ancient silver and bronze metallurgy …ancient silver and bronze metallurgy studies by micro-pixe...

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.


^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 1Point : (468.8, 537.1) (um from centre)

+CuK

β

CuKα

PbLβ

AuL

α

PbLα

AuL

β

SnL

PbM

Cou

nts

Energy (keV)

PbLγ

ZnKα

AgK

α

SnK

α+A

gKβ

SnK

β

AgL

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 2Point : (468.8, -459.0) (um from centre)

FeKα

+CuK

βCuK

α

PbL

β

SbK

α

PbL

α

SnL

PbM

Energy (keV)

PbL

γ

ZnKα

AgK

α SnK

α+A

gKβ

SnK

β

AgL

Silver Dacian bracelet Oradea – point Pb

Cou

nts

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


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 3Point : (1093.8, 712.9) (um from centre)

FeKα

+CuK

βCuK

α

PbL

β

SbKα

PbL

α

SbK

β

SnL

PbM

Cou

nts

Energy (keV)

PbL

γ

ZnKα

AgK

α

SnK

α+A

gKβ

SnKβ

AgL

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

Fig. 5 – Silver Dacian bracelet Oradea (section) – elemental maps.


To study the microstructure of this strange alloy (Ag-Cu-Pb-Sn-Zn), we also performed point spectra (Figs. 2–4). The point spectrum in tin (Fig. 2) revealed the presence of gold (probably a small particle accompanying silver). GUPIXWIN gave the following composition: Ag 55.89%; Sn 24.65%; Cu 4.69%; Pb 1.33%; Zn 0.31%; Sb 0.50%; Au 0.31%; Bi 990 ppm.

The point spectrum in lead (Fig. 3) revealed the relation lead-tin (probably a common mineral) and the presence of relevant traces of zinc and antimony. GUPIXWIN gave the following composition: Ag 10.25%; Sn 36.36%; Pb 33.74%; Cu 3.85%; Zn 0.13%; Sb 0.90%; Au 0.12%; Bi 461 ppm.

The point spectrum in copper (Fig. 4) illustrates all the elements: copper, zinc, lead, silver, tin, and antimony.

The elemental maps obtained for the section of the disk cut from the bracelet are presented in Fig. 5.

Fig. 6 – Silver Dacian bracelet Oradea (section) – map spectrum.

The segregation of lead (localized in direct correlation with silver absence) and copper is very strong, zinc being also slightly segregated. Tin and antimony are homogeneously distributed in the alloy. All these aspects suggest a primitive metallurgy, the knowledge of technological aspects (especially the control of melting temperature) being reduced.

Using SEM-EDS, we analyzed an area on the silver Dacian Bracelet fragment from Oradea, on the side, obtaining secondary electron images and elemental maps (Fig. 7), where we found a Ag-Sn-Pb-Cu-Zn alloy. The segregation


of lead was observed in direct correlation with the absence of silver – see Ag and Pb maps (top left and right). An important presence of lead was detected in the middle of the maps.

Fig. 7 – Photo (by secondary electrons) and elemental maps on silver bracelet Oradea (lateral side)

using SEM-EDS.

Fig. 8 – Spectrum on silver bracelet Oradea (lateral side) using SEM-EDS.


The composition of the analyzed area (weight %) is: O 16.24%, Al 0.62%, Si 0.83%, P 0.24%, Cl 1.09%, Fe 0.45%, Cu 2.74%, Zn 1.33%, Ag 23.15%, Sn 40.34%, Pb 12.70%. The provenance of oxigen is surface silver oxidation. The presence of aluminium, silicon, iron is due to dust particles. Chlorine probably is generated by a typical silver corrosion.

To illustrate the strong in-homogeneities of the alloy we also measured 6 points (Fig. 9 and Table 1).

Fig. 9 – Point spectra on silver bracelet Oradea (lateral side) using SEM-EDS.

Table 1

Elemental composition on measured points

Elements Point 1 Point 2 Point 3 Point 4 Point 5 Point 6 O Al Si Cl Fe Cu Zn Ag Sn Pb Bi

7.41 0.11 0.16

0 0

1.25 0

73.87 13.14

0 0

11.40 0 0 0 0

3.01 0

47.27 37.60

0 0.47

18.38 0.64 0.69 1.63

0 1.95

0 22.21 37.30 16.66

0

15.59 0.42 0.73 0.95 0.54 1.83 1.91

30.00 39.70 8.10

0

16.38 0.54 0.92 1.22

0 3.02 2.00

10.81 56.93 7.86

0

12.18 0 0 0 0

0.65 0

41.36 38.13

0 0


The most spectacular case is the evolution of silver content from 73.87% in point 1 to 10.81% in point 5. Lead is present only in points 3, 4 and 5. The in-homogeneities suggest a separate alloying of silver with lead, copper and tin.

We also investigated silver Roman denarii – Republican and late Imperial. Three less “artistic” valuable items were cut and their section analyzed by micro-PIXE 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.

We illustrate with elemental maps in the case of a Republican denarius (Figs. 10 and 11) a strong segregation of copper practically concentrated in the “core” of the coin. As a consequence, the composition obtained by PIXE from the coin surface is strongly under-evaluated due to copper surface corrosion and to its concentration in the “core”.

Fig. 10 – Silver coin – Republican denarius; Cu-Pb segregation.


Fig. 11 – Silver Republican denarius – map spectrum.

The progress of Roman metallurgy is illustrated by the practical absence of segregation for a late Imperial denarius (Gordianus I) (Figs. 12 and 13).

Fig. 12 – Silver coin Denarius Gordianus I – elemental maps.


Fig. 13 – Silver Imperial Denarius (Gordianus I) – map spectrum.

3.2. BRONZE CASE

The local trade between the Greek colonies (Histria/Dobrogea, Olbia/Southern Ukraine, Apollonia/Bulgaria) and the “barbarian” Scythians, Getae, Thracians got more intense during the 7th–6th centuries BC and was characterized by the use of monetary signs: arrowhead-shaped items were typical [2]. The analyzed by us bronze objects were arrow-heads, their main metallurgical problem being the important presence (few percents) of antimony and manganese in their alloys. Besides the typical copper-tin-(lead) bronze, containing variable amounts of tin (to increase hardness) and lead (to facilitate casting), we also identified two unusual types: Cu-Sn-Mn-Pb and Cu-Sn-Sb-Pb. To explain the presence of antimony as copper minerals component or as intentionally added metal to bronze three less “good looking” arrow-heads were cut and their section analyzed to obtain elemental maps illustrating an eventual segregation of antimony with respect to copper – which was indeed observed. The explanation of manganese presence in the arrow-heads is more complicated. Up to now, the unanimous opinion was manganese can be only superficially present in ancient bronze. The possibility manganese (iron also) from the flux used for smelting copper could become a real bronze “bulk” component must be considered.

Figures 14 and 15 illustrate the case of a section cut in Floriile 11 sample (it is a Cu-Sn-Pb bronze which also contains manganese). The presence of manganese inclusions in the bulk demonstrated Mn is a component of bronze alloy.


Fig. 14 – Floriile sample 11: Cu-Mn segregation maps.

Fig. 15 – Floriile sample 11: Cu-Mn-Pb-Sn case.

The composition of sample 15 Sinoe-Zmeica is more complex (Figs. 16 and 17). Sn-Sb-Pb seem to be well-mixed, nickel is correlated with copper, but anti-


correlated with iron. Very probably iron and nickel are from the flux used to produce the bronze, but a copper mineral containing a nickel can’t be excluded.

FeCu

Pb Sb

500 µm

500 µm500 µm

500 µm

Ni

Sn

500 µm

500 µm

Fig. 16 – Cu-Fe-Sn-Sb-Ni-Pb maps (2.5 mm × 2.5 mm by size), sample Sinoe-Zmeica 15.

Fig. 17 – PIXE spectrum for sample Sinoe-Zmeica 15.

An important aspect to investigate is connected to the origin of the metal, and this can be discussed trying to answer the question about how antimony and


manganese can be present in the alloy. Antimony is a component of poly-metallic ores; its presence in the alloy indicates the use of a secondary sulfide (Fahlerz) ore, which contains copper, arsenic, antimony, and in small quantities also silver, nickel and bismuth. Authors of [10] and [11] connect arsenical and antimonal bronze in Southern Russia with sources in the Southern Caucasian Pyritic copper mines. As such, a plausible hypothesis is to consider these items are related to the Scythian presence in the Northern Black Sea area, Scythians possibly obtaining antimonal bronze by co-smelting Cu minerals with stibnite.

The problem of manganese is even more mysterious and awaits an explanation. Could it be related to some other unknown sources of metal rich in manganese, or some specific technological recipe? An explanation could be the use of manganese oxides as flux necessary to smelt oxidized ores. It is the case of Timna (Sinai) – ores occurring in a highly siliceous gangue must be fluxed with an iron mineral such as hematite or limonite (both often impurified with manganese oxides) [12]. Our hypothesis is a similar type of copper ores smelting in the region of Nikolaev on Dnepr, very rich in manganese minerals – an area known with a significant Scythian presence. The copper minerals from Northern Bulgaria and Serbia (closer to Romania) do not contain manganese or antimony [13], thus these sources are excluded in our case.

4. CONCLUSIONS

Analytical techniques based on quantitative characteristic X-rays measure- ments such as micro-PIXE and Energy-dispersive X-ray spectroscopy (EDS) coupled to electron microscopy give the opportunity to perform a complete characterization of ancient metallic items, for example to investigate metallurgical aspects of silver and bronze alloys. In our cases, segregations of secondary metal components as copper, lead and manganese were put in evidence.

Our results demonstrate the important role of coupled micro-PIXE and SEM-EDS elemental analyses to bring useful insight into the microstructure of archaeological items; especially elemental mapping of the samples helps in the discussion of various hypotheses related to the provenance of alloys and the skills/technological know-how available to the populations living in a historical-geographical area of interest.

Acknowledgements. We gratefully acknowledge funding from the HORIZON 2020 grant agreement EU ENSAR2 – INFN programme. Many thanks go as well to our colleagues G. Talmatchi and F. Constantin for their help.

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ancient silver and bronze metallurgy …ancient silver and bronze metallurgy studies by micro-pixe...

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