Opaque red glass has been extensively studied over the years, but its compositional complexity and variability means
that the way in which it was manufactured is still not fully understood. Previous studies have suggested the use of
metallurgical by-products in its manufacture, but until now the evidence has been limited. SEM-EDS analysis of glass
beads from the early Anglo-Saxon cemetery complex at Eriswell, southeast England, has provided further insights into
the production and technology of opaque red glass, which could only have been possible through invasive sampling. The
matrix of the red glasses contains angular particles of slag, the main phases of which typically correspond to either
fayalite (Fe2SiO4) or kirschsteinite (CaFeSiO4), orthosilicate (olivine-type) minerals characteristic of some copper- and
iron-smelting slags. This material appears to have been added in part as a reducing agent, to promote the precipitation of
sub-micrometer particles of the colorant phase, copper metal. Its use represents a sophisticated, if empirical,
understanding of materials and can only have resulted through deliberate experimentation with metallurgical by-products
by early glass workers. Slag also seems to have been added as a source of iron to colour ‘black’ glass. The compositions
of the opaque red glasses appear to be strongly paralleled by Merovingian beads from northern Europe and Anglo-Saxon
beads from elsewhere in England, suggesting that this technology is likely to have been quite widespread.
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