Early medieval tesserae in northwestern Europe. Long distance trade or local supply?

In northwestern Europe, between the Merovingian and the Carolingian period, a real shift
occurs in craft organization. Artisans left the previous central places and relocated in the
countryside, in the monasteries and in the emporia (Henning 2007, Theuws 2007).
Considering glass production, it is noticeable that this relocation goes hand by hand with
one of main technological modification: the change of flux from soda lime to wood ash
(Wedephol et al. 2011). In that regard, architectural glass will be particularly interesting.
Indeed, if window glass is among the first to experience recipes transformations (Van
Wersch et al. 2016), the tesserae used in the mosaics will long be made with material in
the roman tradition (Neri 2016).
As Byzantine or Italian buildings, early medieval churches north of the Alps were also
decorated with mosaics and small glass cubes have been found in several excavations.
These were assembled to compose walls decorations but they were also a source of
colored glass. In some workshops, they were fused in order to make other objects,
especially window glass as attested by the monk Theophilus (Schibille & Freestone
2013). Written sources also testify for their recycling and their trade around the
Mediterranean as well as between Italy and the Carolingian court (Neri 2016). However,
up until now, no workshop has been found (James 2017). This paper offers to make an
assessment and to draw the role of tesserae in the glass production, especially the one
of architectural glass at the beginning of Middle Ages in northwestern Europe.