Why and When to Perform a Metallographic Examination
-A metallographic examination can provide extremely strong scientific evidence of age, or the lack thereof, as well as revealing the method of manufacture, and the presence of natural or artificially induced corrosion. It should be considered essential in the authentication of certain types of artifacts.
-Once an object has been examined to identify areas of likely restoration or tampering, samples can be removed or, if possible, an in situ examination can be performed. If there is any question of potential tampering, a radiograph is generally recommended.
- It may be desirable to sample some restoration material, to determine if the work was performed in antiquity, or in more modern times.
A metallographic examination can find evidence of age, structural stress and even the methods used to manufacture a metal object. It is often a conclusive tool in identifying metal forgeries, though the information is often combined with a metallurgical analysis.
Advantages
1) A minimally invasive type of examination can be performed in situ on a polished site on some objects.
2) When removal of a metal sample is required, it can often be quite small, and taken from an area where it is not readily noticeable.
3) It can provide extremely strong evidence of advanced age, or the lack thereof, and may even be the only test necessary to identify some forgeries.
4) The cost of the equipment and the subsequent test used can be reasonable, and well within the means of many dealers and collectors.
5) There is a large database for comparison.
6) The samples are not destroyed in testing, and the same sample can be used for certain other types of metallurgical analysis.
Disadvantages
1) It is a destructive test, generally requiring the removal of a sample. To date, no completely non-invasive comparable procedure exists.
2) It requires significant training to correctly interpret some samples, particularly high-quality forgeries.
3) The cost of the test can be moderately expensive, when contracting examination services with some agencies.
4) Preparation of some samples may require toxic or dangerous chemicals.
One of these two bronze bulls is a forgery, and the other is genuine. antiquity.
Both appear quite similar on the surface. Comparison of metallographic samples from each, however, immediately identifies the fake.
A sample from the above fake, shows minimal pitting, corrosion, and no significant internal corrosion.
Above is a bronze sample showing minimal pitting, corrosion, and no significant internal corrosion. Additionally, the arrows point to a copper-phosphide phase in the bronze sample. The presence of this phase was not possible in bronze of the proposed age. Combined with the minimal corrosion, this was conclusive evidence of the use of a modern alloy.
Precious metals can be particularly difficult to examine metallographically, and might offer little to no evidence of their true age. While this is often a problem with high purity silver, some silver alloys can potentially develop discontinuous precipitation in which copper in the alloy migrates toward the grain boundaries. There are several morphologies of this phenomenon, some of which may be related to heating and are, thus, not necessarily indicative of advanced age, although neither are they necessarily indicative of forgery. One form, however, is not known to be inducible by modern science, and its presence is cited as evidence of antiquity.
This is a heavily cast gilt silver bowl, not the more common beaten silver sheet work of the Late Tang/Early Liao period (circa 9th - 10th century) . Rare, historical important and in near mint condition, this premier example warranted multiple examinations.
Pictured above is a micrograph of a sample from the gilded silver bowl with the dragon & phoenix motif. Shown is the discontinuous precipitation of this example. This phenomenon appears as very irregular, wiggly, grain boundaries. In conjunction with a metallurgical analysis, a metallographic examination generally provides evidence of the general age of a metal object.
A rare pair of very large Bronze Fang Hu Form Vessels of the Spring & Autumn Period
(770-481 BCE). These vessels have very rare and pleasing, pierced and crenelated petal-form upper rims. They also feature a very unusual double scale pattern encircling the bronzes in two large bands, with bold, applied zoomorphic handles.
This sample was taken from the casting flash between two tabs on the crenellated top of Hu vessel B. Corrosion has penetrated to a significant depth, with principal attack of the alpha phase. Much of the delta phase has been preserved within the corrosion zone. There is some redeposited elemental copper, which is not found exclusively on the surface. These changes combined can substantiate great age.
