Non-Destructive, Specular Laser Reflectometry and X-Ray Fluorescence Analysis Applied to Coins of the Gallic Roman Empire

Academic Editors: Paola Fermo and Giuseppe Politi Received: 7 April 2025 Revised: 18 May 2025 Accepted: 26 May 2025 Published: 1 June 2025 Citation: Toschke, Y.; Wolke-Hanenkamp, S.; Wolf, E.; Lichtenberger, A.; Martin, K.; Nieswandt, H.-H.; Imlau, M. Non-Destructive, Specular Laser Reflectometry and X-Ray Fluorescence Analysis Applied to Coins of the Gallic Roman Empire. Heritage 2025,8, 202. https:// doi.org/10.3390/heritage8060202 Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/). Article Non-Destructive, Specular Laser Reflectometry and X-Ray Fluorescence Analysis Applied to Coins of the Gallic Roman Empire Yannic Toschke 1, Steffen Wolke-Hanenkamp 1, Eugen Wolf 1, Achim Lichtenberger 2, Katharina Martin 2, H.-Helge Nieswandt 2and Mirco Imlau 1,* 1 Institute of Physics, Department of Mathematics/Informatics/Physics, Osnabrück University, Barbarastraße 7, D-49076 Osnabrück, Germany; [email protected] (Y.T.); [email protected] (S.W.-H.); [email protected] (E.W.) 2Institute for Classical Archaeology and Christian Archaeology, Münster University, Domplatz 20-22, D-48143 Münster, Germany; [email protected] (A.L.); [email protected] (K.M.); [email protected] (H.-H.N.) *Correspondence: mir[email protected]; Tel.: +49-541-969-2654 Abstract: Non-destructive, specular laser reflectometry, an industrially used and easily accessible method, is adapted in numismatic research to the inspection of coins of the Gallic Roman Empire with the objective of the allocation of mints and/or the identification of different minting techniques. For this purpose, the laser-reflectometric fingerprints of three series of coins each consisting of five antoniniani (or radiates) of the Gallic Roman Empire originating from two mints —Trier and Cologne— are systematically determined, analyzed and correlated with the corresponding XRF element analysis. The results show that the use of an inexpensive red-emitting laser system (wavelength 632.8 nm) with a beam diameter d< 0.5 mm and a sample mount that can be adapted to the individual shape and thickness of the coins leads to signal intensities that can be systematically recorded over a large angular range with a very good signal-to-noise ratio (SNR > 10). While the signals cannot be used to assign individual coins to mints, we discuss the possibility of a statistical analysis. Although each coin set consists of only five samples and thus requires further study, the results here suggest that the sets can be distinguished from each other, that there is a correlation to the silver concentration and that it is possible to estimate the refractive index n. Keywords: Roman coins; surface inspection; non-destructive laser method; X-ray fluorescence PACS: 78.40.-q; 78.68.+m; 78.70.En 1. Introduction X-ray photoelectron spectroscopy (XPS), X-ray fluorescence spectroscopy (XRF), Laserablationand Multicollector-inductively coupled plasma-mass spectrometry (LA-ICP-MS, LA-MC-ICP-MS) are well-established methods in numismatics to qualify samples regarding their chemical or atomic composition [ 1 – 3 ]. In particular, extensive work has been carried out on provenance determination to mint of origin using elemental and isotopic composition of silver via XRF [ 4 – 6 ]. There are also corresponding XRF handheld instruments for use at the excavation site after the coins have been cleaned, or in local museums abroad [ 7 ]. Due to their relatively high price point of up to a few Eur 10.000, however, they are only available to a limited extent. For this reason, the cooperation behind this project Heritage 2025,8, 202 https://doi.org/10.3390/heritage8060202 Heritage 2025,8, 202 2 of 20 pursued a first attempt in the adaptation of a non-destructive, cost-effective laser-based method for the use in numismatics that was originally introduced for the inspection of metal surfaces in industry by Rischmueller et al. [ 8 ]. The technical complexity of the method is comparatively low, so that the transformation towards a handheld tool, especially for use in a museum or field use seems very likely. Specifically, the original method enables the determination and differentiation of (ultra-)thin dielectric layers on aluminum parts, so-called conversion layers, and plays an important role in the quality control of corrosion protection of aluminum components. For this purpose, the laser-based method makes use of the physical phenomenon of laser light interference and is capable of differentiating layer thicknesses well below a thickness of 100 nm, and even below 10 nm on surfaces with roughness in the range of ≈ 300 nm. It was demonstrated that the method is sensitive to the totality of the surface morphology and the detected signal therefore contains information about various surface properties such as roughness, absorption and scattering centers, defects and the index of refraction. Against this background, the idea arose to adopt the principles of this method to inspect the surface layer system of ancient coins with the goal of developing a portable, low-cost measuring instrument for the allocation of mints and/or the identification of different minting techniques on site at excavation sites. As an example, the subject of investigation is three series of Roman coins, each consisting of five antoniniani/radiates of the Gallic Roman Empire (=fifteen coins in total). The Gallic Roman Empire existed from 260 to 274 AD as a separate state 1 and was ruled by emperors from the Roman military and had its capitals in Cologne and Trier. The emperors of the Gallic Roman Empire minted their own coins following the typical Roman minting tradition. One of the emperors was Victorinus, who ruled from 269 to 271 as Imperator Caesar M. Piavonius Victorinus Invictus Augustus 2 . He was a successful general and became emperor in 269. His ruling was accepted in the provinces of Gallia, Germania and Britannia. In 271, he was killed in a private conflict close to Cologne and subsequently was consecrated. Victorinus minted gold coins (mainly aurei) and silver denominations (very few denarii and lots of antoniniani/radiates3). From a typological point of view, the fifteen coins under study in this contribution can unambiguously be classified and attributed to two different mints (called Mint I and Mint II in what follows), assigned with a high degree of probability to Cologne and Trier. Details of the geographical location, the positions of Mint I and Mint II and of the hoards are shown in the maps in Figure 1. Heritage 2025,8, 202 3 of 20 Figure 1. (a) Geographical location of Cologne and Trier in Western Europe. (b) City map of Trier c. 450–800 AD . : Position of Mint I, archaeologically confirmed. : Position of the two hoards FMRD IV, 3/4, 3068.3 and 3068.8 (Petzholdt [ 14 ], p. 37). (c) Map 3: Geographical location of Cologne and Brauweiler. : Position of Mint II, not archaeologically confirmed. : Position of the Brauweiler hoard. Although the antoninianus was claimed to be a silver coin, it, at this point in time, in fact was billon containing a very low degree of silver and consisting mainly of bronze. Therefore, due to weathering and wear, the coins primarily show their corroded bronze core, today. Technically, the coins were minted from pre-fabricated flans. The silver coating was applied by white boiling, a chemical process that brings the few silver elements in the metal to the surface (Klosterkämper [ 15 ]; Zwicky-Sobczyk, Stern [ 16 ], pp. 394–395; Kraft [ 17 ], pp. 17–21). Two dies were used for minting: One die, which was fastened in an anvil, was for the obverse. The other die, which was held by hand, was on the hammer side and produced the reverse impression. Engravers produced the dies manually—as typical for pre-industrial production—so that they should display specific individual characteristics that impacted on the product, the coin. Considering the sensitivity of the laser-based method, it is expected that these characteristics can be distinguished in the detected signal so that it becomes possible to assign coins to their original mint. The lack of detailed text sources on the Gallic Empire underlines the importance and relevance of coins as direct historical testimonies. Thus, as there are almost no mint marks for identification, the question of mint localization is not trivial. By identifying the mints and naming them, a ranking already becomes obvious. The attribution and the judgment as ‘main mint’ have implications for the understanding of the organization of the Gallic Empire, for instance, resulting in the question whether there is a capital near the limes or more inside the Empire (cf. [ 12 ], pp. 26 f.). Circular arguments complicate the matter: by defining one city as the capital of the Empire, one consequently locates the main mint here. Or the other way round: the main place of minting is considered the Empire’s capital. The two different mints were usually distinguished by formal criteria (style, lettering, reverse motives and, most crucial, portrait types) (Bland [ 18 ], p. 65 and p. 70; Besley, Bland [ 19 ], p. 62; Bland, Burnett [20], p. 290 f).4: • Mint I struck radiates with draped and cuirassed busts (type D1: the emperor wears a cuirass covered by a military coat held together with a fibula at the front shoulder). The mint was organized in two workshops (officinae), which were occasionally marked by additional mint marks in the field. In the study, only coins from officina B were included. Heritage 2025,8, 202 4 of 20 • Mint II issued cuirassed portraits which might show a bulge of a mantle at the back shoulder (type B1). The coins were produced by a single workshop (officina). The precise identification of the two minting sites was and is heavily disputed in research (cf. Mairat [ 12 ], pp. 23–50, chapter 2) and the attributions changed or were modified as represented by Figure A1. It is only in recent years that a certain consensus seems to have emerged, based primarily on hoard find analysis. Over a long period, Cologne was seen as the important and main mint of the Gallic Empire, whereas the other, the southern mint,Mint II or subsidiary mint 5 was usually located at Trier 6 . Hoards and archaeological finds found in the areas around these proposed minting places in recent decades now make the opposite view plausible 7 . Decisive are the proportional ratios for the places in question and numismatic objects (copper bars, flans, uncirculated radiates, etc.) from the official Trier mint 8 . Therefore, currently, Trier is regarded as the place of Mint Iand Cologne as place of Mint II (cf. also Table 1). For the study, coins with the reverse type of Aequitas were selected from the Cologne mint. They stem from two different issues (phase I and phase II). Table 1. Allocation changes and modifications for the two minting sites, Mint I and Mint II, respectively. Numbers for Victorinus coins, taken from the tables in Mairat [12], pp. 43 and 44. Hoard Mint I Mint II Brauweiler (near Cologne) 36% 63% FMRD IV no. 3064.3 (Trier) 85% 15% FMRD IV no. 3064.8 (Trier) 88% 12% 2. Materials and Methods 2.1. Roman Coin Series The coin collection of the Archaeological Museum of Münster University holds a significant collection of coins of the emperor Victorinus. In this study, three series of Roman coins, each consisting of five antoniniani/radiates of the Gallic Roman Empire, were characterized by specular laser reflectometry and X-ray fluorescence analysis. In addition, for reference, a European five-cent coin of the 21 st century was inspected, as well, which consists of a steel core with copper coating and was produced in an industrial minting process. Since the die of the 5 cent coin is industrially produced, unlike the Roman coins, this coin can be used as a reference in this study. Details of the fifteen coins are given in Figure A1. The types of the obverses and reverses are classified in accordance with the common reference works from Besley [ 19 ], pp. 73–81, or Mairat [ 12 ], pp. 282–294, as given in Table 2. Table 2. Type classification of the obverses and reverses. Obverse Types B1: Cuirassed bust of the emperor right, radiate D1: Draped and cuirassed bust of emperor right, radiate Reverse Types Aequitas 1: Aequitas (personification of equity) standing l., holding scales in her right hand and cornucopia in her left arm Pax 1: Pax (personification of peace) standing l., holding olive-branch in her right hand and a transverse scepter in her left arm Heritage 2025,8, 202 5 of 20 The photographs of Table 3(taken with a digital camera, type EOS50, Canon Deutschland GmbH, Krefeld, Germany) show an example of one Roman coin from each of the three series. Photographs of all coins can be found in Appendix A. It is rather obvious that the individual coins not only differ in shape, thickness, legend and type, but also in the optical color impression and brightness. We here note that all photographs were taken under the same conditions of lighting and digital camera settings (exposure time, sensitivity, etc.) by the photographer of the Archaeological Museum Robert Dylka. Table 3. Photographs of an example of one Roman coin from each of the three series taken under the same conditions of lighting and digital camera settings. Differences in color and brightness can be concluded from an optical inspection by eye. The inventory numbers accord with the data given in Figure A1. Trier Cologne Phase I Cologne Phase II M 5709 M 5769 M 5775 obverse reverse 2.2. Specular Laser Reflectometry and X-Ray Fluorescence Analysis The coins were investigated by specular laser reflectometry as well as by X-ray fluorescence (XRF) for chemical analysis. The optical beam path for specular laser reflectometry is principally sketched in the image of Figure 2a). A linearly polarized laser beam is reflected at the surface of the Roman coin and the magnitude of the intensity is measured using a photodetector. As illustrated in Figure 2b), the magnitude of the detected signal is characteristic for the macroand microstructure of the surface and the surface layer of the coin as it loses light due to scattering, absorption, interference and defects. We here note that we assume the presence of a three-layer system, i.e., a copper core with silver coating and an oxide layer on the top. The latter may be silver oxide and, thus, is transparent to the incident laser light. By detecting the scattering intensity as a function of angle and light polarization, it becomes possible to deduce information about the layer properties—such as thickness, absorption loss or chemical elements at impurity concentration level—as well as from the metal/layer interface, particularly about the index of refraction and absorption loss. It is assumed that these properties are highly characteristic of the coinage and thus enable the mint to be identified. It is noteworthy to mention that this measuring concept can be transformed into a particularly user-friendly measuring device. As a battery-operated and portable handheld system, it is possible to directly output the origin of the coin or at least one probability value (e.g., on a scale of 1–10) on a display, thus completely eliminating the need for the user to analyze measurement data. For users who are familiar with 3D printing and minicomputers (e.g., from the MAKER movement [ 29 ]), it is even possible to set up the entire technology in a do-it-yourself measuring device themselves. This is mainly due to the small number of Heritage 2025,8, 202 6 of 20 components required and enables multiple systems to be set up. A laser pointer as a light source and a smartphone camera as a detector are sufficient to reduce costs. Figure 3shows the technical realization of the optical beam path in a laboratory setup. Its basic design follows the one introduced by Rischmüller et al. [ 8 ] for the inspection of aluminum surface coatings and was adapted to the optical analysis of Roman coins for the study of the present work. (a) air interference absorption θθ oxide silver copper sca ering (b) Figure 2. (a) Optical beam path for specular laser reflectometry. (b) Sketch of the expected surface layer on the Roman coins and physical phenomena that can be expected from the interaction of coherent laser light with the metal–layer–air system in specular laser reflectometry: (1) copper core, (2) silver layer and (3) oxide layer. M M 60 cm 30 cm P  /2 PH PH  2  in out  0,7 0,3 Figure 3. Experimental setup for investigating Roman coins at a wavelength of 632.8 nm as a function of angle of incidence θ and polarization state. P: polarizer; λ/ 2: waveplate; PH: pinholes; BS: beam splitter; D: Si-photodiode; M: mirrors. Here, we use a low-power helium–neon laser at a wavelength of 632.8 nm (average power P0= 1 mW, laser class 2) as the light source, i.e., only a single laser color for inspection. The wavelength is chosen in accordance with the pronounced reflectivity of copper metals in the red spectral range and because of its high beam quality (low beam Heritage 2025,8, 202 7 of 20 divergence, nearly Gaussian intensity profile) at low costs ( ≪ EUR 200). The laser power is comparable to a laser pointer device to ensure eye safety and is much too small for laser-induced phenomena on the metallic coin surfaces, such as for laser ablation, so that a non-destructive interaction is present. The beam is directed at a large angle of incidence θin to the surface of the Roman coin and the reflected light intensity R of the specular beam is detected using a silicon photodiode. Geometrically, the direction of the specular beam follows Snell’s law, i.e., the angle of incidence equals the angle of reflection θin =θout =θ , so that the detector is positioned at an angle of 2 θ with respect to the direction of the incident beam. The specular reflection becomes affected by light scattering, absorption and interference processes and, thus, R(θ) becomes especially characteristic for each individual Roman coin. The method further makes use of the angular dependence of R(θ) according to Fresnel equations (cf. Bass [ 30 ], Hecht [ 31 ]) and its pronounced dependence on the light polarization. For this purpose, two independent rotation stages (Newport Corporation, Irvine, CA, USA, model M-URM80CC) are employed in a θ− 2 θ configuration to determine the signal R(θ) automatically. The two rotation stages are adjusted with the same axis of rotation in relation to each other, with stage 1 rotating the sample at angle θ and stage 2 moving the detector at an angle of 2 θ . Thus, an angular range of ≈ 10 ◦ up to ≈ 85 ◦ is achieved with angular limitations that are due to spatial restrictions of the mechanical components. The precise control of the angle of incidence ( θ ) and detection angle (2 θ ) is achieved by angular encoders. Along the beam path, the laser beam is directed through a combination of a Glan–Thompson polarizer (P) and a λ/ 2 wave retarder plate in order to adjust the light polarization (either orthogonal or parallel to the plane of incidence, i.e., s - or p -polarization). A beam splitter (BS) is used to control the power of the incident laser beam via detector D2, thus also enabling a compensation for laser fluctuations. The light scattered from the coin passes two pinholes (PHs), positioned on either side of the sample to limit the solid angle of the specularly reflected light. The detected signals for s - and p-polarized laser light, Rs(α)and Rp, are used to determine the reflectometric ratio: ∆R(θ) = Rs(θ) Rp(θ)(1) that serves as a central measure throughout this study. A special feature of the setup is the sample holder that was developed for this study (cf. photograph of Figure 4) and produced using 3D filament printing technology (filament: polylactic acid, PLA) to account for the various shapes and thicknesses of the individual samples at low costs. The holder is designed for a simple exchange of the coins and can also be easily adapted to various sizes, thicknesses and shapes by using a series of 3D-printed, exchangable holders. The laser spot with diameter of ≈ 2 mm was positioned at a flat region of the surface in order to retain the low laser beam divergence in the reflected signal. From a practical viewpoint the measurement protocol comprises the following five steps which can be carried out on site at the excavation location or at the museum: (1) Select an appropriate sample mount; insert and fix the coin; mount the holder onto the rotation stage. (2) Rotate the polarizer in vertical position ( s -polarization) and perform an angular scan to determine Rs(θ). (3) Rotate the polarizer in horizontal position ( p -polarization) and perform an angular scan to determine Rp(θ). (4) Determine and plot the ratio ∆R(θ)according to Equation (1). (5) Unmount the Roman coin and continue from the beginning with the next one. Heritage 2025,8, 202 8 of 20 Figure 4. Photograph of the sample holder with a Roman coin. The incident laser beam is visible by the red light spot in the center of the coin. The 3D-printed holder enables mounting of coins with different sizes, shapes and thicknesses, and can be easily exchanged with a series of different holders. The duration of this measurement cycle for one coin depends on the chosen measurement parameters, particularly on angular resolution, angular positioning time and integration time per angular position. It can thus certainly reach tens of minutes per coin. If realized as handheld tool, however, the duration can be optimized to the inspection of 1–2 coins per minute to enable the measurement of a larger set of coins per hour. Naturally, the measurement process can be further automized up to the case that besides inserting and removal of the coins the resulting reflectometric ratio ∆R is displayed upon pressing a button. The dimension of the laboratory setup depicted in Figure 3measures (width × height × depth) 50 cm × 20 cm × 40 cm at a weight of ≈ 5 kg. For use at the excavation site or in the museum, a transformation into a compact (size of a shoe box) and portable tool is possible. This type of transformation to an automatized out-of-the-lab measurement device was already successfully demonstrated for a very similar setup in Toschke [32] that was used for the study of embossed aluminum small components. Additionally, chemical composition analysis is performed for all coins using a XRF spectromter (type Axios, Malvern Panalytical Ltd., Malvern, UK) equipped with the Omnian SW LTU software package and the Helium Archive for data fitting and quantification. This technique enables the non-destructive elemental analysis of samples by detecting characteristic secondary X-rays emitted from the elements present. The main elements investigated include Ag, Al, Ca, Cl, Cu, Fe, Na, Ni, P, Pb and S. During the measurement, a primary X-ray beam excites the atoms in the sample, causing the emission of elementspecific fluorescence radiation. The helium purge system is employed to reduce absorption effects from air, thereby enhancing the detection of light elements such as Na and Al. 3. Results 3.1. Specular Laser Reflectometry Figure 5shows plots of the reflectometric ratio ∆R (cf. Equation (1)) as a function of angle θ sorted according to the three series Trier (left), Cologne phase I (middle) and Cologne phase II (right). The data represent an angular range of 10 ◦<θ< 87 ◦ and were measured at a wavelength of λ=632.8 nm in all cases. In the first step of the data analysis, it is necessary to limit the size of the data set to a few (here: three) parameters with which characteristic properties of the plots shown are described. As these extracted parameters reduce the plots to essential information, they (potentially) simplify the identification of possible correlations between the parameters and the respective mints. In purely qualitative terms, a comparable signal curve can be seen in all plots: a signal builds-up with increasing measurement angle, passes through Heritage 2025,8, 202 9 of 20 a maximum and disappears for very large measurement angles. This means that each plot can be reduced to three parameters: (1) the maximum signal strength, (2) the angular position of the signal maximum and (3) the angular width between the rise and fall of the signal maximum. The next step is to analyze the data, taking into account the numerical values and units, in order to find and define suitable parameters. For instance, all 15 data plots exhibit a similar overall shape with ∆R≈ 1 at the smallest and largest angles of θ= 10 ◦ and θ≈ 87 ◦ , i.e., beam geometries in which the laser beam strikes the coin surface almost vertically (commonly called normal incidence) and parallel to it (grazing incidence), respectively. In between, ∆R is always larger than unity with a peak position θmax (cf. inset of Figure 5) at ≈ 60 ◦ in all cases. The width of the peak, characterized by the full width at half maximum (FWHM, see inset), is ≈ 10 ◦ . Clear differences between the series are found regarding the peak values ∆R(θmax) ; that particular rise from ≈ 15 (Trier), via ≈ 24 (Cologne phase I) up to ≈ 37 (Cologne phase II), i.e., ∆R(θmax) , is indicated as a promising parameter to identify the provenance of coins. The detailed list of the parameters peak angle θmax ( ◦ ), peak width FWHM(◦) and peak values ∆R(θmax)is given in Table 4. 0 5 10 15 20 25 30 35 40 ∆R Trier Cologne phase I Cologne phase II Angle of Incidence (°)θ 20 30 40 50 60 70 80 20 30 40 50 60 70 80 20 30 40 50 60 70 80 θmax ∆R( max)θ FWHM Figure 5. Reflectometric ratio ∆R according to Equation (1) as a function of angle θ in the range 10 ◦<θ< 87 ◦ at a wavelength of λ= 632.8 nm and sorted according to the three coin series: Trier (left), Cologne phase I (middle) and Cologne phase II (right). The inset (Trier plot) schematically shows the parameters θmax , FWHM and ∆R(θmax) that are determined from the data sets, listed in Table 4and used for further analysis and discussion. Table 4. Parameters for the reflectometric peaks depicted in Figure 5: Peak angle θmax ( ◦ ), peak width FWHM( ◦ ) (full width at half maximum), and peak value ∆R(θmax) for all coins and sorted according to the three series. Errors reflect the precision in the determination of the respective values from the data set. Average values with the respective standard deviations are added in the last line of each series (bold printed values). Mint Sample θmax(◦)FWHM(◦)∆R(θmax) Trier M 5709 56.8 ±0.6 15.9 ±0.8 16.3 ±0.5 M 5711 60.3 ±0.6 21.9 ±0.9 08.5 ±0.4 M 5712 65.5 ±0.6 19.7 ±0.9 10.8 ±0.5 M 5713 56.6 ±0.6 18.1 ±0.9 13.0 ±0.5 M 5714 57.9 ±0.6 19.2 ±0.9 11.3 ±0.5 Average 59.4 ±1.7 19.0 ±1.0 12.0 ±1.3 Heritage 2025,8, 202 16 of 20 that other metals have a suitable coating as it is the case for conversion layers on aluminum, so that an expansion of the study to hydroxide layers of gold or oxidized silver (ancient) coins remains promising. Finally, however, it should be emphasized that all the results shown here are based on the limited series of coins of five samples each and are therefore of a pilot nature. A validation of the obviously strongly statistical methodology with more extensive sample sets is absolutely necessary in the next step, but is very promising. Author Contributions: Conceptualization, Y.T., A.L., K.M., H.-H.N. and M.I.; methodology, Y.T.; software, Y.T.; validation, Y.T.; formal analysis, Y.T.; investigation, Y.T.; resources, K.M., H.-H.N., A.L. and M.I.; data curation, K.M., H.-H.N., A.L. and M.I.; writing—original draft preparation, Y.T., K.M., H.-H.N. and A.L.; writing—review and editing, S.W.-H., E.W., A.L., K.M., H.-H.N. and M.I.; visualization, Y.T., S.W.-H. and E.W.; supervision, A.L. and M.I.; project administration, A.L. and M.I.; funding acquisition, A.L. and M.I. All authors have read and agreed to the published version of the manuscript. Funding: This research is part of the projects Optocubes and quantumexpedition and was funded by the German Federal Ministry of Education and Research (BMBF) and German Federal Ministry of Research, Technology and Space (BMFTR) within the funding programs “Photonics Research Germany” with contract number 13N15229 and 13N15230 and “Quantum aktiv” with contract number 13N16719. The authors are responsible for the contents of this publication. Data Availability Statement: The original data presented in the study are openly available in the OsnaData repository of Osnabrück University at https://doi.org/10.26249/FK2/QO3ASW. Acknowledgments: The authors thank Karsten Kömpe (Inorganic Chemistry Division, Department of Biology/Chemistry of Osnabrück University) for providing X-ray fluorescence spectra and for discussion and Robert Dylka from the Archaelogical Museum for taking the photographs of all coins under study. Conflicts of Interest: The authors declare no conflicts of interest. Heritage 2025,8, 202 17 of 20 Appendix A Inv.-no. Measurements Legend and Type of Obverse & Reverse RIC V,2 Elmer 1941 Beachy Head (= Bland 1979) Brauweiler (= Ziegler 1983) Trier (= Demo 1984) Cunetio (= Besley – Bland 1983) Normanby (= Bland – Burnett 1988) Mairat 2014 RIC V,42 5709 2.18g; 23.0mm; 7h 5711 2.56g; 19.8mm; 12h 5712 2.75g; 19.8mm; 6h 5713 2.81g; 19.6mm; 5g 5714 3.05g; 20.2mm; 7h 5769 3.02g; 20.1mm; 7h 5770 2.87g; 19.6mm; 8h 5772 2.77g; 20.3mm; 1h 5773 2.69g; 21.3mm; 2h 5774 2.85g; 21.9mm; 8h 5775 2.56g; 19.1mm; 7h 5776 3.91g; 21.2mm; 7h 5777 2.65g; 20.7mm; 2h 5778 2.67g; 22.5mm; 8h 5779 2.51g; 20.3mm; 8h p. 172 no. 526 [Trier] … p. 117 no. 573-601 [Cologne] … p. 44 nos. 175-177 [Cologne] … p. 148 no. 2530 (“issue 3, phase 2“ mintI or principal mint] … p. 337 no. 1406 [mint I =principal mint =Trier] … no. 588 (issue 3, phase 2) (c. late 269 – mid 270) [Trier, workshop B] “Mint I” = Trier Obv. IMP C VICTORINVS P F AVG. Victorinus [D1] Rev. PA-X AVG / V - *. Pax1 … no. 118 (AD 269-271) [Cologne] … p. 68 no. 682 (“6. Emission, etwa gegen Ende 269”. [Cologne] p. 67 table 3: “issue III, 1 st phase”; p. 98 nos. 17051788 [ Cologne , officina A] p. 184 no. 565 [Cologne] … p. 72 table 4 and p. 99 no. 2746: “issue I, 3rd phase” [Trier] – … p. 44 nos. 182-183 [Trier] … p. 150 no. 2562 ( mint II or branch mint issue 1c”) p. 184 no. 567 [Cologne] … p. 337 no. 1428 [mint II = Cologne] … no. 641 (issue 1/phase 2, c. mid 269) [Cologne] “Mint II” = Cologne phase I “Mint II” = Cologne phase II … p. 337 no. 1427 [mint II = Cologne] … no. 639 (issue 1/phase 1, c. mid 269) [Cologne] Obv. IMP C PI VICTORINVS P F AVG. Marius [B1] Rv. AEQVITAS AVG. Aequitas 1 … p. 72 table 4 and p. 99 nos. 2743-2745: “issue I, 2 nd phase” [Trier] – – … p. 150 no. †2561 (mint II or branch mint, issue 1b”) Obv. IMP C PI VICTORINVS P F AVG. Victorinus [B1] Rev. AEQVITAS AVG. Aequitas 1 … no. 41 (AD 269-271) “Portrait of Victorinus” [“uncertain or Southern Gallic mint ”] … p. 70 no. 701 (“2. Emission, Ende 268 – Mitte 269 / frühe Phase”) [Trier] … p. 70 no. 702 (“2. Emission, Ende 268 – Mitte 269 / spätere Phase”) [Trier] Figure A1. Tabular listing of the mints under investigation of this study. We note that an important argument for the actual attribution of the main mint I to Trier and mint II to Cologne is the evidence of hoards from Brauweiler and Trier (see Table 1in this article). It is by coincidence that the samples analyzed by optical means (types [ 21 ], nos. 682 and 701–702) are not that clear or even seem to contradict the otherwise obvious conclusion. Heritage 2025,8, 202 18 of 20 Table A1. Photographs of all Roman coins from the three series taken under the same conditions of lighting and digital camera settings. Differences in color and brightness can be concluded from the optical inspection by eye. The inventory numbers accord with the data given in Figure A1. Trier M 5709 M 5711 M 5712 M 5713 M 5714 obverse reverse Cologne Phase I M 5769 M 5770 M 5772 M 5773 M 5774 obverse reverse Cologne Phase II M 5775 M 5776 M 5777 M 5778 M 5779 obverse reverse Heritage 2025,8, 202 19 of 20 Notes 1On the Gallic Roman Empire cf. König [9]; Drinkwater [10]. 2Cf. Kienast et al. [11], p. 237. 3 For the coins cf. Mairat [ 12 ], pp. 82–84, 163–181 and 648–724, and RIC V,4 [ 13 ] pp. 165–194. Radiates are coins with a value of two denarii, also named “antoniniani” after the emperor M. Aurelius Antoninus (Caracalla) who established this new over-rated coin in AD 215. The name “radiates” originates from the rays on the emperor’s head instead of the regular laurel wreath; this formal character usually doubles the value of a coin. 4 Already Elmer [ 21 ], p. 14, describes different styles: “Die Hauptmerkmale für Trier sind: höheres Relief, aufgelockertes Haar, breiterer Schrötling und größere Buchstaben, für Köln: flacheres Relief (seit 267), kleinere und zierlichere Buchstaben und auch kleinerer Schrötling”—variations, that are often hardly recognisable. Only two types of Victorinus’ predecessor Postumus mark Cologne (Colonia Claudia Ara Agrippinensium) as minting place: Webb [ 22 ] (Postumus) no. 286 = Mairat [ 13 ] no. 425 (COL CL AGRIP) and Webb [ 22 ] (Postumus) no. 285 = Mairat [ 13 ] no. 426 (C C A A). The reverse motif of a personification holding scales and cornucopia is subsequently attributed to Cologne. 5Cf. the estimation in Bland [18], p. 78 “the mint officials of Trier, who were always slightly more cut off from the mainstream of political events in the Gallic Empire than their Cologne colleagues”. 6 Introduced by Elmer in [ 21 ], pp. 9, 12 f. Cologne had been a military mint for the Central Empire before. For Trier as the second mint, see ibidem p. 14 following scarce literary and epigraphic evidence. 7 Mairat [ 13 ], pp. 9–19; see also Mairat [ 12 ], esp. pp. 35 f. 42–46. 49 f. or Bland, Besley, Burnett [ 23 ], pp. ix f. and xii f.; Besley, Bland [19], pp. 57 f. already argued for a re-evaluation but “prefer to leave open the question of mint-localization” (p. 58). 8 Mairat [ 12 ], pp. 35 f. Cf. 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