We often find pieces of Romano-British window glass at our Colemore dig site - always fragments and never large pieces. Indeed I understand that no complete panes of window glass from the time have survived anywhere in the UK. We know that pieces of such glass were also found in the villa at Stroud during the 1906 excavation. These are photos I took of window glass finds from Stroud (courtesy of Hampshire Cultural Trust):
This is very typical of the glass that we find at Colemore too: bluish green, translucent rather than intended to be able to see through, full of lots of bubbles and impurities, and with a 'teardrop' curved edge. It is also very smooth on one side - the bottom slightly curved surface as seen above - and rough like sandpaper on the opposite side.
I used to believe - and indeed I have been telling visitors to the Finds tent - that such glass was formed by heating a lump of glass in a furnace and then pouring it into a mould. This was based on my reading of a 1982 book by Susan Frank called 'Glass and Archaeology' in which she said (at page 20):
"The Roman glassmakers did not make only vessel glass: window glass first came into widespread use during this period [~ 200 AD].Pompeii, a city famous for its luxurious style of living, boasted windows glazed with large sheets of glass. The bath house windows, for example, were of thick glass measuring about 40 x 30 inches. Much Roman window glass was of a greenish-blue colour, small pieces being fitted into a more or less richly ornamented wooden frame divided into many sections. It was probably cast as blocks, the hot glass being poured or pressed into flat open clay moulds or even poured out upon flat stones."
This method was also proposed by S. J. Fleming in his 1999 book 'Roman Glass; reflections on cultural change' which is cited in Wikipedia. The article here states:
"The earliest panes were rough cast into a wooden frame on top of a layer of sand or stone but from the late 3rd century onwards window glass was made by the muff process, where a blown cylinder was cut laterally and flattened out to produce a sheet."
However, I have just found an article by Dr Denise Allen, the well-known expert on Roman glass who ran a day school for us on identification of Roman glass last year. Denise was enthusiastic about experimental archaeology relating to Roman glass-making techniques. She found that the mould method of manufacture was impossible. The lump of molten glass cooled and solidified immediately on coming into contact with a mould or other flat surface and could not be spread out.
If you want to know how Roman window glass was probably made, it is well worth reading this article by Denise and looking at this web page by the glassmakers who did the experiment, which has some photos of their results.
It is not clear where the glass was manufactured and relatively few glass-making kilns have been found in Britain, mostly I believe in London. It is thought that most of the vessel glass in Britain was imported from what is now Germany, especially around Cologne. In Rome itself glass bowls, cups, dishes etc became so common that they were considered a sign of relatively low status; the rich used vessels of gold and silver. From 200 AD the glassmakers in Rome were banished to the suburbs because they had become so numerous and their furnaces created so much pollution. However, I suspect window glass must have been made in Britannia and I gather that current thinking is that by the 3rd century, given our climate, most villas of any significance had at least some glass windows.