The History of Window Glass Manufacture

The first window glass manufactured in Britain, in Roman times, was broadsheet glass in which an elongated balloon of glass was blown, the ends cut off and the resulting cylinder was split and flattened on an iron plate. Glass produced like this was of very poor quality and at best translucent and due to the small size of the sheets was made into leaded lights.

The production of broadsheet glass declined and by the early 14th century crown glass was being imported from France, though it was not manufactured in Britain until the late 17th century, this being one of the reasons for the very high cost of glass.

Earlier in the 17th century blown plate was manufactured by very laboriously grinding broadsheet glass, which by now could be produced in larger pieces. This was very expensive and so not used much for windows in buildings but mainly for mirrors and carriages.

Crown glass was made by blowing a sphere of molten glass, opening the end opposite the blowpipe while still molten and spinning it out into a circular sheet. While it still contained air bubbles and concentric ripples, the quality of this glass was much better than that of broadsheet, though the size of panes cut from it was still quite limited, so windows were all many-paned. The central pane cut from these "bullions" contained the bulls-eye, the thickened area where the glass was attached to the "punty", the rod used to spin it.

In the late 18th century the manufacture of polished plate glass was introduced into Britain. The process consisted of casting a sheet of glass onto a table and then grinding and polishing it by hand, superseded at the beginning of the 19th century by steam powered machine-grinding and polishing. Large panes of very good quality glass could be produced, but it was a very expensive process, so this was generally only used for the windows of the best rooms in larger houses.

In 1834 an improved cylinder sheet process was introduced from Germany. This was similar to the process for making broadsheet glass, but technological advances meant that much larger sheets of good quality glass could be produced. The withdrawal of duty on glass in 1845 led to a great increase in demand as the price dropped by 75% and this method became the main means of manufacturing window glass until the early years of the 20th century.

A little later, rolled plate obscure glass with a ribbed pattern was manufactured, but by 1888 had been largely placed by machine rolled obscure glass with a variety of patterns and ten years later wired cast glass was invented by Pilkingtons.

Laminated glass, invented in 1903, by incorporating a thin plastic film between two sheets of glass increased the safety and security of much larger windows which could be glazed undivided by glazing bars.

During the 20th century new continuous mass-production techniques developed leading to cheaper ways to produce a more consistently high-quality glass in larger and larger sheet sizes. Most glazing glass today is made using the float process in which molten glass is allowed to float on a bed of molten tin while the upper surface is polished using pressurised nitrogen.

In the late 20th century, as part of the drive to reduce fuel bills by improving the energy efficiency of windows, the double-glazing sealed unit was developed, which in which moisture absorbent spacers are sealed between two panes of glass leaving an insulating air-space. Further reductions in the transmission of heat through these units have been achieved by filling the space with argon and by the use of special coatings or films to reflect infra-red radiation.