Glazing techniques have changed over the years.
Natural glass has existed since the beginning of time - formed when certain types of rocks melt as a result of high-temperature phenomena such as volcanic eruptions, lightning strikes or the impact of meteorites.
The earliest man-made glass objects, mainly non-transparent glass beads, are thought to date back to around 3500 BC, with finds in Egypt and Eastern Mesopotamia.
The major breakthrough in glassmaking was the discovery of glassblowing some time between 27 BC and AD 14, attributed to Syrian craftsmen. However, it was the Romans who began to use glass for architectural purposes, with the discovery of clear glass (through the introduction of manganese oxide) around AD 100.
The 11th century saw the development by German glass craftsmen of a technique for the production of glass sheets. By blowing a hollow glass sphere and swinging it vertically, gravity would pull the glass into a cylindrical "pod". While still hot, the ends of the pod are cut off and the resulting cylinder cut lengthways then laid flat.
Other types of sheet glass included crown glass (also known as "bullions"). For this technique, a glass ball was blown and then opened outwards on the opposite side to the pipe. Spinning the semi-molten ball then caused it to flatten and increase in size. This technique could only produce a limited diameter.
The production of flat glass was originally made by pouring molten glass onto large tables then rolled flat into "plates", cooled, ground and polished. The major breakthrough in flat glass came in 1905 when a Belgian named Fourcault managed to vertically draw a continuous sheet of glass of a consistent width from the tank.
Around the end of the First World War, another Belgian engineer, Emil Bicheroux, developed a process whereby the molten glass was poured from a pot directly through two rollers. Like the Fourcault method, this resulted in glass with a more even thickness, and made grinding and polishing easier and more economical.
An off-shoot of evolution in flat glass production was the strengthening of glass by means of lamination (inserting a celluloid material layer between two sheets of glass). The process was invented and developed by the French scientist Edouard Benedictus who patented his new safety glass under the name "Triplex" in 1910.
In America, Colburn developed another method for drawing sheet glass. The process was further improved with the support of the US firm Libbey-Owens and was first used for commercial production in 1917. The Pittsburgh process, developed by the American Pennvernon and the Pittsburgh Plate Glass Company (PPG), combined and enhanced the main features of the Fourcault and Libbey-Owens processes, and has been in use since 1928.
The float process developed after the Second World War by Britain's Pilkington Brothers Ltd., and introduced in 1959, combined the brilliant finish of sheet glass with the optical qualities of plate glass. Molten glass, when poured across the surface of a bath of molten tin, spreads and flattens before being drawn horizontally in a continuous ribbon into the annealing lehr.