Sunday, October 9, 2011
The Eagle cleaned and examined
Above is a screen grab from Christie's website showing the eagle with the shiny black patina it had when I first saw it and bought it.
Above is a detail of the eagle after cleaning showing the patina, which is of the varied dark and lighter green with reddish undertones that you would expect from an ancient patina. Also visible are the dents and damages to the forward edge of the wing, and you can also see where the wing was joined back to the body. The round hole was done at the time of the repairs I think.
The examination by Susan White took several steps. First was a close microscopic look at the surface to see what the patina is made of. Then she also took it to a facility that has an XRF machine, which uses x-rays to determine the exact composition of the metal. This is useful to determine if something is a pastiche; made up of disparate parts but not belonging to each other, as is possible in a piece like this. Then Susan also took small samples, etched them with acid and examined the slices under a microscope to see if there is intergranular corrosion, where the corrosion products penetrate into the crystalline structure of the metal.
The initial examination under low powered magnification, 1x-7x, revealed a varied patina layer, thin in parts and thick and compacted malachite in others. The thin areas which in places reveal the metal underneath made the piece look as if it does not have age, but the thicker compacted areas indicated an object of some age.
Due to the evidence of reattachment of the wings and tail, Susan arranged to have the piece analyzed by X-ray Flourescence, XRF, which determines the exact composition of the alloys, and would reveal wether this was a pastiche or all of a piece. Several areas were chosen to scan, one on each wing and tail, and another on the body of the eagle. The scans of the different parts confirmed that they all belonged, they were statistically identical, proving they were cast from a single batch of alloy. It also revealed that the eagle is cast not of the usual bronze alloy, which is copper tin, but instead is copper zinc, in other words, brass. The exact alloy revealed by XRF is the following:
Trace amounts of lead were found, no greater than 1.7% in the snake, but averaging about 0.6%.
The iron is unexpected in a modern brass alloy, but the ability to smelt pure zinc was not developed until the Renaissance. The source ores for copper and zinc also contained trace amounts of iron, which got incorporated into the alloy if you cannot refine out the pure metals. Lead was often added to copper alloys to help with the flow of the alloy during casting, so is a perfectly normal element in such an alloy.
Brass is known to have been used in antiquity and in fact, I suspect that many objects described as being of bronze are in fact different alloys, of which copper zinc is one. Some art historians have taken to substituting "coppery alloy" for bronze when describing objects, as there were different alloys used with copper. One alloy in use in very early "bronzes" is copper arsenic, something that sounds positively dangerous to us now.
The metallographic examination consisted of Susan taking three small samples from the piece, one from the back of the left wing, a second from the back of the neck of the eagle, and third from the tail. An effort was made to ensure that the samples were not close to the areas of repair, as the heat from the soldering would alter them. The samples were mounted in epoxy resin and polished very finely to reveal their crystalline structure. After etching with 1:1 ammonia, hydrogen peroxide wash to bring out the crystals, the samples were looked at under high (25x) magnification which clearly showed the massive cuprite on the surface of the sample on the left wing, and the migration of the cuprite into the sample cross section along a grain boundary.
The conclusion of the results of the metallographic analysis suggest an early date to the First Millenium A.D., which would support a Roman date. However, as the nature of science, it can only provide support but cannot prove when it was made.
While the report seemed to bear out my hunch, I still needed to find parallels, if they exist, and overcome the doubt that pervades the field. An example of this; the head of the conservation department of the Metropolitan museum stated that there are no Roman brass cast sculptures, period. I doubt very much that this is true, the Romans certainly used brass, their coins were brass often, and coins were melted down to make sculpture and visa versa. When I go through the Metropolitan Museum's collection, I see many "bronzes" that I bet are in fact brass. Below is photo taken in the study collection of the Greek and Roman wing, of what looks like a brass object. I wonder how many of the Met's bronzes were XRF'ed or otherwise tested for their exact alloy composition. My bet is very few, so how can anyone categorically say that there are no Roman brass cast sculpture?
Above is a fitting from a chariot, with the head of medusa, cast of a bright brassy looking metal, which was striped of its patina, or never developed one. I have not had it analyzed but bet no one else has. I also would guess it is not a standard copper tin alloy, but cannot know that without testing it. I make these observations to illustrate how varied "bronzes" are from antiquity, in composition and patina.
Later posts will look at other factors in attempting to determine the age of my Eagle.