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ANCIENT SILVER AND BRONZE METALLURGY ANCIENT SILVER AND BRONZE METALLURGY STUDIES BY MICRO-PIXE AND SEM-EDS D. CRISTEA-STAN,1 P. MEREUTA1, B. CONSTANTINESCU,1 D. CECCATO2 1 Horia Hulubei

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  • ANCIENT SILVER AND BRONZE METALLURGY STUDIES BY MICRO-PIXE AND SEM-EDS

    D. CRISTEA-STAN,1 P. MEREUTA1, B. CONSTANTINESCU,1 D. CECCATO2

    1 Horia Hulubei National Institute for Nuclear Physics and Engineering, P.O. Box MG-6, RO-077125 Bucharest-Magurele, Romania, E-mail: [email protected];

    E-mail: [email protected]; E-mail: [email protected] 2Università di Padova, Dip. Di Fisica G. Galilei and INFN, Laboratori Nazionali di Legnaro, I-35020

    Legnaro (Padova), Italy, E-mail: [email protected]

    Received September 4, 2017

    Abstract. We studied ancient silver and bronze alloy compositional in- homogeneities and the correlation of various elemental components (segregations) in order to draw some conclusions on the metallurgical skills mastered by the issuers of the artifacts. We analyzed Dacian and Roman silver objects – adornments and coins (Ist Century BC – IIIrd Century AD) and various Histria bronze monetary items – warfare arrowheads and arrowhead-shaped monetary signs found in Dobrogea. As analytical methods micro-PIXE at AN2000 accelerator of Legnaro (2 MeV protons beam) and Energy-Dispersive X-ray Spectroscopy (EDS) coupled to Scanning Electron Microscope (SEM) – gave the opportunity to perform a complete characterization of ancient metallic artifacts. Segregations of secondary metal components as copper, lead and manganese were put in evidence.

    Key words: ancient artifacts, silver, bronze, micro-PIXE, SEM-EDS.

    1. INTRODUCTION

    Archaeometry – the use of physical-chemical methods to study ancient artifacts – essentially helps archaeologists to authenticate, to determine the provenance (geological deposits, workshops and commercial relations) and to find the more adequate procedures for restoration-conservation. In the case of metallic items elemental composition of the alloys must be accompanied by a metallurgical investigation of alloys’ micro-structure [1].

    The purpose of this work was to determine ancient silver and bronze alloy compositional in-homogeneities and the correlation of various elemental components (segregations) in order to draw some conclusions on the metallurgical skills mastered by the issuers of the items. We analyzed Dacian and Roman silver objects – adornments and coins (Ist Century BC – IIIrd Century AD) and various Histria bronze monetary items: warfare arrowheads and arrowhead-shaped monetary signs found in Dobrogea [2].

    Romanian Journal of Physics 63, 204 (2018)

  • Article no. 204 D. Cristea-Stan et al. 2

    In the case of silver we studied a so-called “four-metals” (silver-copper-tin- lead) Dacian spiraled bracelet found in North-West of Transylvania (Oradea County) and some silver Roman denarii (coins) – Republican and late Imperial. For the bracelet the question was to understand if silver is alloyed with bronze or separately with copper, tin and lead and, very important, to study elemental segregations of lead, tin and copper in silver.

    The bronze objects were mainly arrow-heads, used as monetary signs by Greeks and local population in Dobrogea (VIIth–VIth Centuries BC), the main metallurgical problem to be solved being the important presence (few percents) of antimony and manganese in their alloys.

    2. EXPERIMENTAL

    After a preliminary investigation of the objects using XRF (X-Ray Fluorescence) method [3], to avoid corrosion-wear influence on items’ surface, two disks from a bracelet fragment and six less “artistic” valuable coins were cut and their section analyzed by micro-PIXE (Proton Induced X-ray Emission) and SEM–EDS (Scanning Electron Microscope with Energy Dispersive X-ray Spectroscopy) as elemental maps. The segregation phenomenon is directly connected to the quality of the metallurgy, e.g. the temperature of alloying and the preliminary hammering, homogeneous materials being obtained at higher temperatures, not always available in those times. Lead, copper and zinc segregations in relation to silver were put in evidence. The presence of manganese and antimony in Dobrogea monetary signs was also put in evidence.

    The samples were measured using 2 MeV proton beam based micro-PIXE method at AN2000 accelerator of Laboratori Nazionali di Legnaro (LNL), INFN, Italy – 6 µm × 6 µm beam area, maximum beam current ~1000 pA [4, 5]. The characteristic X-rays were measured with a Canberra HPGe detector (180 eV FWHM at 5.9 keV). 2.5 mm × 2.5 mm maps and point spectra were acquired. The quantitative analysis was performed using GUPIXWIN software [6]. Considering the penetration depth of 2 MeV protons and the absorption of X-rays the average thickness of analyzed layer was about 50 microns in silver-based alloys and about 60 microns in bronze.

    The results on the silver Dacian bracelet were confirmed by an investigation based on a Scanning Electron Microscope (SEM) Zeiss EVO MA15 [7] with Energy Dispersive X-ray Spectroscopy (EDS) module provided by Thermo Scientific [8]. The settings used in the SEM-EDS analysis were 15 KV accelerating voltage (EHT), 850 pA current (Iprobe) at a working distance (WD) of 10.5 mm. Energy-dispersive X-ray spectroscopy (EDS) was performed using the system to make elemental maps but also point analysis (point ID) on the samples. The EDS system uses a Silicon Drift Detector (SDD), with a resolution of 129 eV FWHM at 5.9 keV. The active surface of the detector is 30 mm2 having a better collection of

  • 3 Ancient silver and bronze metallurgy studies by micro-PIXE and SEM-EDS Article no. 204

    data either over the entire scanned surface or in point ID (for this type of analysis the electron beam is concentrated in a single point on the surface of the sample with a spot size area smaller than 100 nm). The raw grey image is formed using secondary electrons and the elemental maps detecting X-ray emission.

    The scanned area was approximately 1.25 mm2 with the electron beam hitting in the central area of the sample. Considering the formula for calculating

    depth penetration of incident electrons (µm) [9] and the metals

    existent in the alloy (Cu, Pb, Sn, Fe, Ag) the average depth penetration was about 1.2 µm allowing a good characterization of corrosion-wear phenomena. The analysis was performed in high vacuum (HV) and the current of 850 pA used in order to get enough events for a better precision in determining the elemental composition of the samples. The quantitative analysis was performed using the Pathfinder V 1.1 software.

    3. RESULTS AND DISCUSSIONS

    3.1. SILVER CASE

    For silver Dacian metallurgy we illustrate with a spiraled bracelet found in Oradea County, Transylvania. The questions were to understand if silver is alloyed with bronze or separately with copper, tin and lead and, very important, to study elemental segregations of lead, copper and zinc in silver.

    Fig. 1 – Silver bracelet Oradea lateral side – elemental maps.

  • Article no. 204 D. Cristea-Stan et al. 4

    ^0

    ^1

    ^2

    ^3

    2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

    D:\Micro-PIXE DATE ANALIZE\2015\LNLdecembrie2015\PIXE\727092P2._ : Sample 32 map1 point 1 Point : (468.8, 537.1) (um from centre)

    +C uK

    β

    Cu Kα

    Pb Lβ

    A uL

    α

    Pb Lα

    A uL

    β

    S nL

    Pb M

    C ou

    nt s

    Energy (keV)

    Pb Lγ

    Zn K α

    A gK

    α

    S nK

    α +A

    gK β

    S nK

    β

    Ag L

    Silver Dacian bracelet Oradea – point Sn

    Fig. 2 – Silver Dacian bracelet Oradea – Sn point spectrum.

    ^0

    ^1

    ^2

    ^3

    ^4

    2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

    D:\Micro-PIXE DATE ANALIZE\2015\LNLdecembrie2015\LMF\727093P2._ : Sample 32 map1 point 2 Point : (468.8, -459.0) (um from centre)

    Fe K α

    +C uK

    βC uK

    α

    P bL

    β

    S bK

    α

    P bL

    α

    Sn L

    P bM

    Energy (keV)

    P bL

    γ

    Zn K α

    A gK

    α S nK

    α +A

    gK β

    S nK

    β

    Ag L

    Silver Dacian bracelet Oradea – point Pb

    C ou

    nt s

    Fig. 3 – Silver Dacian bracelet Oradea – Pb point spectrum.

  • 5 Ancient silver and bronze metallurgy studies by micro-PIXE and SEM-EDS Article no. 204

    A fragment from this bracelet was analyzed on its lateral side. Elemental maps and spectra show the segregation of lead (localized in direct correlation with silver absence), copper being also slightly segregated; tin is homogeneously distributed in the alloy (Fig. 1). All these aspects suggest a primitive metallurgy.

    ^0

    ^1

    ^2

    ^3

    ^4

    2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

    D:\Micro-PIXE DATE ANALIZE\2015\LNLdecembrie2015\LMF\727094P2._ : Sample 32 map1 point 3 Point : (1093.8, 712.9) (um from centre)

    Fe K α

    +C uK

    βC uK

    α

    P bL

    β

    Sb K α

    P bL

    α

    S bK

    β

    S nL

    P bM

    C ou

    nt s

    Energy (keV)

    P bL

    γ

    Zn Kα

    A gK

    α

    S nK

    α +A

    gK β

    Sn Kβ

    A gL

    Silver Dacian bracelet Oradea – point Cu

    Fig. 4 – Silver Dacian bracelet Oradea – Cu point spectrum.

    Ag Cu

    500µm 500µm

    Zn

    500µm

    Sb

    500µm

    Pb

    500µm

    Sn

    500µm

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