Pseudomorphs on Metal Objects from Ur
A closer look at U.14097 and U.9134 (Penn Museum Nr. 31-17-241 and B17476)
Chisels from PG 1653 and PG 537
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This month’s Blog entry is written by researcher Kyra Kaercher with technical assistance from conservator Tessa De Alarcon. Special thanks to the Conservation Department for the macro photo details of the objects.
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As the Kevorkian Fund Research Assistant working on the Ur Digitization Project, I work with the objects from the excavations and try to match them to Woolley’s records. The past few months I have been focused on the metal tools and weapons, specifically the copper alloys (copper and bronze). When I first started on this project, I noticed the copper alloys fell into two distinct categories. One was reddish-brown, metallic, and had pitting on the surface. The other was blue-green with different textures on the surface. After talking to our conservator, Tessa De Alarcon, I learned the reddish-brown pitted metals are probably caused by an electrolytic reduction treatment of the copper alloys, and the blue-green metals have not been treated with this treatment. I have looked at about 45% of the metals from Ur that are located at the Pennsylvania Museum, and 48% of them were probably electrolytically reduced.
Electrolytic reduction treatment was used in order to protect the copper alloys from corrosion. The treatment uses an electric current run through a ionic solution. This method removes the cuprite, or corrosion caused between the copper alloy and the soil minerals. Sometimes a thin layer of new metal is laid on the surface of the object if the solution is not changed regularly. This makes it hard for more chemical analysis to occur, especially surface analysis such as XRF.
Of the remaining 52% of copper alloy objects examined, 33% have pseudomorphs. Pseudomorphs are a mineral compound that appears in an atypical form. Common pseudomorphs are petrified wood and fossils. In metals, pseudomorphs appear where metal has replaced the organic material that is either part of the object, or that the object was lying on when buried. The pseudomorph retains the appearance and dimensions of the original material. The most common pseudomorphs on the metals from Ur are wood, leather, rope, textiles and basketry. The mineralization of the material sometimes contains enough detail to specify the genus and species of the organic.
The two objects I looked at more in-depth were U.14097 (31-17-241) and U.9134 (B17476). These objects are both chisels found in personal graves. U.14097 was found in PG 1653. It is the only object from this grave, and Woolley publishes neither the object nor the grave. U.9134 is one of several objects from PG 537. This grave included spearheads, daggers, a mace, and a saw. Woolley dated this grave to the Early Dynastic III Period (roughly 2600 – 2400 BCE).
The fiber pseudomorphs that appear on the chisel from PG 1653 can tell us much about the way rope was manufactured and used at Ur. These pseudomorphs are an example of positive replacement where corrosion products replace the fiber as it decays, forming a positive fiber cast. The close-up view shows us a small string that is braided. Most natural fibers are spun as either an S-spin which is to the right, or a Z-weft which is to the left. This fiber is an S-spin in the single threads that combine to form a three string braid. This object was probably laid on or in a coil of the fiber because it is in a random pattern around one end of the chisel. If the fiber was attaching something to the chisel, or wrapped around one end, a pattern would appear. The preservation of the pseudomorphs is to such a degree that the fiber can be further analyzed, and researchers might be able to identify the species of the fiber.
Above are two examples of textile pseudomorphs from the copper chisels. On the left is an example of positive replacement, and on the right is an example of negative replacement. The positive replacement creates a cast of the textile, whereas the negative replacement creates an impression of the textile. In U.14097, the textile is woven, as one can see the individual strands are interwoven with each other. In a better sample, the pseudomorphs may point to a difference between wool and plant fiber (like linen) material. In U.9134, the textile left a very strong vertical impression, and there is some horizontal impressions as well, pointing to a woven textile. These textiles point to the chisels being laid on or near the body probably in association with clothing or they were possibly wrapped in some sort of cloth. The portions with pseudomorphs are not large enough to get a pattern to see if the wrapping was intentional or not.
Above is an example from U.14097 which includes pseudomorphs of basketry or matting. In comparing this specimen with the textiles, we can see that both are woven. However, this is made from a thicker material like reeds rather than fiber. The thicker material points to a plant fiber, which is found in basketry or matting. With the help of a specialist, this plant fiber might be identified down to the species.
Wood pseudomorphs like those found on U.9134 can be determined by the pattern they leave on the metals. The wood pseudomorphs replicate the cell walls of the wood, leaving an exact copy of the structure. The wood species can be identified based on this replica, and changes to the wood and the soil around the wood can also be detected. Like the fiber, textiles, and basketry, it looks as if the chisels were laid on the wood, rather than being embedded in the wood. On objects such as the axes, wood pseudomorphs appear in the sockets, showing the objects were buried hafted to a wooden handle.
One object may have multiple types of pseudomorphs, showing how the object is made or used. The two chisels shown above were laid on different organics that were then preserved on the chisel itself. Pseudomorphs are useful in that they show organics, that have deteriorated, to such a degree they can be identified down to the species level. This gives us the opportunity to learn much more about the use of these objects, as well as the material used to create these objects. Our website, www.ur-online.org, is up and running, but is still a work in progress. It contains many more images of these objects as well as the other objects in PG 537.