While we primarily work on Egyptian materials in the Artifact Lab, we occasionally work on objects from other cultures as well. (http://www.penn.museum/sites/artifactlab/2013/12/21/ch-ch-changes-in-the-artifact-lab/) Recently, two new objects were brought to lab. They are two glass vessels from Cyprus, which were discovered in the archaeological site of Kourion. Their date is unknown.
First of all, what is glass made of? Generally three materials are mixed together:
- A former, being the main component: silica, usually found in sand;
- A flux, lowering the melting point of the glass mixture, the melting point being the temperature at which the glass mixture becomes a liquid (from 1600-1713 Celsius for raw silica alone to 800 Celsius for silica + a flux); this material is an alkali or soda.
- A stabilizer, inserted inside the chemical structure of the glass to strengthen it; usually lime.
- A fourth material, metal oxides, can be added to obtain a specific color (manganese for purple, gold for red, silver for yellow…).
This composition and the percentages of each substance change according to times and places. Moreover, glass can take a wide range of different shapes.
Here is a picture of the objects before treatment:
The two glass objects before treatment.
Both are glass vessels. The vessel on the left was restored in the past; a coating was applied on its whole surface and it was glued with that same substance. This adhesive is now flaking off the object, leaving thin and transparent films. This become more obvious when observed under ultraviolet light.
The object viewed under UV light. The bright white-yellow material is the old adhesive.
The old adhesive is pretty obvious now, with its white-yellowish color. This substance is also soluble in acetone. These properties allowed us to conclude that it is cellulose nitrate, a well-known material used to restore glass objects in the past. In addition to not aging well, this adhesive was applied very thickly on the edges, preventing the fragments from being joined together correctly.
Example of a problematic cellulose nitrate deposit on the
edge of a fragment.
Both glass objects also show evidence of delamination of their surfaces. It takes the form of a white layer, which flakes off the object.
This phenomenon, called delamination, can start in the burial environment especially when the object undergoes weathering. This weathering changes the refractive index of glass as well. Each glass artifact has a specific refractive index, indicating how the light passes through it. According to this, our eye will perceive the object a certain way. Any change in the material, such as delamination, will alter this refractive index and thus our perception of it.
Here is an illustration directly on the object itself:
Delamination of the glass; the delaminated layers are white whereas the ‘glass substrate’ show a brown amber color.
This process, if not stopped, can end up delaminating the whole object, layer by layer, resulting in the loss of this artifact. Conservation treatment, and good environmental controls, can prevent this from happening.
We’ll write more about the treatment of these glass vessels in our next post!