For the sake of precision, it would be more appropriate to speak of tin-rich bronzes, arsenic-rich coppers, or tumbaga (copper-rich gold alloys used in pre-Columbian art).
Metals (especially copper alloys such as bronze) cannot be dated directly. So we have to look for exploitable chronological markers, such as technical clues that can provide indications of the object's authenticity. Our laboratories then interpret the results to authenticate the materials.
CIRAM laboratories, for a global approach to bronze objects
The most relevant approach for metal objects remains the observation and study of the chemical composition of alloys and their degree of corrosion (patina) using optical microscopy and scanning electron microscopy coupled with an elemental analysis system using energy dispersive X-ray spectroscopy (SEM-EDS).
What information is obtained from the chemical analysis of the metal?
The first important piece of information to emerge from microscopy concerns the metal's microstructure. A good understanding of the metal's microstructure provides valuable clues as to how the object was manufactured. For example:
- The presence of dendrites characterizes a melt,
- Flattened, aligned inclusions are evidence of hammering,
- If this alignment is too perfect, it corresponds to a modern laminating process.
Examination of the internal structure can therefore reveal technological clues and, by extension, help to pinpoint the period of creation of the work in question. The same applies to the analysis of alloy composition.
Determining the age of an object by analyzing the composition of an alloy
The study of concentrations of copper, tin, zinc, lead, etc., while not sufficient to determine the age of an object, can be very useful in establishing its modernity.
For example, the presence of aluminum, phosphorus, chromium or manganese (from 0.2 to 0.3%) are formal signs of modernity. Of course, these elements are natural and have always been present on the planet, but their reasoned and deliberate use in the manufacture of metal alloys dates back to the end of the 19th century (and even the beginning of the 20th century for phosphorus).
Indeed, if we refer to the work published by the international scientific community, these elements have never been detected in ancient metals, except in trace amounts (of the order of 0.01%). Furthermore, aluminum cannot correspond to pollution originating from the melt core, for example, as pollution forms inclusions distinct from the metal, and these are not completely dissolved. What's more, aluminum never stands alone in a cast iron core, but is always associated with other elements: silicon in kaolinite; sodium, potassium or calcium in feldspars. So, if you detect 0.5% aluminium in brass, you'd have to detect silicon too, for it to be core pollution. And yet, this is never the case! This proves, if proof were still needed, that the presence of aluminium corresponds to the use of modern metals.
The same applies to phosphorus, manganese and chromium.
Examination for corrosion, or "patina
The final step in characterizing a metal object is to analyze its corrosion.
We usually talk about its patina. While this term implies a surface approach, the study of corrosion focuses in particular on the nature of surface corrosion products (the patina proper), but also on the development of corrosion processes within the alloy.
Why does the study of patina tell us how old an object is?
A copper or silver alloy that is several hundred years old will have been subjected to numerous environmental attacks: humidity, temperature variations, the development of micro-organisms... These elements will lead to the degradation of the metal, its corrosion. Among the most characteristic signs of corrosion of natural origin, developed over several centuries, we can cite the following:
- Deep metal weathering;
- Preferential degradation of copper-rich zones - which are the most fragile;
- Multiple corrosion products (cuprite, azurite, malachite, atacamite, nantokite, tin oxide, etc.), a combination of sediments, or the absence of recurring chlorine or sulfur.
On the other hand, if the corrosion remains very superficial and parallel to the surface of the object, if the metal is attacked homogeneously or if chlorine is detected in all the corrosion products, we can establish that the alteration is artificial and modern. This false patina will therefore indicate that the object has been intentionally degraded, in order to simulate its age.
Finally, it's important to point out that even if an object has been heavily cleaned and its patina removed, microscopic study is still possible. Corrosion processes penetrate the material, leaving traces that even violent abrasion cannot remove.
It is crucial to bear in mind that the study of a metal's chemical composition and degree of weathering provides indirect technical and chronological clues. It is impossible to obtain quantifiable chronological information on metal. Consequently, we cannot tell the difference between an object made 2,000 years ago and one made only 1,000 years ago. These investigations will only enable us to assess the compatibility of the elements studied with the presumed attribution.
CIRAM, a specialist in the dating and authentication of your materials, offers a complete interpretation service. We share our results and discuss their interpretation with you, to explain the relevance of the research, particularly for metal objects.
