The Glass in the Staffordshire Hoard
September 21, 2012
The Anglo-Saxon Hoard found in Ogley Hay in July 2009 is made principally of gold objects, many of which are cloisonné work objects. To date, all the red transparent stones observed in the Staffordshire Hoard are believed to be garnets, as confirmed by the analysis on a number of objects carried out by PIXE at the Louvre Museum in Paris in 2010. However, the inlays in the Staffordshire Hoard were not made of garnet alone. A number of objects feature glass inlays.
Glass is a combination of three components: soda (sodium carbonate Na2CO3) –silica (silicon dioxide SiO2) and lime (calcium oxide CaO). This type of glass is called soda-lime glass, and it has been in use since ancient times.
Soda-lime glass can be of two varieties:
- low in magnesia & potash (potassium oxide) – predominant in Roman times
- rich in magnesia & potash – like much medieval European glass.
The first variety contains natron as the source of soda. The second variety contains wood ash or ash from plants grown in saline soil (halophytes), as the source of soda.
The quality and workability of a glass depends greatly on the ratio of its components (2). Lime serves as a stabiliser and too much or too little of it will make the glass prone to corrosion and soluble in water. A higher quantity of silica makes for good quality glass. Much of the glass from antiquity is high in silica and low in potash and lime, while Medieval European Glass is higher in potash and lime and lower in silica, therefore more unstable (2).
Studies on Anglo-Saxon glass at the British Museum (1) show that glass used in the 5th to mid 6th centuries was of the natron type, very similar to Roman glass and had quite possibly been produced as a raw material in the form of cullet or raw chunks of glass in the Mediterranean and then imported into Britain. From the mid-6th century some Anglo-Saxon glass has been found to contain a quantity of plant or wood ash (1), maybe produced to supply the shortage of good quality natron glass during this period. The composition of glass from 7th to 9th centuries indicates an attempt to extend the glass available in Britain by adding old Roman glass (1), perhaps recycled from buildings in the Mediterranean area. The availability of raw material to North-Western Europe and its influence on glass objects production may have depended on political instability in the Mediterranean affecting trade routes at the time.
Deterioration and appearance
The main factors affecting the preservation of archaeological glass are; physical damage, its composition and the environment (3). Water is one of the major agents of ancient glass deterioration; through a complex chemical reaction water causes the alkali in the glass to leach out. This usually makes the surface weaker, and can even cause it to shrink and flake, producing the rainbow-like effect you can see on ancient glass vessels in museums (3). pH is also a major factor of deterioration: if acidity is allowed to be too high, this can cause the leaching of the silica itself, with total breakdown of the glass surface.
Most of the glass used in the Staffordshire Hoard has survived burial in good condition but some show signs of weathering and deterioration. The Hoard contains examples of transparent coloured glass (red and green), as well as various colours of opaque glass. The original appearance may not always be obvious due to deterioration and scratching of the surface.
Glass in the Hoard
To date, 17 objects have been found to contain glass inlays. The following photographs illustrate the glass inlays found in some of the objects in the Hoard.
Why is the state of preservation of blue glass inlay in the Eagle mount K72 so different from those in the Pyramids K377 & K462?
Why has the green inlay K744 retained much of its transparency while the two repairs on K674 look so dull? K744 appears to have a branching or crazing effect on the surface: is this physical damage or due to a chemical change?
The central blue inlay in K 1226 has suffered much disruption: is this due to physical damage caused by impact or to the weakness of this particular glass?
Further analysis can tell us if there are compositional differences between these inlays and that may help to explain their different levels of preservation. It could also potentially help date the Hoard, as we know that glass composition has varied through history. The chemical composition of some of the glass was analysed together with the garnet inlays using PIXE and the compositional data is currently being interpreted as part of the Staffordshire Hoard research project.
By Deborah L Magnoler
Staffordshire Hoard Conservator
Birmingham Museum & Art Gallery
- Freestone IC , Hughes MJ & Stapleton CP, (accessed 2012) The composition and production of Anglo-Saxon Glass in The Catalogue of Anglo-Saxon Glass in the British Museum , free PDF at http://www.britishmuseum.org/pdf/2%20Composition%20and%20Production%20ed%20bibliog.pdf
- Freestone, I.1991, Looking into Glass, in Science and the Past, (ed. Bowman, S.), 37-57, London: British Museum Press.
- Tennent N H, The Conservation of Glass and Ceramics (1999), Heritage List.