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Conservation of copper alloys artefacts from archaeological excavation

Authors:
  • Istituto Centrale per il Restauro

Abstract and Figures

The conservation work on the pilgrim's flask from Casale del Fosso necropolis is a part of more comprehensive study project, which aims to examine issues that are connected to archaeological metals and its conservation problems. From the conservative point of view the main interest is directed toward the testing of new molecules, already developed by the National Research Council, in the stabilization phase of archaeological artefacts. In order To perform this study a group of objects in copper alloy, including the flask, from the necropolis of Veio-Casale del Fosso have been already selected and analyzed
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CONSERVATION OF COPPER ALLOYS ARTEFACTS
FROM ARCHAEOLOGICAL EXCAVATION
M M*, V B*, G G*,
M V*, M P C**,
G M I**, L D***, E G ****
* Istituto Superiore per la Conservazione ed il Restauro, Rome (mauramereu@gmail.com)
** CNR-ISMN, Istituto per lo Studio dei Materiali Nanostrutturati
*** Sapienza, University of Rome
**** University of Catania, Department of Chemistry
Abstract
e conservation work on the pilgrims ask from Casale del Fosso necropolis is a part of more
comprehensive study project, which aims to examine issues that are connected to archaeological
metals and its conservation problems. From the conservative point of view the main interest
is directed toward the testing of new molecules, already developed by the National Research
Council, in the stabilization phase of archaeological artefacts. In order To perform this study a
group of objects in copper alloy, including the ask, from the necropolis of Veio-Casale del Fosso
have been already selected and analyzed
Keywords: Copper alloys, Corrosion, Inhibitors, EDXRF, XRD
INTRODUCTION
All archaeological artefacts as a vehicle of information are a moyen d’accès privilégié à
la conoissance du passé1. In particular, the metallic objects exert a dual role; an intrinsic
historical value and also an evidence of the advanced skills required for their construction.
Every conservation work is aimed at the acquisition of such information that will be used
as a means to provide the long-term preservation of nds and at the same time, their
readability and presentation.
It is not always easy to achieve these objectives. Since at the time of discovery the metal
nds are usually heavily modied due to corrosion: volume, shape, weight and mechanical
properties change during the period of burial, most often are illegible, the contact with the
new environment causes the change of the previous balance and thus the reactivation of
corrosion.
1 Berducou 1990, p. 15.
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At this point there are two main problems: cleaning (exactly the right level of cleaning which
stop) and the stabilization of the nd material about the corrosive processes in progress.
Regarding cleaning, the right level coincides to the identication of the original area, a
concept that has been the focus of several studies. in this context is associated to a physical
structure that can reveal information such as decorations and qualications of surface
processing signs, signs of use, etc.., and therefore must be preserved and studied. e
diculty of this identication is mainly due to non-uniformity of corrosion processes and
the morphology of corrosion products, which can cause lags or even “mixed up” levels
which constitute the original area. All of these problems contribute to make complex the
determination of the layers of corrosion products. Because exists a limit beyond which
you cannot nd the original surface, its exact identication is conducted with reference
to various parameters that are related to: formation of corrosion products (some are oen
located near of the original surface), presence of decorations or surface treatments, contact
between dierent parts of the object or objects nearby. In the cases where recognition of these
indices was dicult, then the location of the area will be established as a hypothesis.
e stabilization of the material, ie those treatments nalized to slow down the natural
process of corrosion is one of the most dicult phases of conservation. Very oen the
archaeological copper alloys have a pronounced electrochemical corrosion caused mainly
by the presence of unstable species (active chlorine). In general, these species are located
in depth, at the interface between the core metal and corrosion products. In environmental
conditions of high humidity these species reactivated the mechanism of degradation. e
state of conservation of artifacts is so irremediably compromised.
e corrosion is manifest in attacks very focused and deep (so-called pitting), whose
main characteristic is just to develop under the corrosion products, penetrating deeply
into the alloy, with typical pinpoint forms (pit) or-and craters appear on the surface. e
treatment options are varied and can work directly on the pitting phenomenon or extract,
as much as possible, chlorides and then apply a treatment of inhibit corrosion. For this
purpose the corrosion inhibitors are used, a group of substances that are theoretically
able to stabilizing the active corrosion process. Some problems were identied for their
use as the impossibility of application on all types of ndings or the inconstancy of their
ecacy and especially the risks to health.
However, in this period there is a wide eld of studies in continuous evolution based on the
problem of the stabilization. In most cases, research are still conned to an experimental
context. Industrial compounds are tested rst on standard samples and only aer on
original nds, with a proposed action protocol. ese experimentations have the objective
of evaluate the eects, interactions and mode of application of the new compounds that
could be corrosion inhibitors. is is the way followed by the laboratories of the Institute
for the Study of Nanostructured Materials of CNR, engaged in the development of new
molecules, that promising good results.
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EXPERIMENTAL
e study was performed at the Superior Institute for the Preservation and Restoration
(Rome) which provided an opportunity to start a project of cooperation between the dierent
institutions this study tried to get the archaeometrical data pertinent to the archaeological
copper alloy selected for the pilot phase. It is a homogeneous group of artifacts, including
the pilgrims ask, dating from the end of the 8th century BC, from the exceptional
burial area of Casale del Fosso (Veio). e conservation interest of these important
documents of the material culture is joined to a study of technology and exchange that
occurs the metallurgical production.
Every conservative approach needs appropriate planning, which includes the choice
of most appropriate interventions as a next step to the documentation and diagnostic
tests. e intervention, dened in relation to product with the objective of provide its
long-term conservation, the readability, the presentation, and allow the acquisition of
information of the object. is implies a large knowledge of the object through the macro
and micro structural characterization.
e macrostructural characterization starts with a detailed examination of the object through
an accurate visual observation. e aim is to identify the manufacture, its function ,its use
through the identication of its technological characteristics (size, shape, traces to decoration,
presence to organic material, traces to work and use, old restorations, completeness of its
archaeological prole) and its conservation status (study of the constituent materials, analysis
of degradation phenomena and the characteristics of corrosion products). e observation
must always accompanied from the record to every detail considered important, through a
detailed photographic and graphics documentation. In this rst analytical step is important
the realization of cleaning tests to identify, through the stratigraphy of the corrosion products,
the original surface and determine the most appropriate cleaning method.
e micro structural characterization provides important information about the materials of
metal archaeological artifacts.
It involves the use of qualitative and quantitative analysis and comparison of data with
those obtained for similar types of artifacts and historical context. In the Italian context, the
archaeo-metallurgic sector is a recent eld of research that had attracted interest and limited
production to specic cultural and historical contexts, one of these is the case of as copper
alloy nds mainly worked to hammer of the early Iron Age that are o of interest of this
research. Important here is the work of Cristiano Iaia who collected all “the available data on
the composition of metal hammer made objects (...) from central Italy (Etruria, Lazio,Marche)”
from “both contexts of the rst Iron Age that early in the second Iron Age, or Orientalising”
adding that “the large chronological gap in the sources and the dierences of methods of
analysis used in these cases certainly inuencing the comparability of data and decrease its the
value.From his studies emerged as the laminated parts are made of a suciently ductile
and malleable alloy that should not lead to breakage either during the hammer working, or
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in use”2, in which the average percentage of tin is 8,6%, a level relatively high and even
higher for the nds from Veio, which however supplied to the league the best characteristics
to be submitted to mechanical stress.
On all selected nds has been carried out the energy dispersive X-ray uorescence (EDXRF)
on three of the six samples have been observed with metallographic microscope while the
other three were submitted to X-ray diraction. All investigations has been conducted at
the Laboratory of Chemistry of Superior Institute for the Conservation and Restoration in
Rome. e nds were initially chosen according to the supposed technique of production and
were divided in two groups, one consisting of artifacts obtained by melting, the other group
composed of artifacts obtained by alternating cycles of mechanical and thermal processing.
e rst group consists of three bulae (T1000) and one ring (T857), the second group,
largest, consist of two spiral bracelets (T1000), one piece of coating lamina, part of a wagon
(T872), six big studs (T872), (one, the n. 3, was submitted to metallography). Finally, the
pilgrim ask from the grave 962.
RESULTS
e Energy Dispersive X-ray Fluorescence (EDXRF) has been performed on 15 objects
described above. e faleres have been analyzed in two dierent sites, since it has
hypothesised a dierent composition between the lamina and the nail in the middle. e
Table 1 reports the results of XRF with qualitative and semi-quantitative characterization
indicated in percentage by weight. We know how the technological advancement of Etruscan
bronze alloy is evident by well-dened composition and depending of the processing
technique. Generally we can found for the laminates a content of Sn around 8%, while for
the melted objects 8-12%. e data shown in the Table 1, for all of the samples, are not
compatible with these standards, certainly distorted by depth of corrosion. erefore, in
order to understand the correct data, the quantitative analysis needs to consider the size
and the extension of the corrosion present on these objects. Considering the percentage
of copper (Cu) present in the bulae made by melting, the amount reached in the case of
No. 36233 equal to 56.1% is very signicant. Instead we can see that the composition of
faleres is comparable to the standard percentage of copper between 80,3 % and 88,8%.
e results of EDXRF analysis (Table 2) performed on samples of the pilgrim’s ask conrm
the presence of a binary copper-tin alloy accompanied by anomalous values of zinc, which
must be attributed to the restoration materials. A control analysis has been even carried out
on the fragment in the back where there are residues of substances attributed to previous
restoration. It show very high values of zinc, accompanied by signicant percentages of
arsenic.
2 Iaia, 2005, p. 24.
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Table 1. Studied ndings
N° Inv. Findings Analysis N° Inv. Findings Analysis
36233
(T 857)
bula
XRF
36498
(T 872)
stud-nail
XRF
37144
(T 1000)
bula
XRF
36498
(T 872)
stud
XRF
37173
(T 1000)
bula
XRF
36497
(T872)
stud
XRF
metallog-
raphy
36230
(T 857)
ring
XRF
36496
(T 872)
stud-nail
XRF
36232
(T857)
stripe
XRF
metallog-
raphy
36495
(T872)
stud-nail
XRF
37150
(T 1000)
bracelet
XRF
36494
(T872)
stud-nail
XRF
37152
(T1000)
bracelet
XRF
metallog-
raphy
36493
(T 872)
stud-nail
XRF
36500
(T 872)
fragment
XRF
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Table 2. EDXRF results
Finds Cu% Sn% Pb% As% Ag% Sb%
36233 (T857) – bula 56.1 34.7 6.2 1.4 0.2 1.3
37144 (T1000) – bula 89.4 8.8 1.5 0.3 0.0 0.0
37173 (T1000) – bula 83.1 15.8 0.8 0.3 0.0 0.0
36230 (T857) – ring 79.8 17.8 1.8 0.5 0.0 0.0
36232 (T857) – stripe (P1) 74.1 25.1 0.6 0.2 0.0 0.0
37150 (T1000) – bracelet 69.3 25.6 5.0 0.0 0.1 0.0
37152 (T1000) – bracelet (P2) 78.8 20.7 0.5 0.0 0.1 0.0
36500 (T872) – fragment 86.1 13.2 0.6 0.1 0.0 0.0
36498a (T872) – stud (nail) 85.9 13.6 0.5 0.0 0.1 0.0
36498 (T872) – stud 80.9 16.4 2.6 0.0 0.1 0.0
36497 (T872) – stud (P3) 82.3 17.2 0.4 0.0 0.1 0.0
36496a (T872) – stud (nail) 81.2 17.2 1.5 0.0 0.1 0.0
36496 (T872) – stud 85.9 13.2 0.9 0.0 0.1 0.0
36495a (T872) – stud (nail) 82.9 15.8 1.2 0.0 0.1 0.0
36495 (T872) – stud 88.8 10.5 0.7 0.0 0.0 0.0
36494a (T872) – stud (nail) 80.6 17.7 1.7 0.0 0.0 0.0
36494 (T872) – stud 81.6 17.1 0.9 0.4 0.0 0.0
36493a (T872) – stud (nail) 80.3 18.1 1.6 0.0 0.0 0.0
36493 (T872) – stud 86.8 12.5 0.5 0.2 0.0 0.0
e analysis of the patinas conducted by X-Ray Diraction (XRD) on a piece of nail
(probably a stud) (Fig. 1), on a fragment of bula (Fig. 2) and on a fragment of the ask
(Fig. 3) showed a uniform condition which demonstrate the presence of cuprite [Cu2O] and
malachite [Cu2CO3(OH)2]. the homogeneity of the patinas is attributed to the environmental
characteristics of the discovery context. e analysis performed on the restoration
material (Fig. 4) of the sample No. 19 showed the presence of copper (II) acetoarsenite
[Cu(C2H3O2)2·3Cu(AsO2)2] (an emerald green articial pigment also known as Paris green,
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which has a very good covering power and mainly used in the restoration in the ‘50s) gypsum,
illite-montmorillonite, calcite, quartz and zincite.
On the two groups of nds have been performed three samples of the alloy that have to be
submitted to metallographic observation. Crystal structures with polygonal grain and traces of
geminate are visible (Fig. 5), which testify that occurred a thermal and mechanical processing.
In addition to information of technology has been possible to determine the state of
conservation of objects, identifying the type and the entity of corrosion. Its possible to
observe the thickness of corrosion products and the size of intergranular and intragranular
corrosion processes (Fig. 6). We know that the rst are particularly insidious and dangerous
because of their invisibility on the surface and also because they may compromise the
mechanical strength of the object. Furthermore this type of corrosion, especially if accompanied
by intragranular corrosion, imposes an adequate intervention to guarantee the preservation of
the manufacture.
e studies carried out in this context provides an excellent opportunity to start and
explore the conservation problems we discussed above and the systematic analysis of the
constituent materials and manufacturing techniques.
e artifacts analyzed in this study are an homogeneous group of copper alloy nds, dating
to the second half of the 8th century BC and coming from a single context, the necropolis of
Casale del Fosso (Veio). is is a wide and important burial area where have been found graves
Fig. 1: XRD on a piece of nail Fig. 2: XRD on a fragment of bula
Fig. 3: XRD on a fragment of ask Fig. 4: XRD of restoration material
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belonging to people of upper society, involved in the management of trade and the movement
of assets (metals, livestock, slaves, wine), who remarked their qualication status through a
military outt and at the same time by exhibition of exotic products like the pilgrims ask.
It represents the most signicant ndings and at the same time the most compromised by
the conservative point of view. For this reason we decided to submit it to a long and complex
restoration work still in progress.
RESTORATION PROJECT AND CONCLUSIONS
e operation become dicult for two reasons: the conservation status of the constituent
materials and the previous restoration, which was damaging for the preservation of
the object. e pilgrim ask can be dates from the end of 8th century BC. It was made by
assembly of several thin sheets of embossed copper alloys. e thickness of the sheets is 0.5-
1.2 mm. e ask has been found in fragments during the excavations of the necropolis of
Veio, in 1915-1916. erefore it has been restored following the mimetic principles. Today
we have two parts of the sides of the ask, a large fragment of the back face full of gaps,
some fragments of the lateral parts and of the neck. All these parts have been pasted on a
circular paperboard that reproduces the original form (Figs. 7-8).
e conservation status of the ask is very critical. e sheet is mineralized and cracked in
several places. On its surface there are a lot of dirt and materials from the previous restoration.
Furthermore, portions of the base are deformed and this caused the detachment of
fragments of the sheet (Figs. 9-10).
e plan for treatment, still in progress, including the removal and restoration of the original
fragments of the ask and creating a new base, it is needed for the preservation and exposure
of the object.
Fig. 6: Phase-contrast image of the thick-
ness of corrosion products of the nd No.
36232 (50X).
Fig. 5: Crystal structure with polygonal
grain, geminate and intergranular corro-
sion of nd No. 37152 (500X).
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e surface was completely cleaned using a scalpel under stereo microscope (Fig. 11). In this
way has been possible to eliminate extraneous material surface, revealing the original color
of the patina, dene the limits of integration and nd the technical expedients used in the
previous restoration. In order to reproduce the embossed point of decoration they used a
small concave hemisphere of lead then covered with gypsum and pigments (Fig. 12).
e component parts of bronze small sheet have been temporarily bonded with thin
strip of Japanese paper soaked by ethanol and placed on the surface of the sheet. e
component parts small sheet of bronze have been temporarily bonded with thin strip of
Japanese paper soaked by ethanol and placed on the surface of the sheet (Fig. 13).
In the next step we would create a cast of the back of the ask in order to make the tilting
of this side and cleaning easier. at that time it was possible to move from its base. Before
doing this, the back surface has been protected by strips of Japanese paper. Two tests have
been performed to seal the protected area and make the cast (Fig. 14). In the rst test was
realized using microcrystalline wax, in the second test we used a very thin layer of wax.
Subsequently, silicone rubber has been poured on the ask in order to obtain an accurate cast
of the decorations.
Fig. 7: Front face. Fig. 8: Rear face.
Fig. 9: Deformation of the base and loss of
some parts of the bronze sheet.
Fig. 10: e detachment of fragments and
highlighting of the base.
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All steps have been documented by digital photos and drawings created by AutoCAD
soware.
is work proves that the analysis performed on ndings before restoration in order to
understand their nature and problems can contribute substantially to the choice of most
appropriate treatment to be made.
Fig. 11: Clearing surface by scalpel. Fig. 12: Technical expedients used in the
previous restoration.
Fig. 13: Fixing the broken points using glued
and Japanes paper.
Fig. 14: Two tests for the creation of a cast.
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