On the surface silver enrichment of ancient high silver
alloys: an insight of the Portuguese silver coins from
the age of the Discoveries
R.Borgesa,b,c*,R.J.C.Silvaa,M.F.Araújob,A.Candeiasc,L.Alvesb
a
i3N/CENIMAT,DepartmentofMaterialsScience,FacultyofScienceandTechnology,Universidade NOVAdeLisboa,CampusdeCaparica,2829-516Caparica,Portugal
b CentrodeCiênciaseTecnologiasNucleares(C2TN),InstitutoSuperiorTécnico,UniversidadedeLisboa,CampusTecnológicoeNuclear,EstradaNacional10(km139,7),2695-066 BobadelaLRS,Portugal
c LaboratórioHércules,UniversidadedeÉvora,PaláciodoVimioso,LargoMarquêsdeMarialva8,7000-809Évora,Portugal
*corresponding author:[email protected]
Abstract
MethodsandTechniques
CorpusofAnalysis
High silver surface analytical data have been so far regarded as reliable for
Interdisciplinary approach focused on the study of (1) surface composition and
original bulk composition of silver coins which are accordingly referred as
morphology/distribution of existing phases in the near surface silver matrix
being produced from very pure silver alloys. However, the extent in which the
200 coins minted by the sixteenth century
surface elemental composition analytical results are influenced by surface
Portuguese monarchs selected among
silver enrichment has not been described in coins alloys with silver finenesses
those existing in the Lisboa Mint House
greater than the silver content indexed to the maximum value of copper solid
numismatic museum collection (Imprensa
solubility in silver – 91.2% Ag. Microstructural and compositional
Nacional Casa da Moeda, SA), aiming that
characterization of Portuguese 11 dinheiros coins was carried out with
emphasis on the microstructure and what it can reveal about the monetary
process, and on the surface and subsurface composition and its impact on
microstructure, of (2) coin core fineness, of (3) compositional gradients of elements
along the thickness, and to (4) investigate and gather information of the coin
manufacturing process.
•
Elemental composition characterization through determination of majority,
the coin surface presented the lowest
minority and trace elements by Energy Dispersive X-Ray Fluorescence, EDXRF, and
surface alteration, i.e., corrosion or
Proton Induced X-Ray Emission, PIXE, in Campus Tecnológico e Nuclear.
elemental surface analysis. The combination of different methods of analysis,
patina. The coins show generally a good
EDXRF, PIXE, SEM/EDS and LA-ICP-MS, shows that surface analytical
state of preservation with a more or less
Microscopy with Energy Dispersive X-Ray microanalysis, SEM-EDS, in i3N/CENIMAT
techniques do not obtain data very close to the original bulk compositions for
dark patina on the surface with an
and Laboratório Hércules, by X-Ray Difraction, DRX, in i3N/CENIMAT, and by PIXE
these higher silver content coins and should be taken cautiousness together
unknown thickness, featuring different
nuclear microprobe, in Campus Tecnológico e Nuclear.
with historical context and information. It is also highlighted on these higher
•
degrees of engraving wearing.
fineness coins the presence of an important subsurface microstructural layer
depleted of copper-phase which primarily results from intergranular corrosion
during the coin manufacturing process.
•
Metallographic and compositional cross-section examination by Scanning Electron
Investigation of elemental concentration depth profiles between the surface and
A small number of coins were purchased
the bulk of the coins by Laser Ablation Inductively Coupled Plasma Mass
for destructive analysis.
Spectrometry, LA-ICP-MS, in Laboratório Hércules.
www.cenimat.fct.pt
Results
Coincomposition
Surfacesilverenrichment
Ag Kα/Lα ratios determined using the
intensities of EDXRF Ag Kα and Lα
X-ray lines on the anverse and reverse
of a representative group of coins
minted during the sixteenth century
period by D. Manuel I, D. João III and
D. Sebastião I, are very close to the
ratio acquired in the reference alloy
(92.745% wt Ag), not allowing making
any surface silver enrichment
consideration, and not explaining the
up to 98% wt Ag results found.
EDXRF/PIXE composition results are
comparable in all the coins from the
different chronologies and mint
houses, but in large disagreement
with the 11 dinheiros silver fineness
fixed by the known sixteenth
century monetary laws. Deviations
are higher than 4% and up to 7% wt.
On a 92% wt Ag alloy, the information depth for Ag Lα intensity does not exceed a thickness of about 2 µm and 90%
of the Ag Kα intensity comes from a 40 µm thickness layer.
Surfacevs.bulkcomposition
Mintingprocesseffect
Fine dark copper-rich phase in a banded matrix structure,
elongated in perpendicular direction to the thickness
reduction force caused by the hammering work on the metal
during production, before the striking of the coins.
SEM/EDS Ag and Cu compositional lines show an
homogeneous elemental distribution from coin bulk to the
surface with no concentration gradient.
However, PIXE microprobe analysis indicate an important
copper depletion on the near surface region, resulting in
copper composition differences of approximately 50%,
between the surface and the bulk of the coins. The surface
copper depletion originates an overestimation 3 to 5%
higher of the superficial measured silver.
%
Coin Ag
JPV2 Anverse 94.65
Bulk
89.95
RCB4 Reverse 95.98
Bulk
92.94
JC1 Anverse 95.64
Bulk
91.35
n.d.–notdetected
Cu
4.00
8.73
2.44
4.78
4.02
7.99
25 to 40 μm deep subsurface region virtually
free of the presence of copper-rich phase, and
with different copper to silver phase
proportion, when compared to the core of the
coin.
ppm Bullion
Zn Ni Au
Hg Pb
Bi
(%)
n.d. n.d. 7524 n.d. 5834 n.d.
n.d. n.d. 7052 n.d. 6237 n.d. 91.27
284 n.d. 7961 n.d. 7608 n.d.
439 n.d. 10249 n.d. 12008 n.d. 95.18
52 37 165 49 1267 1856
n.d. n.d.
n.d. n.d. 2233 4400 92.01
Microstructure morphology resulting from
multiple hammering, annealing and quenching
steps involved in thickness reduction of the
silver bar after casting.
Depletion of copper on subsurface layer
impacts the Ag fineness obtained by superficial
analytical methods, which get information
from this microstructural arrangement.
LA-ICP-MS subsurface copper profile was found to be similar on the
coins minted in Lisboa (JPV2, JC1) in different reigns and on different
chronological periods of currency emission, indicating a distinctive
effect of the monetary production process and showing a consistency
of the minting executed in this mint house over time. Au, Hg, Pb, Bi,
minor elements have superficial contents different from the internal
layers, which are not representative of the original bulk composition.
Conclusions
•
When analyzing higher silver alloys important metallurgical information may be missed relying only on the
judgment of surface analytical methods, as EDXRF or PIXE, which could incur in potential erroneous interpretations
of the numismatic and the economic history.
•
In higher silver alloys, the minting process induces a subsurface microstructurally modified layer, deprived in
copper phase and with an unknown thickness and elemental compositional gradient, which deep could vary
from 25 to 60 µm.
•
This subsurface copper depleted layer is probably originated from high temperature intergranular copper
corrosion, by formation of copper oxides from surface to the interior, associated to the preferential lixiviation of
the copper phase during minting process.
•
Duetotheunpredictablevariabilityofthissubsurfacemicrostructurearrangement,thecompositionaldifferences
betweenthesurfaceandthecoreofthecoinscanonlybedeterminedbyusingadditionaldestructiveorinvasive
analyticaltechniques.
•
Albeitmoreanalysesareneeded,itseemsthatthePortugueseLisboaminthousehadrigorouslyimplementedthe
11dinheiros standardimposedbythemonetarylaw.
Acknowledgements
Rui Borges acknowledges the FCT grant SFRH/BD/79532/2011. This work is funded by FEDER funds through the COMPETE 2020 Programme and
National Funds throught FCT - Portuguese Foundation for Science and Technology under the project number POCI-01-0145-FEDER-007688, Reference
UID/CTM/50025. Authors are thankful to the Imprensa Nacional Casa da Moeda SA, namely to Maria João Gaiato, for providing access to the study of
the silver coin collection belonging to the Numismatic Museum.