You can tell a lot about a glass vessel from its colour, and even the tiniest sherd can help an archaeologist work out when it was made, what shape it was, and even something about its rarity. In turn, this can help us understand the nature of a settlement and its contact with other sites.
Glass vessels from the Middle Anglo-Saxon period (c. AD 700-900) are usually only found in fragments. Neverthless, the range of colours is spectacular and making them was a complicated art; as well as managing the conditions of the furnace and the molten glass, Saxon glass makers had to carefully balance colourants and decolourants. These vibrant vessels represent a little known, but incredibly colourful technological, cultural and economic achievement. All in all, glass-makers didn’t just bring colour to the Dark Ages – they’re an excellent prism through which archaeologists can understand Anglo-Saxon society itself.
Making clear glass was harder than you’d expect. Iron impurities mean that without intervention, most glass is actually bluey-green. Anglo-Saxon glass workers had to use decolourants, like antimony or manganese, to counteract these impurities. Clear glass seems to have been especially valued for making cone beakers in the 7th century, and then funnel beakers from the mid-8th century onwards. Both were types of unstable drinking vessels that drinkers couldn’t set down on the table until they were empty!
Deep red was usually made by adding copper-rich material to the melt. Deep red is one of the colours most strongly associated with globular beakers, as most of the deep and bright colours are. Globular beakers were smaller and rounder vessels which, unlike cone or funnel beakers, could be set upon a table. They were also used for drinking, but perhaps in different circumstances.
Amber is dependent on the amount of iron present, but also on the creation of a reduction atmosphere inside the furnace (i.e. deliberately creating a very low oxygen environment). A shorter period at high temperature and fewer cycles of remelting the glass also change the final colour. Amber is typical of late 6th and 7th century Anglo Saxon glass, and – just like deep red – is widely associated with globular beakers, but also bowls.
A dash of tin oxides was the usual ingredient for this very unusual colour. Unlike the others, this kind of yellow is always opaque and, rather than making up the vessel’s main body, was much more commonly used for trailed decoration – the technique of winding coloured glass around the vessel as it is made. Only two sherds of glass with yellow body colour have ever been found in England.
Glassware is most commonly found ranging from pale blue to pale green. As striking as they are, these are actually the ‘natural’ colours of glass caused by the presence of iron impurities in the sand used as one of the principal ingredients.
Olive green glass was also probably not deliberately coloured, but the result of impurities in the glass. Intriguingly, this colour was more dominant before and after the Middle Saxon period, when higher quality materials were available.
The best way to make this colour is to add a tiny amount of cobalt. Deep blue is another of the colours most often used for globular beakers, especially the seventh century variety known from princely graves such as Prittlewell, Essex.
Generally-speaking, a large proportion of manganese creates purple glass, whereas as smaller amount works as a decolourant, although furnace conditions and interactions with iron present are also factors. Purple is incredibly rare in England – only two examples are known, but both are claw beaker sherds (a bit like the olive-green example above), which may well have been imported as complete vessels.
If anyone finds another purple sherd, or indeed any other Anglo-Saxon glass sherd, please let me know!
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