Consolidating and reconstructing glass objects

* A new post from former Artifact Lab graduate intern Laura Galicier, contributing from a distance in Paris!

Reconstruction of a fish-shaped vessel from ancient Afghanistan (picture from a video of the British Museum, that can be viewed at https://www.britishmuseum.org/channel/exhibitions/2011/afghanistan/video_glass_blowing.aspx

Reconstruction of a fish-shaped vessel from ancient Afghanistan
- picture from a video on the British Museum website, that can be viewed by  following this link

Two glass objects from Cyprus were previously introduced to you. After an initial examination, several treatment steps were decided.

First, the surface showed evidence of delamination and was slightly flaking. We chose to consolidate the surface because if this destructive process went on it could lead to the complete loss of the object. An acrylic resin (Paraloid B72, that you’re now pretty familiar with) in acetone was chosen to do this light stabilization.

Then, we tried to see if the pieces from each object could be built up. We found that while the jug (n.63-1-196) would be able to reconstructed, the fragments of the bowl (n.63-200) didn’t fit together.

On the right: the glass jug    On the left: the glass bowl

The glass jug                                                                         The glass bowl

So the building work for the jug began! We had to find where every fragment was supposed to go. If you read our blogpost about the Egyptian Demotic jar, you’ll realize that building up a glass object is very different. Of course, the size of these glass fragments is considerably smaller than the jar fragments. Besides, the edges of a ceramic are irregular, which can help with reassembly, whereas the edges of glass are smooth.

Glass fragments glued together

Glass fragments glued together

In terms of thickness, a glass object can be very irregular, especially after deterioration, such as delamination of the surface. Generally, you hope that two fragments of similar thickness belong to the same area of the object, but with glass, delamination makes it possible for two fragments of very different thickness to fit together. Moreover, compared to ceramic, glass fragments have a very different way to adjust to each other.

Despite these differences, the methodology to reconstruct glass and ceramic has some similarities: it is necessary to map out the joins so as to know precisely where each fragment goes.

After a bit of work the fragments were put in the order to be joined.

The fragments arranged in the right order

The fragments arranged in the right order

Then, the fragments were temporarily reconstructed using scotch tape. Taping the joins clarifies where each fragment goes and exactly in which order to build them up. This order isn’t always the most obvious but if it isn’t respected, a fragment could prevent another one to fit.

The fragments were built together with scotch tape.

The fragments were built together with scotch tape.

Then, the scotch tape was removed and the fragments were glued with an adhesive (Paraloid B72). Three groups of fragments were reconstructed: fragments of the top, fragments of the bottom and a few fragments that should be placed in-between. The in-between fragments couldn’t be glued to the top or to the bottom because there’s a wide gap between them and the other fragments. This is why it was necessary to make fills so as to support these before going any further.

The three groups of fragments reconstructed and glued together.

The three groups of fragments reconstructed and glued together.

The fills will be explained in a post to come!

 

A new material in the Lab

loadimg.phpWhile 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.

Untitled-1Capture

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.

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 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.

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.

New Picture (3)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.

Untitled-10Here 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.

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!