Conservation of Glass
Alex Zappa (Objects Conservation Intern) and Rob Law (Antiquities Technician)
The Antiquities Department is currently undertaking a project to rehouse Greek and Roman glass vessels in its collections. This gives conservators and technicians the opportunity to assess the condition of the collection, to identify objects requiring conservation treatment, and to ensure all objects are stored safely.
Iridescent - Has it always looked like that?
An important part of the care of glass objects in the collection is protecting them from physical harm. But glass can also undergo chemical changes that leave it vulnerable. Glass is primarily composed of silica, an alkali (soda or potash), and a stabilizer (often lime). In archaeological environments the alkali component is easily leached from the glass when it is in contact with water. The loss of the alkali leads to the iridescence often seen on archaeological glass. This process takes place over a long period of time and leaves distinct layers of silica on the surface of the glass. These interfere with the light, creating the shimmering appearance on the objects shown here. The tall ungentarium (GR.GL.21) has iridescence on its interior which appears to have formed as a result of water running down the inside of the neck of the object.
Iridescent layers can easily become detached from the surface of the object (see dish of flakes above) leaving the glass thinner and more fragile. Glass compositions vary depending on where and when they were made, meaning that objects can react differently and suffer different levels of deterioration. This too can be seen in the variety of iridescent appearance on objects in the gallery.
Broken - Do we put it back together?
The more obvious type of damage that occurs to archaeological glass is breakage. When an object is broken, curator and conservator together have to make a decision about whether or not to put the pieces back together. Often, they may choose to reassemble broken vessels as it can be more difficult to store the individual broken pieces in a way which prevents them from coming to further harm. It is also easier to understand what a vessel looked like and how it might have been used once it is reassembled.
However, if it is safe to do so, there can also be advantages leaving an object in pieces. The unguentarium above (GR.1.2011) has been left in two halves allowing observers to see the second tube within the outer layer.
Conservation of GR.GL.21
If it is decided to reassemble a broken glass object, the conservator must make sure that enough of the object remains to allow this to be done safely. If too many pieces are missing, or pieces are missing in key areas, the object might not be structurally stable when put back together . One way of determining this is to reassemble some, or all of the pieces, using low-tack adhesive tape, avoiding areas of iridescence which could be damaged by the tape. This also allows the conservator to determine the order in which the pieces should be glued. The object is then reassembled in the identified order with a conservation-grade adhesive. It is important to note any areas of missing glass which make the object vulnerable to further damage.
On the unguentarium above (GR.GL.21) there were two areas which required additional support (see Figures 2 and 3). This was achieved using a film made from a high quality epoxy resin, which will not deteriorate over time. While liquid, the resin was poured onto a thin sheet of plastic and allowed to set. The resultant film was cut to shape and draped over the area in which it will be used while still flexible or shaped once set using a hair dryer. It was attached with the same adhesive used to reassemble the vessel. The film adds extra support to the identified areas, protecting them in case they are accidently knocked or damaged.
Glass Display and Storage
Ancient glass, as seen throughout our galleries, is usually unproblematic in terms of display. The vessels and jars are mostly free-standing and once placed upon a shelf within a case, need little or nothing in the form of mounts to support or stabilize them. However, due to the nature of ancient glass and the risk of de-lamination of its surfaces, storage of our glass has been more challenging, with the need to reduce handling and subsequent wear or damage being of paramount importance.
Double Unguentarium (GR.1.2011)
This type of blown glass vessel, known as a double unguentarium, was made somewhere in the Eastern Mediterranean between AD 101-300(?) to hold scented oil, perfume or make-up. Glass blowing developed during the first century BC around Syria or Palestine and was used by the Romans to create a diverse range of containers and tableware. In this technique, which is still in use today, molten glass from a crucible in a furnace is gathered on the end of a blow-pipe and is then inflated and shaped.
The double ugentarium is made from two hand-blown round-bottomed glass tubes fused together, onto which has been added trailed molten glass decoration and three looped handles (see Figure 4.1-4). Understanding how this type of vessel is made can be useful when creating support for the object in storage (see Figure 6.)
Making a glass support for storage
As part of the rehousing project, two large cabinets with drawers of varying depth were purchased to store the glass, and individual ‘trays’ were designed and built for each item using Correx (a conservation-grade rigid plastic board) for the base and Plastazote (foamed polyethylene sheets, with some raised ridges) to gently support and hold the glass vessels in place.
Most of the Museum’s Roman glass can fit on one of four different-sized supports. The size of the support is determined by the shape and size of the vessel and whether it needs to be stored upright or horizontally. In the case of the unguentarium illustrated in Figure 6, its rounded bottom means that it is best stored horizontally, taking care to support and protect the looped handles. To protect the fragile surface of many of the glass vessels there needs to be minimal contact between surface and support.
Step 1: Pieces of Plastazote and Correx (see above) are cut to size using a sharp knife and attached to one another using conservation-grade double-sided tape (see Figure 6 a-b). The vessel is placed on top of the foam sheet so that areas which need support can be identified.
Step 2: Pieces of Plastazote are cut to make the supporting structure, their size and shape again determined by the size and shape of the vessel. The unguentarium needed to be raised slightly (Figure 6 c) and prevented from moving from side to side (Figure 6 d). Further support was added to protect the handles. The pieces of Plastazote are held in place by wooden cocktail sticks cut to size using snippers. The vessel is now ready to be placed in storage.
Using this solution we are now able to safely remove any object from any drawer without actually touching its surface. It is also now possible to view any of the glass vessels clearly in situ.