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. 

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