Analysis of gold objects

Studies of ancient metals generally focus on the chemical nature of the metal or alloy and the production technique, as well as identifying corrosion processes and analysing surface deposits.  
 
Gold objects are analysed on the basis of solid samples, surface replicas, and elemental chemical analysis by directly scanning the object's surface (PIXE) or, when possible, using LA-ICP-MS



Contributions

Gold is a non-degradable material, so determining its age is a complex matter, usually achieved by indirect observation, focused on the weathering of other elements in an alloy. 
Weathering of a gold alloy generally results in dissolution along the grain boundaries and gold enrichment on the surface. These phenomena are related to the gradual disappearance of less-stable elements (copper, silver, etc.) present in the metal.  
These phenomena may be natural or artificial. Their appearance and the presence or absence of corrosive chemicals provide evidence for distinguishing between different types of weathering.  
 
Identification of the process used to make the object and analysis of the composition of the metal or alloy and any non-metallic inclusions (copper sulphide and oxide, etc.) are compared with archaeometallurgy data to verify that there are no technical anomalies.  
Analysis of the deposits and their interaction with corrosion products on the surface of the alloy make it possible to identify the object's preservation environment and any ancient or recent surface treatments.  



Methodology

The analysis methodology is similar to that used for copper alloys and silver objects. 
 
Stereomicroscope examination of the object is always advisable to determine its structure, identify the production techniques used, and assess its general weathering status. Special attention is paid to the marks left by tools during the production process (swaging, burnishing, and rolling of the metal), which may be significant for assessing the object's age.  
Surface examination may be complemented by X- or gamma-ray radiography.  
 
Chemical analysis by direct scanning of the object's surface (PIXE) is used to determine its precise composition and reveal the presence of trace elements in the gold, which may provide significant indications of its origin. It is also possible to realise a LA-ICP-MS analysis, a more accurate method, with a micro sample of metal
 
Scanning electron microscope (SEM) examination of the surface of a solid sample detects any ancient corrosion phenomena (dissolution along the grain boundaries) and makes it possible to determine the nature of any deposits or corrosion products.  
 
Inverted optical microscopy and SEM-EDX examination of a microsection may reveal a weathering gradient between the metal matrix and the surface (depletion of certain elements, dissolution along the grain boundaries, weathering cracks, etc.), as well as detecting any surface treatments (e.g.: gold or silver enrichment, Pre-Columbian "tumbaga" alloys, etc.). 
This process also verifies the purity of the metal by examining and characterizing any non-metallic inclusions.  



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Although there may be considerable uncertainty with U/Th-He dating, it determines the most recent date the gold was cast and, therefore, when the object was made. This recent technique is still relatively experimental and reliable dating of some gold objects is impossible, generally due to excessive helium in their structure.

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