Indeed, though glass appears in floor, wall and ceiling mosaics throughout the Roman empire, there are only a handful of examples from Britain – invariably in work of outstanding quality.
In London, interestingly, far more loose glass tesserae have been found south than north of the Thames, though fewer sites have been excavated there.
The 1st to 3rd century sequence at Union Street included the south-west corner of a masonry building, pits, ditches, a box-lined well and a grave. The tesserae were mostly translucent dark or light blue, with smaller numbers of opaque green, turquoise or other colours.
Each tessera typically measures about 10mm and is irregular in shape. Two-thirds came from a gravel or mortar floor, the remainder were found scattered throughout a number of later Roman deposits. Pre-Construct Archaeology also found large numbers of tesserae when digging the nearby site of 33 Union Street (USS03) in 2003.
The absence of any bonded examples, or of traces of mortar backing, suggests that these tesserae were never used. If so, they probably represent manufacturing debris from a mosaicist’s workshop. This is more likely than them being waste glass collected for recycling or for use in enamelling, for example, or bead or jewellery manufacture.
Other significant finds from the dig include chippings from stone tesserae and a few imported marble veneers. This strengthens the suggestion that a workshop specialising in luxury building-materials once existed on, or close to the site.
A version of this article by Ian Blair and Angela Wardle was first published in Archaeology Matters, Southwark Special, Summer 2005
For information see Mosaicists in Work life.