After a couple of months of treating metal objects, we are now learning how to conserve inorganic artefacts. We were recently given the opportunity to conserve waterlogged medieval window glass from Auckland Castle Gardens. The excavation is run by the Durham University Archaeology Department, who are uncovering the medieval remains of the historic residence of the Durham Bishops.
Archaeological window glass does not resemble the clear, transparent glass we use today, but instead looks discoloured, dark and opaque. The burial environment can cause chemical changes to occur within the glass which can give it a somewhat beautiful iridescent quality. Glass which has been waterlogged for centuries is no longer hard but is soft and spongy, making it very delicate and tricky to handle. If the glass is allowed to dry out without any treatment it will become incredibly fragile and crumble at the slightest touch, therefore we need to conserve it as quickly as possible.
First, we cleaned the glass fragments and removed any mud or residues from the surface, taking note of any decoration or surface coatings such as paint. The fragments are then placed in a water/solvent solution. The method we are using involves gradually increasing the concentration of the solvent solution over several weeks until the glass is submerged in 100% solvent. The solvent diffuses into the glass and effectively replaces the water within, which prevents damage from occurring when it is then left to air dry.
Sometimes though if the fragments are too small for handling, like those in the bowl in the photograph before conservation treatment, they can be slowly air dried over a couple weeks in pierced sample bags. Any additional materials such as samples of surrounding earth, any mortar or lead-linings can also be included to air dry as they will not be affected by the process. These can be seen more clearly in the post conservation photograph.
It is only once the glass has dried that the amount of decay within the glass can be realised, and the level of iridescence becomes apparent. Any fragments which are flaky can be preserved by applying a stabilising solution which may need to be brushed on if the pieces are fragile. This helps to prevent crumbling when the fragments are in storage or when they are handled.
The pieces we conserved had small amounts of grooved decoration which can be seen in the photo above. However, most fragments were undecorated, plain sherds, and therefore may seem uninteresting and unimportant. However, a lot can be learnt from these fragments, and future tests may be performed in order to learn about the chemical composition and manufacturing method of the glass, which can inform us on the wider aspects of the site and its cultural significance. Scientific techniques for archaeological analysis are continuously developing and may one day be able to ask new questions about these objects, therefore the preservation of even plain window glass is of great importance.
Kirsty.