CIRAM Authentication Of Metal Objects - 2013

www.ciram-art.com Europe - Phone +33 5 56 23 45 35 – Mobile +33 6 64 14 24 10 - [email protected]

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STUDY OF METAL ALLOYS Bronze, Brass, Iron



The matérial 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…

It’s deterioration

Healthy metal

Corrosion products

Clay deposits

Heterogeneous material with different phases.

C

S

Corrosion at the surface

Secondary corrosion products



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, …).

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%

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

before 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

after 2003

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



The tool’s marks

• Hammering traces, burnishing, polishing…

• décor by incisions • …

Manual incisions Traces of mechanical burnishing Manual burnishing

Superficial deposits They contain elements which correspond to the surface treatments used (traditional or modern).

Acid attack with the ferric chloride to simulate copper corrosion and burial deposits.

⇒ Modern application on a modern object

Amorphous material constituted of silver and copper that does not correspond to corrosion

products.

⇒ Modern acid attack to simulate the corrosion

modern work with modern tools

⇒ ancient work or

⇒ recent work with an ancient technic

Acid attack with aqua regia: smouth surface and

microporosities.

⇒ Putting in color of gold (traditional)

or ⇒ Modern acid attack



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… SURFACE ANALYSIS, IN SUMMARY ü Preliminary study that allows to exclude the obvious fakes: § Modern shaping, ferric chloride, acid attacks, pigments…

ü Complementary study to cross section analysis, to characterize decorations § And to observe the whole object.

ü 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).

⇒ Natural products of a natural or artificial corrosion

⇒ Application of pigments,

to simulate the corrosion products

Blue azurite

Green malachite

Red cuprite

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

CIRAM exclusivity

White lead

blue ultramarine

Green of Scheele



Copper alloys corrosion 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 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.

The copper rich phase is quite corroded

The copper rich phase is superficially corroded

Sn = 35% wt

Sn = 13% wt

NB : in that case the presence of phosphorus indicates a funeral burial context instead of an attack with phosphoric acid.



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. La corrosion des globules de plomb 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).

Inter-granular corrosion

Corrosion trans-granulaire

Corrosion inter-granulaire et trans-granulaire

Draining of lead to the surface

Corrosion of lead globules and gradual replacing by copper oxides.



The “Patina” The "patina" corresponds to the corrosion products made by the alloy deterioration:

• We detect copper salts cuprite (red copper oxide), malachite (green carbonate), atacamite (green chlorites), azurite (blue carbonate) and blue to green sulphates.

• We detect blue to green tin oxides.

• We also detect silicate deposits associated to corrosion products.

The characteristics of corrosion, in summary ü The intern dendritic corrosion ü The superficial tin increasing ü The inter and trans-granular corrosion ü The lead globule corrosion ü The “patina” ü 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?

Red cuprite

malachite and green tin oxides

azurite bleue

« Healthy metal »

"Noble patina"



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 "smooth". 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.

Silver Alloys Corrosion phenomena observed on silver alloys are similar to those of copper alloys: ü Preferential corrosion of the copper rich phase and intergranular corrosion. ü Development of superficial microfissures and microporosites, embrittlement. ü Formation of silver salts. ü Indications about the manufacturing techniques (hammering, lamination…)

Obvious fake: The alloy is not corroded

"Intelligent" fake: the alloy is corroded, the corrosion products are natural and their colour reminds the "noble patina".

Superficial porosity and embrittlement

The micromorphology multilayered indicates

a lamination phase.



In summary… The authentication research uses a beam of clues that permits to define:

Ø The nature of the alloy

Ø The degree of the corrosion

Ø The origin of the corrosion The surface analysis permits to obtain complementary information:

Ø The tools’ marks

Ø The superficial deposits

Ø The decorations Nevertheless, these techniques of analysis have limits:

Ø They are not dating methods.

Ø They only study slow processes: efficient for objects aged of at least 300 years old.

Ø The samplings are limited compared to the sizes of the object. It would be necessary to define, in complement, the degree of homogeneity and deterioration of the object, with X-ray radiography.

Nowadays, new fakes are made and the only microanalysis is not sufficient any more,

because it can conclude the object is ancient whereas it is not.

The homogeneity degree of the object Case #1

The microanalysis shows that the alloy is naturally and amply corroded.

« Ancient » object Modern montage that associates a recent metal and an ancient one.

The ancient metal is located at the level of the base of the object where samplings

are generally made.



Case #°2

The microanalysis indicates the alloy is not corroded and that the patina is artificial. Case # 3

All this results are compatible with the presumed age of the object.

O

O

« Fake » object The sample was made on a restoration area. A second sample (at the base of the object)

showed results compatible with the presumed age.

.

The X-Ray radiography was performed in first step; The sample was made in a chosen area.

The scanning electron microscopy analysis confirms the object is ancient.

We observe technical elements, fissures and restoration areas.

The microanalysis indicates the alloy is naturally and deeply corroded. We observe a noble patina.



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

-0,20

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

-2 0 2 4 6 8 10

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

Intensité TL (u.a.)

Dose (Gy)

Dose Naturelle

0

1000

2000

3000

4000

5000

6000

7000

8000

50 100 150 200 250 300 350 400 450

NatNat + 6,0 GyNat + 9,0 GyBruit de fond


Température (°C)

recent

-1,80

20000

40000

60000

80000

100000

120000

140000

160000

-3 -2 -1 0 1 2 3 4 5 6 7

intensité TL entre 250 -400 °C Dose naturelle (Gy)


Dose (Gy)

Dose Naturelle

0

1000

2000

3000

4000

5000

6000

7000

8000

50 100 150 200 250 300 350 400 450

NatNat + 3,0 GyNat + 6,0 GyBruit de fond


Température (°C)

ancient



In summary…

The authentication research involves the complementarity of the approaches and of the analysis methods:

Characterization of the interne structure and of the homogeneity of the object Analysis of the constitutive material of the object Analysis of the degree and the nature of the corrosion

CIRAM proposes a wide range of analysis permitting to bring the information necessary, to rigorous search of authentication.



To learn more about CIRAM Services To ask for a Study Visit our website www.ciram-art.com Contact us EUROPE Office & Laboratory, Bordeaux, France

Phone / Fax +33 (0)5 56 23 45 35 e-mail [email protected]

NORTH AMERICA Office, New York, USA

Mobile +1 917 509 5616 e-mail [email protected]

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CIRAM Authentication Of Metal Objects - 2013

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Transcript of CIRAM Authentication Of Metal Objects - 2013