Thin Description No. Sample Description Macroscopic Description Thin Section Overview SectionDescri Thin SectionDescri ¹ ption ption Macroscopic (1)Macroscopic (2) (3) ¹ Minerals 2020, 10, 931No. Sample SampleNo. OverviewSample Sample Overview Thin Section Thin SectionThin Thin 10 of 22 Description Description rock fragments Section Section Minerals 2020, 10, 931 13 of 37 (granitic ¹ ¹ (1) (2) (3) components), qtz, organic Table 4. Optical descriptionMinerals of the 2020,ceramic 10, 931 samples from the study from1 cm the selectedDescri megalithic tombs.12 of 37 components (1) (2) qtz(3) (1) (2) (3) ptionrock Macroscopic No. Sample Sample Overview , Thin Section fragmeThin Sample Description DescriptionDescri No. Sample Macroscopic(alt Description Thin SectionSectionnts 1 Minerals 2020, 10, 931Overview 14 of 37 Thin Sectionption ereqtz Macroscopicqtz (graniti¹ No. Sample Sample Overview Thin Section Thin (1)d), (2)Description (3), c Section fsp(alt (alt compoDescri ¹ (1) (2) (3)ere, ere nents)ption , Macroscopic 1 cm No. Sample Sample Overview mad) d)Thin Section qtz,Thin Description 20080021 20080021 fsp qtz, (altered)fsp fic (1) (2) (3) organicSection 1 20080021200009_22531 200009_2 1 200009_2 ++++++++ + miqtz, +++++ fsp,+ mafic, compo ¹ 253 253 nerma, mineralsma (?) nents (alt alsfic ficqtz rock ere Minerals 2020 , 10, 931 (1) (2) (?),mi(3) mi, 15 of 37 fragme d) 1 cm rocner ner(alt nts fsp kals alsere rock fragments(graniti , Descri fra(?) (?)d) (graniticc ma qtz, ption Macroscopicgm qtz, (altered)fsp components),compo 2 20080015500007_2281No. Sample Sample Overview +++fic + (altered)Thin Section Thin Descriptionent fsp, mafic, qtz, organicnents), 20080015 mi ) fsp, mafic 1 cm Section 20080015 s mineralsma (?), componentsqtz, Minerals 2020, 10, 931 2 500007_2 ner +++ + minerals 16 of ¹37 2 500007_2 +++ + (?) rockfic organic Minerals281 2020 , 10, 931 (?), rock 4 of 9 281 als qtz fragments mi (?) compo (?), fragments Sample , fs ner Descri Description nents No. Sample (1) (2) roc(3)Macroscopic Description(?) Thin Section Overview als ption Thin Section ¹ Macroscopick 1 cm No. Sample Sample Overview (1) (2) Thin(?), (3) Section Thin Descriptionfra Qt roc Section gm z kqtz, ¹ ent qtz, (altered)(altered)fra s (? fsp,1 maficcmfsp,gm mafic 1 cm 20080027 20080027 3 20080027800005_03 3 (1)++ ++(2)++ (3) ++ ++ minerals++ mineralsent (?), 800005_0 Minerals800005_0 2020, 10 , 931 1 cm 11 of 24 20080015 rock(?), s rock 2 500007_2 +++ + Sample fragmentsfragments(?) (?) Description No. Sample Macroscopic Description Thin Section 281 Overview (?) Thin Section ¹ 1 cm (1) (2) (3)

1 cm

1 cm 20080024

4 20080024000005_1254 000005_1 20080024 + + +++ +++ qtz, fsp

25 4 000005_1 + +++ qtz, fsp 25

Minerals 2020, 10, 931 1 cm 17 of 37 rock fragme

nts , Descri (graniti rock fragments fsp ption rock fragments 20080031 Macroscopic c (granitic No. Sample Sample Overview20080031 ++++ (?) Thin Section 1 cm Thin (granitic 5 20080031400002_23255 400002_2 Minerals 2020, 10, 931Description+++++ +++++ +++++ qtz, fspqtz, (?) fsp compo components),6 of 9 5 400002_2 + +++++ +++++ Section components), 325 (?) nents), qtz, organic 325 ¹ qtz, organic Sample qtz, Description No. Sample Macroscopic Description Thin Section componentscomponents Overview organic Thin Section ¹

(1) (2) (3) 1 cm (1) (2) (3) compo nents 1 cm

qtz p 20080024 , f 4 000005_1 + +++ qtz, fsp rock 25 fragme rock fragments 20080118 qtz nts rockrock fragments fragments 20080118 20080118 qtz, fsp (granitic 6 100021_6 ++++ , +++++ + qtz, fsp (graniti (granitic(granitic 6 20080118100021_6036 100021_6 6 100021_6 ++++++ ++++++ qtz,+ fsp (?)(?) components), 03 + fsp (?) c components),components), 03 03 qtz (?) compo qtzqtz nents),

qtz 11 cmcm 1 cm

11 cmcm

20080020 qtz, fsp 20080020 7 100025_1 + +(+) 1 cm (?) 7 20080020100025_11127 100025_1 112 + ++(+) +(+) qtz, fsp (?) 112

rock fragments 1 cm 20080020 (granitic qtz, fsp 8 100013_7 ++ ++ components), (?) 96 qtz, organic components 1 cm

20080020 qtz, fsp 7 100025_1 + +(+) (?) 112

sp (?)

Minerals 2020, 10, 931 18 of 37

Descri ption Minerals 2020, 10, 931 Macroscopic 19 of 37 No. Sample Sample Overview Thin Section Thin Description Section Descri¹ ption Macroscopic No. Sample Sample Overview Thin Section Thin Description (1) (2) (3) Section ¹

Q (1) (2) (3)

Minerals 2020, 10, 931 20 of 37

1 cm Descri

ption Macroscopic No. Sample Sample Overview tz, Thin Section Thin Minerals 2020, 10, 931 Description 11 of 22 fsp Section (?) ¹

Minerals 2020, 10, 931 Table 4. Cont. 7 of 9 (1) (2) qtz(3) , fs Sample Description No.Sample Sample Macroscopic Description Thin Section Description No. Sample OverviewMacroscopic Description Thin Section Thin Section ¹ Overview Thin Section 1 (1) (2) (3) rock fragme

nts (graniti rock fragments Minerals 2020, 10, 931 21 of 37 rock fragments 20080020 1 cm c 20080020 (granitic(granitic 8 20080020100013_7968 100013_7 ++ ++++ ++ qtz, fspqtz, (?) fsp compo components), 8 100013_7 ++ ++ components), 96 (?) nents), 96 Descri qtz,qtz, organic organic qtz, ption componentscomponents Macroscopic No. Sample Sample Overview Thin Section organicThin Minerals 2020, 10, 931 Description p 1 cm 22 of 37 Sectioncompo 1 cm nents¹ Minerals 2020, 10, 931 8 of 9

20080107 qtz Descri 9 20080107300006_13109 300006_1 ++++++Sample +(+) , f+ (+) qtz, fsp ption Description No. Sample (1)Macroscopic (2) (3) Macroscopic Description Thin Section No. Sample310 Sample Overview Overview Thin Section Thin Thin Section ¹ Description (1) (2) (3) Section ¹ Qt Minerals 2020, 10, 931 23 of 37

(1) (2) (3)

Descri

z, ption Macroscopic rock No. Sample Sample Overview fsp Thin Section Thin Description fragme rock fragments 20080107 Sectionnts rock fragments 20080107 (granitic 10 300009_1 ++++ +(+) qtz, fsp (graniti¹ (granitic 10 20080107300009_113810 300009_1 ++++++++ +(+) qtz +(+) qtz, fsp components), Minerals 2020, 10, 931 138 24 ofc 37 components), 138 , f qtz compo qtz (1) (2) (3) 1 cm nents),

Descriqtz 1 cm ption Minerals 2020, 10, 931 Macroscopic 25 of 37 No. Sample Sample Overview20080107 Qt Thin Section Thin Description 9 300006_1 z +++ +(+) qtz, fsp Section

310 1 cm ¹ 1 cm Descri ption Macroscopic No. Sample Sample Overview Thin Section Thin 20080029 (1) (2) (3) Description 20080029 11 600055_2 ++++ +(+) 1 cmqtz, fsp Section 11 20080029600055_222111 600055_2 221 ++++++++ +(+) +(+) qtz, fsp ¹

221 spQt

z (1) (2) (3)

Minerals 2020, 10, 931 9 of 9 20080029 12 20080029600009_200812 600009_2 ++++++ +(+) + (+) qtz, fsp Sample Description 008 No. Sample Macroscopic Description Thin Section Overview Thin Section ¹

(1) (2) (3) rock

fragme

nts rockrock fragments fragments 20080022 20080022 1 cm (graniti (granitic(granitic 13 20080022500017_210613 500017_2 13 500017_220080029 ++ ++++(+) ++++(+ ) ++(+)qtz, fspqtz, fsp c components),components), 106 12 600009_2106 +++ +(+) qtz, fsp compo qtz 008 qtz nents), , qtz fsp 1 cm1 cm

(?) 20080011 qtz, fsp 20080011 14 500010_2 + +++ 14 20080011500010_207714 500010_2 + + +++ +++ qtz, fsp (?)(?) 077 qtz 077 , fsp sp qtz, fsp (?) 20080022 (?) (very (ve 15 500016_2 +++++ ++ small 20080022 ry qtz, fsp (?) 103 ++++ frag- 15 20080022500016_210315 500016_2 +++++++ sm ++ (very small + ments) 103 all frag-ments) 1 Macroscopic descriptions:fra (1) relative amount of matrix to temper phases: +++++ (sample has the max

of matrix of the group)…+ (min of group); (2) larger/smaller mineral (other) fragments in matrix: g- 1 Macroscopic descriptions: (1) relative+++++…+; amount (3) Macroscopic of matrixme identifiable to temper mineral phases: fragments+++++ [17]. (sample has the max of matrix of the group) ... + (min of group); (2) larger/smaller mineralnts, (other) fragments in matrix: +++++ ... +; (3) Macroscopic 3.2. Chemical Compositions (Portable XRF vs. WD-XRF) identiﬁable mineral fragments [17]. fsp) Chemical compositions of identical sherds measured either by WD-XRF or by portable XRF are 1 Macroscopic descriptions:compared (1)in relaFiguretive amount5. The dashedof matrix lines to temp correlateer phases: WD-XRF +++++ (sample results has with the maxportable XRF values of the

of matrix of the group)…+ (min of group); (2) larger/smaller mineral (other) fragments in matrix: same samples. Perfect agreement between WD-XRF and portable XRF exists for those measurements +++++…+; (3) Macroscopicthat fall on identifiable the solid mineral line, i.e., fragments 1:1 correlation [17]. line. The symbols visualize the different portable XRF measurements. To reiterate, WD-XRF measurements were conducted on fused glass disks. Aliquots 3.2. Chemical Compositions (Portable XRF vs. WD-XRF) of the powder used to prepare the glass disks were measured by portable XRF (dark grey squares). Chemical compositionsDuring fused of identical glass disksherds preparation measur ed eitherelements by WD-XRF transfer orin by their portable highest XRF oxidation are state. Thus, compared in Figureadditional 5. The dashed aliquots lines of thecorrelate powder WD-XRF were heated results to with1050 portable°C for 3 h XRF prior values to portable of the XRF measurements same samples. Perfectto test agreement for any influencebetween WD-XRFon XRF results and portable if elements XRF were exists existent for those in theirmeasurements highest oxidation state (open that fall on the solidsquares). line, i.e., As 1:1 portable correlation XRF liarene. often The usedsymbols as a visualizenon-destructive the different technique portable in archaeology XRF by analyzing measurements. To reiterate,fracture surfaces, WD-XRF the measurements present fracture were surfaces conducted of theon fusedsherds glass were disks. measured Aliquots before grounding the of the powder usedsherds to prepare to powder the glass for furtherdisks were processing measured (grey by diamonds).portable XRF (dark grey squares). During fused glass diskThe preparation following observations elements tr canansfer be made:in their highest oxidation state. Thus,

Minerals 2020, 10, 931 12 of 22

The limitations of macroscopic observation of the surface are weathered rinds and adhered dirt. Robust statements regarding clay and temper material are barely possible; but ornament and decoration features can be identified. Fractured surfaces provided at first glance impressions of used clay and temper of the ceramics, visible as matrix and fragments inside the matrix. In addition, different mineral and rock fragments could be seen, although minerals like e.g., quartz and feldspar were not easy to distinguish and the portion of feldspar might be overestimated. Further identification of rock fragments was limited by magnification of hand lenses and binocular loupe. More detailed observation was possible by thin section microscopy albeit this being an abrasive method. Not only inorganic temper phases could be identified (i.e., mineral phases and rock fragments, including rock determination), also organic temper components could be detected. However, some experience was needed to distinguish various mineral phases, as more than solely quartz was most likely present as temper in addition to rock fragments and organic components.

3.2. Chemical Compositions (Portable XRF vs. WD-XRF) Chemical compositions of identical sherds measured either by WD-XRF or by portable XRF are compared in Figure5. The dashed lines correlate WD-XRF results with portable XRF values of the same samples. Perfect agreement between WD-XRF and portable XRF exists for those measurements that fall on the solid line, i.e., 1:1 correlation line. The symbols visualize the diﬀerent portable XRF measurements. To reiterate, WD-XRF measurements were conducted on fused glass disks. Aliquots of the powder used to prepare the glass disks were measured by portable XRF (dark grey squares). During fused glass disk preparation elements transfer in their highest oxidation state. Thus, additional aliquots of the powder were heated to 1050 ◦C for 3 h prior to portable XRF measurements to test for any inﬂuence on XRF results if elements were existent in their highest oxidation state (open squares). As portable XRF are often used as a non-destructive technique in archaeology by analyzing fracture surfaces, the present fracture surfaces of the sherds were measured before grounding the sherds to powder for further processing (grey diamonds). The following observations can be made:

1. Potassium, Rb, Ti, V, Y, Zn, and Zr contents agreed well independent of technique used (WD-XRF and portable XRF) and portable XRF measurements on powder, heated powder or break planes. 2. Portable XRF measurements resulted in slightly higher Fe concentration compared to WD-XRF, but measurements on powder, heated powder or break planes agreed well. 3. Portable XRF measurements led to Cr values that were distinctly higher compared to WD-XRF, but again measurements on powder, heated powder or break planes agreed well. 4. Aluminium, Ca, Mg, Si, P, and Sr measured by p-ed XRF were lower compared to WD-XRF by element-specific factors. Agreement existed between powder and heated powder measurements, but measurements of break planes disagreed with powder measurements. 5. Niobium did not show an off-set that could be clearly related to over- or underestimated values. This might be due to the fact that Nb-contents were close to detection limits. Agreement existed again between powder and heated powder measurements, but measurements of break planes provided random contents of the respective elements. 6. With the exception of the 3 elements (Al, Cr, and Si), the quantity of all elements measured with the portable XRF device could be corrected by multiplying the results by the relevant coefficients from the regression equations. Minerals 2020, 10, 931 13 of 22 Minerals 2020, 10, 931 15 of 25

Figure 5. Comparison of portable XRF and WD-XRF measurements. All chemical data are available as a supplementaryFigure 5. Comparison ﬁle (Table of portable S1). XRF and WD-XRF measurements. All chemical data are available as a supplementary file (Table S1). The limited reliability of the chemical analyses by applying the technique to fracture surfaces [55] was proved true. Measurements of powders provided satisfying data with regard to accuracy and precision for K, Fe, Rb, Ti, V, Y, Zn, and Zr. Iron revealed slightly higher contents in measured powder, but still acceptable values compared to WD-XRF measurements. We recommend correction for powder measurements to derive reliable concentrations for all elements except K, Rb, Ti, V, Y, Zn, and Zr, as portable XRF measurements of those elements agreed well with WD-XRF measurements. However,

Minerals 2020, 10, 931 14 of 22 the values of single elements in diﬀerent sherds measured with an identical and routinely calibrated portable XRF device could be easily compared and grouping of sets of sherds based on their chemical bulk compositions was possible. In addition, publishing the analysis of reference materials along with the results obtained from the samples would provide all necessary information to evaluate possible deviationsMinerals in2020 the, 10, measurements, 931 regardless of the technique used. 16 of 25

3.3. Characterisation3.3. Characterisation of Componentsof Components and and Porosities Porosities byby 2D and and 3D 3D Sections Sections FigureFigure6 shows 6 shows examples examples of of the the comparison comparison ofof variousvarious temper temper components components and and non-plastic non-plastic inclusionsinclusions in two in neolithictwo neolithic samples samples and Standard and Standard I. Bone I. fragments, Bone fragments, organic organic material material and idiomorphic and mineralidiomorphic phases (in mineral this case phases mica (in mineral this case biotite) mica can mine beral precisely biotite) detectedcan be precisely by µCT. detected When examining by µCT. the thin sections,When examining it was not the clear thin whether sections, they it was were not cavities clear whether or non-plastic they were inclusions. cavities Butor non-plastic it is quite clear inclusions. But it is quite clear from the µCT scans that straws, present only as imprints, can be from the µCT scans that straws, present only as imprints, can be identiﬁed and are non-interchangeable identified and are non-interchangeable with cavities due to e.g., manufacturing techniques (Figure 6, with cavities due to e.g., manufacturing techniques (Figure6, middle). middle).

Temper No./ Component / µCT-Model Microscopic View Name Non-Plastic Inclusion

No. 19- 200800123 Bone 00012_231 4

No. 12- 200800123 Organic: 00012_231 Straw 4

Standard Biotite I

FigureFigure 6. Examples 6. Examples of temperof temper components components andand non-plasticnon-plastic inclusion inclusion in intwo two neolithic neolithic samples samples and and Standard I. a,b: bone fragment in sample No. 19-20080012300012_2314 (a: X-ray microtomography Standard I. (a,b) bone fragment in sample No. 19-20080012300012_2314 (a: X-ray microtomography (µCT) reconstruction, b: cut surface); c,d: straw as non-plastic inclusion in No. 12- (µCT) reconstruction, b: cut surface); (c,d) straw as non-plastic inclusion in No. 12-20080029600009_2008 20080029600009_2008 (c: µCT reconstruction, d: photomicrograph of thin section); e,f: mineral phase (c: µCT reconstruction, d: photomicrograph of thin section); (e,f) mineral phase biotite (e: µCT biotite (e: µCT reconstruction, biotite is in red, f: polished surface). reconstruction, biotite is in red, f: polished surface). The advantage of µCT compared to thin-section analysis is not only the non-destructive analysis of the samples. Thin sections allow only a small 2D section of the sample, but µCT scans allow the entire object to be analysed. However, it is only possible to differentiate between temper components as long as the density of various components can be distinguished in grey values. The grey values

Minerals 2020, 10, 931 15 of 22

The advantage of µCT compared to thin-section analysis is not only the non-destructive analysis of the samples. Thin sections allow only a small 2D section of the sample, but µCT scans allow the entire object to be analysed. However, it is only possible to differentiate between temper components as long as the density of various components can be distinguished in grey values. The grey values will then provide information aboutTableTable the 5. 5.X-ray X-ray quantity microtomography microtomography of the scans phasesscans of ofneolithic neolithic or componentssamples samples and and morphometric morphometric and the information. information. objects can be visualized in a 2D and 3D model. TotalTotal ObjectObject PorosityPorosity No.No.1 1 CategoryCategory 2 2 3D3D Section Section of of1 × 1 1 × cm 1 cm 2D 2D (Detail) (Detail) Slice Slice of of3D 3D Section Section The µCT scans of the neolithic samplesVolumeVolume providedin in% 3% 3 in addition information on the porosities of the ceramic structures, or more generallyin inmm³ mm³ of sample structures. Can porosities be used to indicate the typological classification of the finds or even the type or shape of the vessel? Table5 displays 3D reconstructions of X-ray microtomography scans of the investigated neolithic samples, cross sections as 2D reconstructions, information about the total objects’ volumes and the MineralsMinerals 2020 2020, 10, 10, 931, 931 2 2of of15 15 porosities of those volumes. Data processing was done with the SkyScan software CTAn and CTVol. Temper components as well as the variousTotalTotal shapes of the voids are visible in the 2D cross sections. ObjectObject PorosityPorosity No.No.1 1 CategoryCategory 2 2 3D3D Section Section of of 1 ×1 1× cm1 cm 2D 2D (Detail) (Detail) Slice Slice of of 3D 3D Section Section In addition, information about the typologicalVolumeVolume in horizonsin % 3% 3 and vessel types are given in Table5 (see also Table1 for more details). inin mm³ mm³

Table 5. X-ray microtomography scans of neolithic samples and morphometric information.

MineralsMinerals 2020 2020, 10, ,10 931, 931 3 of3 of15 15 1 2 Total Object Porosity in 2D (Detail) Slice of 3D No. Category 3 3 3D Section of 1 1 cm Volume in mm TotalTotal % × Section ObjectObject PorosityPorosity No.No.1 1 CategoryCategory 2 2 3D3D Section Section of of1 × 1 1 × cm 1 cm 2D 2D (Detail) (Detail) Slice Slice of of3D 3D Section Section VolumeVolume in in% 3% 3 in inmm³ mm³

MineralsMinerals 2020 2020, 10, 10, 931, 931 4 of4 of15 15 6 36 6 20623 3 20622062 6.656.656.65 TotalTotal ObjectObject PorosityPorosity No.No.1 1 CategoryCategory 2 2 3D3D Section Section of of1 ×1 1× cm1 cm 2D 2D (Detail) (Detail) Slice Slice of of3D 3D Section Section VolumeVolume in in% 3% 3 in inmm³ mm³

11 31111 2791 3 3 2791 2791 5.945.945.94

1 41 1 17834 4 17831783 5.015.015.01

MineralsMinerals 2020 2020, 10, ,10 931;, 931; doi:10.3390/min10100931 doi:10.3390/min10100931 www.md www.mdpi.com/journal/mineralspi.com/journal/minerals

7 47 7 57014 4 57015701 10.8810.8810.88

MineralsMinerals 2020 2020, 10, 10, 931, 931 5 of5 of15 15

TotalTotal ObjectObject PorosityPorosity No.No.1 1 CategoryCategory 2 2 3D3D Section Section of of1 ×1 1× cm1 cm 2D 2D (Detail) (Detail) Slice Slice of of3D 3D Section Section VolumeVolume in in% 3% 3 in inmm³ mm³

MineralsMinerals 2020 2020, 10, ,10 931, 931 6 of6 of15 15

TotalTotal ObjectObject PorosityPorosity No.No.1 1 CategoryCategory 2 2 3D3D Section Section of of1 × 1 1 × cm 1 cm 2D 2D (Detail) (Detail) Slice Slice of of3D 3D Section Section VolumeVolume in in% 3% 3 in inmm³ mm³

Minerals 2020, 10, 931 16 of 22

Table 5. Cont. MineralsMinerals 2020 2020, 10, 10, 931, 931 7 7of of15 15

1 2 Total Object Porosity in 2D (Detail) Slice of 3D No. Category TotalTotal 3D Section of 1 1 cm Volume in mm3 %3 × Section ObjectObject PorosityPorosity No.No.1 1 CategoryCategory 2 2 3D3D Section Section of of 1 ×1 1× cm1 cm 2D 2D (Detail) (Detail) Slice Slice of of 3D 3D Section Section VolumeVolume inin % 3% 3 inin mm³ mm³

8 48 8 56184 4 56185618 7.237.237.23 MineralsMinerals 2020 2020, 10, 10, 931, 931 8 8of of 15 15

TotalTotal ObjectObject PorosityPorosity No.No.1 1 CategoryCategory 2 2 3D3D Section Section of of 1 ×1 1× cm1 cm 2D 2D (Detail) (Detail) Slice Slice of of 3D 3D Section Section VolumeVolume inin % %3 3 inin mm³ mm³

2 52 2 11695 5 11691169 1.481.481.48

MineralsMinerals 2020 2020, 10, ,10 931, 931 9 of9 of15 15

TotalTotal ObjectObject PorosityPorosity No.No.1 1 CategoryCategory 2 2 3D3D Section Section of of1 × 1 1 × cm 1 cm 2D 2D (Detail) (Detail) Slice Slice of of3D 3D Section Section VolumeVolume in in% 3% 3 in inmm³ mm³

12 51212 2321 5 5 2321 2321 2.932.932.93 MineralsMinerals 2020 2020, 10, ,10 931, 931 10 10of of15 15

TotalTotal ObjectObject PorosityPorosity No.No.1 1 CategoryCategory 2 2 3D3D Section Section of of1 × 1 1 × cm 1 cm 2D 2D (Detail) (Detail) Slice Slice of of3D 3D Section Section VolumeVolume in in% 3% 3 in inmm³ mm³

4 f4 4 2824f f 28242824 12.2012.2012.20 MineralsMinerals 2020 2020, 10, ,10 931, 931 11 11of of15 15

TotalTotal ObjectObject PorosityPorosity No.No.1 1 CategoryCategory 2 2 3D3D Section Section of of1 × 1 1 × cm 1 cm 2D 2D (Detail) (Detail) Slice Slice of of3D 3D Section Section VolumeVolume in in% 3% 3 in inmm³ mm³

11671167 3.373.37 5 f5 5 1167f f 3.37

9 f9 9 1638f f 16381638 1.981.981.98

10 f1010 1507 f f 1507 1507 5.995.995.99

MineralsMinerals 2020 2020, 10, 10, 931, 931 1212 of of15 15

TotalTotal ObjectObject PorosityPorosity No.No.1 1 CategoryCategory 2 2 3D3D Section Section of of 1 ×1 1× cm1 cm 2D 2D (Detail) (Detail) Slice Slice of of 3D 3D Section Section VolumeVolume inin % 3% 3 inin mm³ mm³

MineralsMinerals 2020 2020, 10, ,10 931, 931 13 13of of15 15

Minerals 2020, 10, 931 17 of 22

Table 5. Cont. MineralsMinerals 2020 2020, 10, 10, 931, 931 1414 of of15 15

1 2 Total Object Porosity in 2D (Detail) Slice of 3D No. Category TotalTotal 3D Section of 1 1 cm Volume in mm3 %3 × Section ObjectObject PorosityPorosity No.No.1 1 CategoryCategory 2 2 3D3D Section Section of of1 ×1 1× cm1 cm 2D 2D (Detail) (Detail) Slice Slice of of3D 3D Section Section VolumeVolume in in% 3% 3 in inmm³ mm³

14 f1414 2276 f f 2276 2276 1.811.811.81

15 f1515 4076 f f 4076 4076 6.476.476.47

13 c13Minerals13 Minerals 923 2020c 2020 c, 10, 10, 931, 931 923 923 4.954.954.95 1515 of of15 15

TotalTotal ObjectObject PorosityPorosity No.No.1 1 CategoryCategory 2 2 3D3D Section Section of of 1 ×1 1× cm1 cm 2D 2D (Detail) (Detail) Slice Slice of of 3D 3D Section Section VolumeVolume inin % 3% 3 inin mm³ mm³

3 s3 3 2672s s 26722672 4.304.304.30

1 Consecutive1 Consecutive number number (No.) (No.) corresponds corresponds to to Table Table 1. 1.2 Categories2 Categories refer refer to to the the typochronological typochronological 1 2 Consecutive number (No.) correspondsclassificationclassification toof of Tablethe the pottery pottery1. accordCategories accordinging to to the the refer dating dating to horizons thehorizons typochronological 3, 3,4, 4,5 and5 and types types (see (see classiﬁcation Menne, Menne, 2018, 2018, for for of 3 the pottery according to the datingmoremore horizons details details [17]). [17]). 3, 3 4,Porosity 3 Porosity 5 and is typesistotal total porosity porosity (see Menne, of of each each scanned 2018,scanned sherd, for sherd, more temper temper details components components [17]). are are visiblePorosity visible in in is total porosity of each scanned2D sherd,2D (detail); (detail); temper please please note: components note: sample sample no. no. are 19 19 (20080012300012_2314, visible (20080012300012_2314, in 2D (detail); WP WP please43, 43, s) s)is is note:not not listed listed sample as as only only no. a a3D 193D (20080012300012_2314, WP 43, s)reconstruction isreconstruction not listed exist as exist only and and no a no 3Dporosity porosity reconstruction analyses analyses was was made. exist made. and no porosity analyses was made.

Only finds of horizons 3 to 5 were selected in this study as those provide the most precise typochronological classification of finds (see [17], for more details). Although the number of investigated finds are small (7 finds assigned to horizons and 8 finds with only known types of the vessels as those finds are stratigraphically unclassifiable), differences between the different categories of the horizons can be identified. The porosities of finds of horizon 4 (5.01, 7.33, 10.88, i.e., on average 7.74%;) are slightly higher than those of horizon 3 (5.94, 6.65, average 6.30%). Finds of horizon 5 have the smallest porosities (1.48%, 2.93%, 2.21%). The present study shows that the younger samples have a lower porosity compared to the older ones. This may be related to a change in production technology in the younger phase of TRB, whereby the vessels may have become more solid. The collar flask has a porosity of 4.95% and the funnel beakers have a porosity of 1.81–12.20% (n = 6), on average 5.30%; the pottery of the single-grave period has a porosity of 4.30%. The comparison of funnel beakers and bowls (all from categories 3 to 5) shows similar porosities but also reflects the large variations in the porosities of the individual vessel types (funnel beaker 5.30 3.91%; ± bowls 5.73 3.06%). Based on the limited data set of our study, it is not possible to differentiate ± between vessel types and possible differences in production technology of manufacturing. For future studies it remains to be examined whether the use of the vessel types (e.g., beaker for liquid contents

Minerals 2020, 10, 931 18 of 22 or bowls for solid contents) can be distinguished on the basis of porosity. This might indicate whether different clay mixtures have been used at the same time intentionally during a typological horizon. Comparison of the porosities in consideration of the three sites E2, GB7 and WP43 reveal that the ceramics of GB 7 have porosities of 1.81–6.47% (n = 5), on average 4.42%, of E2 1.98–10.88% (n = 5), on average 6.55%, and WP43 1.48–12.20% (n = 5), on average 5.27% (see Table5). The average porosities in E2 are 2.13% higher than in GB7 and 1.28% higher than in WP43. This might be an indication of the different production techniques used at the different sites. Despite the small number of samples, it is not possible to say whether the porosities indicate site-specific production methods. The observed trend requires a larger sample size to be tested in order to confirm this tendency. Even though only 15 finds were studied in detail by µCT, and the great potential of non-destructive µCT method is obvious for distinguishing various finds regarding their typochronology, the sample number is far too small to clearly keep the sites apart and also to provide evidence for different production techniques. More samples have to be investigated to find out if there are exact differences in porosity in bowls, amphorae and cups and to provide answers to questions like “Could the different types of vessels possibly have been produced using different techniques?” or “Could this have been part of the burial rite?”.

4. Concluding Remarks and Outlook Ceramics can be either examined non-destructively or through destruction caused by analytical procedures. These two concepts have been explored and combined by using diﬀerent methodologies. A common procedure in archaeology is to partly destroy the objects for analysis. But often the ultimate examination is only based on thin sections and portable XRF for basic geochemical analysis. This leaves a lot of unused potential for a detailed examination of the materials. The aim of this study is to evaluate the quality and limits of measurement techniques like portable XRF, WD-XRF, and µCT and to apply constraints on their reliability and applicability for archaeological interpretations of ceramic studies in order to draw conclusions about, for example, the production technique of the ﬁnds. This was done in four steps:

1. Destructive method by thin sections and macroscopic investigation

Using the non-destructive method of macroscopic investigation, information on temper components can be obtained at ﬁrst glance. But more detailed and reliable information on ceramic components with thin-section petrography as the standard method requires the partial destruction of samples.

2. Non-destructive method of portable XRF

A first impression of the geochemical structure of the sample can be gained by portable XRF at the fresh fracture, if present. However, it must be considered that the larger the grain size of the components of the ceramic, the more the geochemical data can be falsified. It turned out that the measurements of the fresh fractures did not provide satisfactorily precise results. But it is beyond question that portable XRF gives a quick overview of the chemical composition of ceramics and can be used both non-destructively and destructively. The portable analyses on pulverized samples are a cost-effective and fast alternative to wavelength-dispersive X-ray fluorescence (WD-XRF).

3. Destructive method by pulverized samples with portable XRF and WD-XRF

In comparison to portable XRF measurements, WD-XRF provides more precise data on the geochemical composition. However, WD-XRF is often not possible for cost reasons. The direct comparison of aliquots of identical pulverized samples measured with portable XRF and WD-XRF revealed reliable portable XRF concentration data of K, Fe, Rb, Ti, V, Y, Zn, and Zr, but other elements need to be corrected. Although portable XRF provides reliable data, there are limitations in the performance and evaluation of measurements. Multiple measurements of standard GBW04711 over a period of time of 5 years showed that the quality of the measurements strongly depends on the maintenance of Minerals 2020, 10, 931 19 of 22 both software and hardware as well as on the “Aging Eﬀect” of the X-ray tube. Regular calibration of portable XRF devices is of crucial importance.

4. Non-destructive method of µCT

The non-destructive method of µCT is a suitable complement to the conventional archaeometric method for the identification of the components using thin sections. Cross sections based on µCT scans even allow 2D insights at different individually selected parts of the sherds. Quantitative information regarding modal abundances of phases and porosities of the ceramics can be obtained by µCT scans (3D). Even characterisation of the organic temper components and non-plastic inclusions is possible. The studied neolithic ceramics showed slight differences in porosities. The younger ceramics have lower porosities compared to the older ones. With regard to the various vessel types, differentiation is not possible due to the large variation of porosities in individual vessel types. The same is true for a differentiation based on the sites. The porosities of the 15 neolithic ceramics studied disclosed neither type-specific nor site-specific manufacturing techniques. However, the great potential of the µCT technique to extract valuable information needed by archaeologists to group samples and, if applicable, to deduce details on manufacturing is obvious.

Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/10/10/931/s1. Table S1: Raw data_portable XRF: Appendix_A_Data_pedXRF_Menne et al. Author Contributions: Investigation, J.M., A.H., and C.H.; formal analysis, J.M. and C.H.; resources, J.M.; visualization, J.M., and C.H.; writing—original draft preparation, J.M. and A.H. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: We would like to offer our special thanks to the following Institutes for their assistance and technical support with the analysis of the data: Institute of Mineralogy and Petrography, University Hamburg (WD-XRF); GEOMAR Kiel (thin-section microscopy). Stefan Dreibrodt, Brendan Ledwig, Jan Thomsen, and Katrin Struckmeyer are thanked for their intense discussion, technical support and helpful comments as well proof reading the article. Marianne Talma kindly provided her data of the standard measurements of the new X-ray tube (portable XRF (2019)), which we were allowed to integrate into our results. Furthermore, we want to thank Andrea Kaltofen (Kommunale Bodendenkmalpflege, Landkreis Emsland, Lower Saxony, for her permission to study and sample the ceramics. Last but not least we thank four anonymous referees for their comments. Conflicts of Interest: The authors declare no conflict of interest.

References

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