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 Composition

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.

K16. Eye element made of glass

K16. The spherical eye element in this eagle fitting is glass.

 

K72. “Eagle mount”

K72. “Eagle mount”. One of the two small triangular cells at the top contain the remains of a fragmented light blue glass inlay (a detailed image can be seen further on in the blog).

 

K370. Collar from the seax handle

K370. One of the collars from the seax handle.

 

K377. Sword pyramid

K377. One of a pair of sword pyramids with 3 opaque blue glass inlays on each side.

 

K545. Stud

K545. Stud with domed multicoloured (red/brown, blue, white) opaque glass chequerboard.

 

K674. Pommel

K674. Two cells in a pommel filled with red glass to imitate garnet. Note the difference in lustre between these and the surrounding garnets.

 

K744. Transparent green glass inlay

K744. Transparent Green glass inlay on a silver gilt fragment. To date, this is the only green inlay observed in the Hoard.

 

K1166. Sword pyramid

K1166. One of a pair of sword pyramids with multicolour (red/brown, white, blue) glass chequerboard and X-shaped detail in each white square.

 

K1167. Glass chequerboard and zoomorphic cloisonné on a pommel

K1167. Glass chequerboard and zoomorphic cloisonné on a pommel. Unlike that in the pyramids, this chequerboard does not feature X marks in each square.

 

K1226. Rectangular fitting with soda glass inlay

K1226. Rectangular fitting with pale and dark blue chequer board glass inlay.

 

K72 & K462

K72, the remains of a glass inlay (left). K462, blue glass inlays (right)

Why is the state of preservation of blue glass inlay in the Eagle mount K72 so different from those in the Pyramids K377 & K462?

 

K674 & K744

K674, red glass imitating garnets (left). K744, green transparent glass (right).

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?

 

K1226. Disrupted glass inlay

K1226, disrupted glass inlay.

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

References

  1. 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
  2. Freestone, I.1991, Looking into Glass, in Science and the Past, (ed. Bowman, S.), 37-57, London: British Museum Press.
  3. Tennent N H, The Conservation of Glass and Ceramics (1999), Heritage List.

One Response to The Glass in the Staffordshire Hoard

  1. I am a jeweller and a faceter of coloured stones which is an unusual mix of skills. However it is obvious that the makers of the hoard cut and set the garnets in the same workshop as the gold smithing. But as there is glass repair work been done to replace garnets that had fallen out, I suggest that the workshop where it was made was not local to the people that owned the weapons, and the repair work of adding glass may have been done by local glass workers. Do you think this work was made in the same workshop as the finds in Sutton Hoe? Is Lindasfarn a suspected location? As a cutter who has cut many different types of garnet I am interested in how you think these garnets were cut. Dose the surface scratching show linear marks as from a spinning wheel or are they in all directions as it would be if it was cut and polished by hand in a figure of 8 motion. I am wondering, if these garnets have come from Sri Lanka through the Byzantine Empire, were they cut into thin discs for the mosaic industry? Is there any record of Byzantine mosaics having garnets in around this period around the Mediterranean? I have tried googling it but to no avail. I know the mosaics of this period are famous for being glass with gold leaf. How these garnets were cut into thin sheets is fascinating. The UK dose have corundum in the Hebrides islands which would make it possible for the cutting and polishing to be done in the UK but it would make since that the garnets were traded as thinly cut tiles that were then shaped into the inlay pieces. If the garnets were shaped by chipping the edges then were they were made would have small garnet chips but if they were ground then there would be a lot of slurry which would stay in the ground at the location. I would be very interested to hear your thoughts on these matters.
    Steve

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