CIRAM Authentication Of Metal Objects - 2013

CIRAM Analysis – Works of art
Analysis, authentication and dating
of works of art

CIRAM Analysis
Analysis and dating
of cultural heritage

CIRAM Archaeology
Analysis and dating
of archaeological artefacts

CIRAM Track
Safe marking and traceability
of objects

AUTHENTICATION OF METAL OBJECTS:

SCANNING ELECTRON MICROSCOPY AND X-RAY RADIOGRAPHY

Why developing a crossed approach?

ENSCPB 16, Avenue Pey-Berland – Chem’Innov – 33607 Pessac cedex – France
Tel.: +33 5 57 34 21 18 Fax:+33 5 56 23 45 35

www.ciram-art.com

Contacts : Drs Olivier Bobin et Emmanuel Vartanian
Version UK 1.0 mise à jour le 16 février 2009

CIRAM SARL au capital de 21.000 € ENSCPB – Chem’Innov Tél. & Fax : 00.33.(0)5.56.23.45.35
480 825 652 RCS Bordeaux - APE 7112B 16 avenue Pey Berland Portable : 00.33.(0)6.66.63.80.99
SIRET 480 825 652 00021 33607 Pessac Cedex E-mail : contact@ciram-art.com
N° TVA Intracommunautaire FR 56 480 825 652 France Site web : www.ciram-art.com

Version UK 1.0 mise à jour le 16 février 2009 Metals analysis
Page 2 / 17

The analysis’s steps

X-ray radiography

Manufacturing techniques

Corrosion areas

Restoration areas

« Dating » / Antiquity test

Thermoluminescence on the residual cores
(heated mineral)

Preferentially on closed objects
(man, animals…)

Surface analysis

Tool marks : burnishing, polishing...

Superficial deposits: burial sediments, corrosion products…

Surface treatments : build patina, putting in color of gold,
cleaning, acid attack, …

Material analysis

Composition of the alloy

Degree and kind of the corrosion

Origin of the corrosion

From the necessity of having a crossed approach

X-ray Radiography

Thermoluminescence, if possible

Scanning electron microscopy


Page 3 / 17

The X-ray radiography
It allows to evaluate the structural homogeneity of the piece and reveals:

the restoration areas

the corrosion areas

some technological indications

The metal is quite corroded. We We observe the traces of the vent
observe pieces maintained by holes and of the wax feeder tube
metallical quot;plastersquot;. (“tire-cire”),
between the forelegs.

We observe defects of casting
and/or restoration.


Page 4 / 17

The « dating »
THE THERMOLUMINESCENCE
This technique allows obtaining the approximate dating of the last firing of the object.
The analysis is conducted on the heated mineral: the core.

This approach is efficient only on casting objects.

8000

7000
Intensité TL (u.a.)
6000

5000
Nat
Nat + 3,0 Gy
4000
Nat + 6,0 Gy
Bruit de fond
3000

2000 intensité TL entre 250 -400 °C Dose naturelle (Gy)
160000
1000
140000 Intensité TL (u.a.)
0
50 100 150 200 250 120000
300 350 400 450
Température (°C)
100000

80000

ancient 60000

40000

20000
-1,8 Dose (Gy)
0
-3 -2 -1 0 1 2 3 4 5 6 7
Dose Naturelle

8000
Intensité TL (u.a.)
7000
Nat
6000 Nat + 6,0 Gy
Nat + 9,0 Gy
5000 Bruit de fond recent
4000

3000 intensité TL entre 350 -400 °C Dose naturelle (Gy)

20000
2000
18000
1000 Intensité TL (u.a.)
16000

0 14000
50 100 150 200 250 300
12000 350 400 450
Température (°C)
10000

8000

6000

4000

2000 Dose (Gy)
-0,2
0
-2 0 2 4 6 8 10
Dose Naturelle

Limits
The core must not have been exposed to the light or polluted and its origin is sometimes uncertain.


Page 5 / 17

Characteristics of a metal
THE MATERIAL
Generally, it has to do with an alloy. The use of pure metals is relatively rare.

Bronze: copper, tin, lead …

Brass: copper, zinc, tin, lead…

Gold: often alloyed with copper and silver.

Silver: often alloyed with copper, lead…
S

C
Heterogeneous material with different
phases.

ITS DETERIORATION

Secondary corrosion
Clay deposits products

Healthy metal Corrosion products

Corrosion at
the surface


Page 6 / 17

Characteristics of a metal
Why does the metal’s composition constitute the first step of the study?

⇒ Some elements are modernity indicators.

⇒ The corrosion process depend on the nature of the material (silver, gold, …)
or of the alloy (bronze, brass, …).

The chemical
extraction process of
aluminium was
developed in 1827
by Friedrich Wölher !

But the composition of the metal can not be considered as a fundamental proof of authenticity,
the interpretation can depend on the knowledges …

Zn : 37%

before 2003
The ancient calamine technique does not permit to obtain
more than 33% of Zn: modern process developed around
1820 and spread after 1850.

after 2003
Article of J.M. Welter in Techné (n°18, p.27-36):
Ancient brass with 35 to 40% of Zn
Modern making of brass at 40% of Zn with calamine process


Page 7 / 17

The surface analysis
The tools’ marks
Hammering traces, burnishing, polishing…

decor by incisions

…

Manual incisions Traces of mechanical burnishing Manual burnishing

modern work with ⇒ ancient work
modern tools or
⇒ recent work
with an ancient technic
SUPERIFICAL DEPOSITS
They contain elements which correspond to the surface treatments used (traditional or modern).

Acid attack with the ferric Amorphous material constituted Acid attack with aqua regia:
chloride to simulate copper of silver and copper that does smouth surface and
corrosion and burial deposits. not correspond to corrosion microporosities.
products.
⇒ Modern application on a
modern object ⇒ Modern acid attack ⇒ Putting in color of gold
to simulate the corrosion (traditional)
or
⇒ Modern acid attack


Page 8 / 17

The surface analysis
THE CORROSION PRODUCTS
They result from the interaction between the material and its environment.
They come from long and natural process, or from an acid attack…

Green Malachite White lead

blue azurite

blue ultramarine

red
cuprite Green of
Scheele
⇒ Natural products of a ⇒ Application of pigments,
natural or artificial to simulate
corrosion the corrosion products

SURFACE ANALYSIS, IN SUMMARY:
Preliminary study that allows to exclude the obvious fakes:
Modern shaping, ferric chloride, acid attacks, pigments…
Complementary study to cross-section analysis, to characterize decorations
and to observe the whole object.
Study necessary, but not sufficient to determine the age of an object.

As a complement, analysis by PIXE (Particul Induced X-Ray Emission) external beam (identical to AGLAE,
the installation of the Louvres laboratory C2RMF) permits a nondestructive study of the object and gives
a quantification of the major elements, of the minor ones and of the traces (hundred of ppm ; 0,01% = 100
ppm).

CIRAM exclusivity

This technique permits the authentication, since
the ancient alloys contain many traces elements
(Pb, Pt, Sn, Sb, Fe, As, Bi…), whereas modern
metals are almost pure.

Principally powerful for gold or silver objects


Page 9 / 17

The corrosion of metals

Which are the characteristics of the corrosion according to the metal?

What is called quot;patinaquot;?

Is the corrosion of the metal natural or artificial?

The results form a beam of clues that will permit us to give our opinion on
their compatibility or not with the presumed age of the object.

Typical conclusions

These results indicate that the object has not undergone any corrosion from a natural and long
time, subsequent to the manufacturing phase.

These results are inconsistent with the presumed period of the object.

These results indicate that the object is corroded and indicate the damages correspond to a long
and natural weathering subsequent to the manufacturing phase.

These results are consistent with the presumed period of the object.


Page 10 / 17


Page 11 / 17

Copper alloys corrosion
THE CHARACTERISTICS OF THE DETERIORATION – The intern dendritic corrosion
The intern dendritic corrosion corresponds to the preferential decay of the copper rich phase, in comparison to
the tin rich phase which is more resistant.
It develops along the dendrite and penetrates the interior of the material.

The copper rich phase The copper rich phase
is quite corroded is superficially corroded

THE CHARACTERISTICS OF THE DETERIORATION –The tin-enrichment
The preferential corrosion of the copper rich
phase leads to a superficial increasing of tin and
to the formation of green to blue tin oxydes. Sn = 35% wt
NB : in that case
the presence of
phosphorus
indicates a funeral
burial context
instead of an attack
with phosphoric
acid.

Sn = 13% wt


Page 12 / 17

THE CHARACTERISTICS OF THE DETERIORATION – Inter and trans-granular
corrosion
Inter-granular corrosion corresponds to the preferential deterioration of the grain boundaries and leads to a
superficial decohesion.
Trans-granular corrosion develops throughout the different phases.

Inter-granular corrosion Trans-granular corrosion Inter and trans-granular
corrosion

THE CHARACTRISTICS OF THE DETERIORATION – The lead globule corrosion
The corrosion phenomena involve the draining of lead to the surface.
At the same time we observe the corrosion of the lead globules and their gradual replacing by copper oxides.
(cuprite).

Draining of lead Corrosion of lead globules and gradual replacing by copper
to the surface oxides.


Page 13 / 17

Copper alloys deterioration
THE CHARACTERISTICS OF THE CORROSION – The quot;patinaquot;

The quot;patinaquot; corresponds to the corrosion products made by the alloy deterioration:
We detect copper salts: cuprite (red copper oxide), malachite (green carbonate), atacamite (green
chlorides), azurite (blue carbonate) and blue to green sulphates.
We detect blue to green tin oxides.
We also detect silicate deposits associated to corrosion products.

azurite

malachite and green
tin oxides

Red cuprite

quot;healthy metalquot;

quot;Noble patinaquot;

THE CHARACTERISTICS OF CORROSION, in summary :
The intern dendritic corrosion

The superficial tin increasing

The inter and trans-granular corrosion

The lead globule corrosion

The quot;patinaquot;

The depth of the corrosion penetration

Is it necessary to observe all these criteria,
to establish that a metal object is naturally corroded?

Is it possible to reproduce artificially this process?


Page 14 / 17

ARTIFICIAL CORROSION or FAKE PATINA
To accelerate or simulate the corrosion, one have to use surface treatment with strong acids (hydrochloric,
sulphuric, and phosphoric acid...). It is also possible to bury the object during some months or some years.
⇒ Front of attack is linear and quot;smoothquot;. The corrosion does not penetrate deep into the material.
⇒ Cl, S and P elements attest a chemical attack. We only detect copper chlorides as corrosion products.
⇒ There is no interpenetration between the corrosion products and the superficial deposits.

Obvious fake: quot;Intelligentquot; fake: the alloy is corroded, the
The alloy is not corrosion products are natural and their
corroded colour reminds the quot;noble patinaquot;.

Others kinds of alloy corrosion
SILVER ALLOYS
Corrosion phenomena observed on silver alloys are similar to those of copper alloys:
Preferential corrosion of the copper rich phase and intergranular corrosion.
Development of superficial microfissures and microporosities, embrittlement.
Formation of silver salts.
Indications about the manufacturing techniques (hammering, lamination…).

Superficial
porosity and
embrittlement

The micromorphology
multilayered indicates
a lamination phase.

GOLD ALLOYS
Gold is very resistant to corrosion (it does not oxidize). Only copper and silver phases can be corroded. The
authenticity research will principally base on surface studies and on the traces elements (PIXE external beam).


CIRAM Authentication Of Metal Objects - 2013

Recommandé

Recommandé

Contenu connexe

Dernier

Dernier (20)

En vedette

En vedette (20)

CIRAM Authentication Of Metal Objects - 2013