The compositional homogeneity of potash lime silica glasses in northern Europe from 12th-17th centuries.

This research investigates the compositional homogeneity of potash-limesilica
glasses from the 12th-17th centuries in Northern Europe, and the significance of
this with respect to compositional studies of archaeological glasses. The variables in
the glass making process that influence the formation of a homogeneous glass are
discussed, and investigated using laboratory replication of beech and bracken ash
glasses. The experimental results are compared to archaeological material from glass
production sites at Blunden's Wood, Knightons, Sidney Wood, and Little Birches in
England, and Hils in Germany.
Backscattered scanning electron microscope (SEM) imaging is used to
qualify the extent of inhomogeneity in both the experimental and archaeological
samples. It is confirmed that visually homogeneous glasses can contain
inhomogeneities that are only visible under backscattered SEM imaging. It is seen
that the size and orientation of inhomogeneities is varied, and specific glass artefact
types (such as crucible and waste glass) are more prone to inhomogeneity than fully
formed glass (such as window and vessel glass). Electron microprobe analysis
(EPMA) is used to quantify the extent of elemental variations present in the
inhomogeneous archaeological glasses. The results show that a number of elements
are significantly influenced by inhomogeneity, including those (such as calcium,
magnesium and sodium) which are commonly used to form compositional groupings
of medieval glass.
It is concluded that although a number of variables in the glass making
process influence the formation of a homogeneous glass, specific variables, such as
increased furnace temperature and a high alkali concentration in the ash, appear to be
the dominating factors. The presence of large elemental variations in a number of
the archaeological glasses analysed confirms that inhomogeneity is a vital
consideration in compositional studies of this material, and that particular care must
be exercised when using analytical techniques that require only a small sample size.