A selection of predominantly Roman glass-working waste and glass fragments was analysed by inductively coupled plasma spectrometry. Some analyses were undertaken of glass fragments adhering to remains of a ceramic tank furnace in which they had been melted. The analyses showed evidence for an exchange of chemical constituents between the clay and the glass. The glass was all of standard soda-lime silica composition. However, there were significant differences in the levels of decolourisers, such as manganese and antimony, which relate to iron contents and suggest some preselection of the raw materials.