Tuesday, July 13, 2010

Farewell, Gordion!

We're wrapping up the 2010 conservation season at Gordion, and saying goodbye to our friends and colleagues. It has been a busy and exciting season here, and we've had a great time. Thanks to everyone who has been reading the blog -- we've enjoyed all of the great feedback!

We'll be back next year, with more updates about life
 and conservation at Gordion! 

Sunday, July 11, 2010

Environmental Monitoring

Environmental monitoring has been an important aspect of the study and maintenance of the Midas Mound Tomb since the 1980s. The wooden tomb is potentially very susceptible to changes in the surrounding environment, and these conditions could be impacted by seasonal changes and tourists visiting the tomb. A thesis on the monitoring and conditions of the tomb was completed by Robyn Haynie, of the University College London, in 2009. The long-term monitoring has given us an excellent understanding of the seasonal changes that occur, and the overall environment of the tomb chamber.

Hygrothermographs were used in the past to monitor the conditions in the tomb chamber, but have now been replaced by electronic dataloggers. These dataloggers monitor the fluctuations in relative humidity and temperature, and are downloaded yearly to check the data. 

Richard Liebhart removes a datalogger placed 
inside the wooden tomb. 

The dataloggers have been placed in various areas around and within the tomb to compare micro-environments in the tomb and larger chamber. There are also tell-tale monitors to keep track of the expansion and contraction of the wooden logs making up the structure of the tomb. These indicate that while seasonal expansion of individual logs is taking place within the tomb, the structure is stable overall.

Emily and Elizabeth change the batteries on the dataloggers.

As the dataloggers are only checked once a year, it is important to ensure that they function properly for the duration of the year. We change the batteries each time, just to be sure that this is the case.  

 Jessie analyzes the downloaded data.

The information is dowloaded, and compared with information from previous years. This enables us to ensure that the conditions are behaving as expected, and that major changes to the environment have not occurred.   

Sunday, July 4, 2010

Treating Bronze Objects

Many bronzes have been recovered in excavations at Gordion. The large amount of bronze material, and the moisture in the burial and storage environments result in corrosion on many of these objects. The application of Benzotriazole (BTA), a corrosion inhibitor, is a common treatment for these materials. This is a less invasive treatment options than those often employed in the past, such as chemical stripping.

The objects requiring treatment were selected in the course of our condition survey because of visible active corrosion. The presence of active corrosion often indicates that environmental conditions are affecting the stability of the objects. Past research has been done at Gordion to examine the conditions within the display cases, including Oddy tests and monitoring with dataloggers. The results of this research indicate that these objects are displayed in relatively stable conditions. It now seems likely that the corrosion could be caused by chemical reactions from previous treatments. There are not written records about many of the early conservation treatments, but personal recollections of past practices confirm that this is a possibility.

The standard procedure for treating bronze objects at Gordion follows these general procedures:

If necessary, objects are desalinated to remove soluble salts and 
chemical residues from previous treatments.  

The objects are mechanically cleaned, using a glass bristle brush and
scalpel, and then brushed with ethanol. 

They are then degreased with acetone and placed into a solution of BTA in 
ethanol. The objects are placed into a vacuum chamber to help increase
 the penetration of the solution.

After removal from this chamber, the objects are coated with dilute B-44 acrylic resin. Here we are coating the larger bronze object after treatment with BTA by filling a plastic bag with B-44 solution. This allows us to evenly coat the surface using a minimal amount of resin.

Recently there has been discussion among conservators about the appropriate procedures and effectiveness of BTA as a treatment for bronze corrosion. Julie Unruh, Gordion conservator-at-large, has been actively researching these issues. Questions include the effectiveness of BTA and AMT (another corrosion inhibitor) in combination, the appropriate length of time for BTA submersion, the efficacy of the vacuum chamber, and the possible substitution of corrosion inhibitors with desiccation and environmental controls. If anyone wants to share their knowledge of the effectiveness of these treatments, we would be very happy to hear your thoughts. 

Friday, July 2, 2010

Examination of Archaeological Glass

Researcher Janet Jones has been working on the identification and classification of the glass assemblage found at Gordion. Among the many examples of glass from the site, there are several examples of relatively complete glass vessels. We spent a day helping her with a technical examination of some of the Roman and Hellenistic mosaic glass examples. One of Janet's goals was to examine the patterns and colorants within the glass to get a better sense of their construction. This is useful when determining whether the pieces are all from a single vessel or if there are actually fragments from multiple objects.

The mosaic fragments in their storage housing

One of the difficulties of identifying glass colors is finding a suitable light source for viewing the object.  To facilitate our examination, we set up a temporary light-box. This was made by supporting the microscope on a glass plate, suspended over a lamp in a plastic crate. This created a way to view the piece in transmitted light while under magnification.

Jessie examines the glass on our improvised light table

The surface of the fragments are heavily weathered, obscuring the original colors of the glass. In some areas, we mechanically removed small portions of the weathering layer to determine if the colors of the weathering layers correspond to the colors of the glass underneath. The appearance of the weathering layers on separate pieces did not always correspond, indicating that fragments might have been subject to different burial environments.  In our investigation, we were able to determine that several of the fragments share a repeating pattern of blue, yellow, and aquamarine, suggesting that they are from the same vessel.

The small fragment at the bottom reveals gold foil sandwiched 
between two colorless layers.

One fragment appears to contain gold sandwiched between layers of colorless glass, which is a known technique in ancient glass manufacture. This bowl is Roman and dates to the first half of the 1st century C.E. The gold was observed several seasons ago by a different group of conservators and Janet was interested in looking further at the other fragments to see if more areas with gold could be identified. We weren't able to identify other areas of gold foil, but this would be a promising area for materials analysis of these glass pieces.

The short strip mosaic fragment

While the construction of the majority of the pieces is similar, there are several pieces made with a short strip technique. While stored together, these mosaic pieces appear to come from an entirely separate vessel and likely date to the Hellenistic period. Stylistic features can be useful in the dating and classification of glass, and the technical study of these pieces will assist Janet in her analysis and interpretation of the Gordion glass assemblage.