Metals

Metals cannot be tested using conventional absolute dating methods such as carbon-14. Archaeometallurgy is a science that studies the manufacturing processes of metal objects. Combined with the study of alteration mechanisms and surface patinas, it is possible to highlight characteristics of age, in agreement or otherwise with the supposed age of the object.

COMPOSITION OF METALS AND METAL ALLOYS

Nature of the metal (bronze, brass, silver, gold, etc.) is essential to authentication process.

Analyses are carried out using micro-samples studied in microsections, to allow access to the sound metal beneath the corroded surface.

The semi-quantitative results obtained by energy dispersive X-ray analysis (E.D.X) are compared with the laboratory’s technological resources. A database, results of decades of expertise, complements a scientific library (articles, monographs, etc.) that is regularly updated.

Quantitative analysis of the metal with trace element identification can be carried out using PIXE (Particle Induced X-Ray Emission) or LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) method.

Pre-Columbian gold pendant under PIXE laser for study of trace elements. Trace elements in Pre-Columbian gold are indicative of the ores used and of a cultural area.

Study of corrosion processes

Bronzes and copper alloys

Copper alloys are particularly sensitive to corrosion phenomena. It is important to differentiate between natural long-term deterioration of the metal and artificial ageing.

Bronze specific alterations include:

intra- and intergranular corrosion
preferential alteration of a specific phase of the alloy
Alteration of lead globules, deposition of metallic copper and crystallisation of cuprite.
Formation of a mineralised patina
...

Microsection of a metal sample froma « You » vessel dated to the end of the Shang dynasty (1600-1050 BC). Preferential corrosion of the copper-rich alpha phase of the alloy, revealing the as-cast metal’s dendritic microstructure.

Microsection of a metal sample from a « Ding » vessel dating from the early Shang dynasty. Replacement of a large lead globule by red cuprite.

Microsection of a metal sample from a « Ding » vessel dating from the early Shang dynasty. Tin and copper salts-rich mineralised patina on the surface of the metal. The tin-rich delta phase of the alloy is preserved in this surface corrosion layer.

The absence of alteration on the surface of the metal and the presence of an artificial patina (paint, resin, etc.) allow « rough fakes» to be unequivocally identified.

SILVER

Among the corrosion mechanisms of silver, the appearance of cracks caused by oxidation and preferential leaching of impurities into the metal, or sulphides crystallisation, are characteristic of alteration phenomena associated with silver.

Microsection of a metal foil from an Inca silver goblet. Appearance of surface cracks related to segregation, oxidation and preferential leaching of copper and impurities at grain boundaries.

For silver objects, particular attention is paid to the manufacturing processes (hammering, burnishing, rolling).

Silver foil obtained by successive hammering and annealing: grain structure and presence of annealing twins crossing the grains.

GOLD

Gold is a noble metal very few subjected to weathering. The study of degradation processes is possible via corrosion of alloying elements present in the metal.

Dissolution at the grain boundaries, related to dissolution of the least stable elements, or gold depletion on the surface of the object can be demonstrated.

Microsection of a Pre-Columbian tumbaga alloy sample. Dissolution at the grain boundaries (arrows) on the metal surface.

White to red corrosion products (silver chlorides and copper oxides) developed in the vacuoles of a Pre-Columbian tumbaga alloy.

SCIENTIFIC IMAGING

Metallographic studies can be coupled with radiography, revealing the internal structure of the object and the assembly techniques used.

Tube from a Mochica earring revealing the presence of fastening tabs and edge-to-edge soldering.

Radiography can also be used to assess the overall state of conservation of the object, revealing for instance cracks hidden by restorations…

… or a recent reassembly from old metal fragments.

Han horse restored following alteration and fragmentation of the bronze plate. Fragments repositioned and fixed to a brass support.

DATING

Thermoluminescence to date bronzes

Some bronzes still conserve the clay core used when the object was cast.
Thermoluminescence makes it then possible to carry out an age test on the core.

Thermoluminescence curves of a terracotta core from a Ming Dynasty bronze. The average age of the last firing is between 400 and 500 years.