Window Glass Evolution
According to Wikipedia, the word window originates from the Old Norse ‘vindauga’, from ‘vindr – wind’ and ‘auga – eye’, i.e., wind eye. The word is first recorded in the early 13th century, and referred to an unglazed hole in a home’s roof or walls. Later, windows were covered with animal hide, cloth, or wood. Over time, windows were built using a variety of materials intended to protect inhabitants from weather elements, while still transmitting some light, including paper, flattened pieces of translucent animal horn, and plates of thinly sliced marble.
The first window glass was manufactured in Britain in Roman times, and was called broadsheet glass. Broadsheet glass was made by first blowing an elongated balloon of glass. The balloon ends were then cut off, and the resulting cylinder was split and flattened on an iron plate. Broadsheet glass was of very poor quality, barely translucent, and could be made only in small sizes.
By the early 14th century, crown glass, which was far superior to broadsheet glass, was being made in France. 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. Although crown glass still contained air bubbles and concentric ripples, the quality of this glass was much superior to that of broadsheet glass. The size of individual panes was still quite limited, so all windows were multi-paned.
Early in the 17th century, blown plate glass was manufactured by grinding broadsheet glass, which by now could be produced in larger pieces. This was an expensive and time consuming process, so blown plate glass was used mainly for mirrors and windows in carriages.
In the late 18th century, the manufacture of polished plate glass was introduced in Britain. The process consisted of casting a sheet of glass onto a table and then grinding and polishing it by hand. At the beginning of the 19th century, the process was improved with the introduction of steam powered machine-grinding and polishing. Large panes of very good quality glass could be produced, but it was a very expensive process, so polished plate glass was generally used only for the windows of the best rooms in larger houses.
Laminated glass was invented in 1903, by incorporating a thin plastic film between two sheets of glass. Laminated glass increased the safety and security of much larger windows, which could now be glazed undivided by glazing bars.
During the 20th century new mass-production techniques were developed, leading to a less expensive way to produce a consistently high-quality glass in larger and larger sheet sizes. Most window 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 pressurized nitrogen.
In the late 20th century, when reducing energy consumption became a key goal for the construction and home improvement industries, the double-glazed sealed unit was developed. Recently, window manufacturers have introduced a triple-glazed sealed unit for superior thermal performance.
In both double and triple paned windows, spacer bars are used to separate the glass panes. The space between the glass panes is filled with either Argon or Krypton, inert gases that improve the R-value of the window. R-value refers to an insulating material’s capacity to resist heat flow. The addition of gas between glass panes has given double and triple paned windows the ability to significantly slow heat transfer through the window.