Glass making went through various stages of evolution. The very earliest glass was barely see-through and only small pieces could be made which were fitted into lead cames. The circular “bullions” seen in early glass were a byproduct of the manufacturing process and have been reproduced since to give a cosy fake-old impression in “oldy worldy” pubs etc.
As the technology improved the glass got flatter but was difficult to make into large panes and was still far from perfect. There was a manufacturing process called cylinder glass (c1730-1860) which made glass that was not quite flat so the pane size was restricted for this reason.
The glass making process that made the majority of glazing up to about the mid nineteenth century is known as “Crown” glass (c1690-1850) and it produced the familiar relatively small Georgian panes and slightly rippled and flawed glass that is so characteristic of the period.
In the nineteenth century polished plate glass (c1773-1910) was devised but was initially too expensive for ordinary use so the use of the older technology continued. Then the cost started to fall and window panes got much bigger. People ripped out the glazing bars and fitted plate glass.
Then in the twentieth century people started to admire the earlier glazing patterns and sometimes to reinstate the glazing bars (called astragals in Scotland). Nowadays I think most glass is “float” glass (c1960-present) but glass that mimics the older types is made for conservation work. I think the technology of glass making developed at different rates in different places and it is difficult to find a comprehensive timeline and detailed description of the various processes.
Director, Building Surveyor