The earliest form of glass, slab glass was made by pouring molten glass onto a flat surface.
Broad or cylinder glass
Broad or cylinder glass is an 11th century German invention that made its appearance in the UK during the early 1200s. It consists of glass that was blown to form a bubble, which was then cut into a cylinder shape, reheated, and flattened into sheets. The result was a highly imperfect glass that provided a distorted view through a green tint.
This glass was introduced to England in 1674 and remained popular until the 1830s. Also a blown glass, crown glass was blown into a bubble which was pierced by a rod and then spun to form a disk. The glass was cooled and then cut into panes. The centre piece, where the rod was attached, was usually discarded, although you occasionally see panes of that type in older homes. Crown glass was finer and clearer than broad glass. Although crown glass provided a less distorted view than previously produced window glasses, it still had a slight ripple to it.
During the late 18th and early 19th centuries, crown glass was produced alongside the cylinder type, but it was eventually pushed out of production and the technique for making crown glass was lost. Today, the closest one can get to crown glass is cylinder glass.
Cylinder sheet glass
In a method similar to creating broad glass, cylinder sheet glass starts its life with the same blown glass technique but then the cylinder was swung in a trench to increase its size. As with broad glass, this larger cylinder is then cooled and cut before being reheated and flattened. As well as allowing larger panes to be made, the resulting product also provides a superior surface quality compared to broad glass.
Cast glass is a product of the late 17th century and created by pouring molten glass into a mould. The cast glass process is used for multiple purposes, including creating glass sculptures and mirror glass. Because the process is labour intensive, in glazing applications cast glass is typically reserved for statement windows – often including a texture or design.
Invented by Emile Fourcault in 1904, the drawn glass process places a slot in a tank of molten glass and then ‘draws’ sheets of glass through it over water-cooled rollers and into a cooling chamber.
Around the same time, Irving Colburn introduced the Colburn machine, which used paper making as its inspiration. The sheet of glass is first drawn vertically from the surface of the molten glass and then gradually bent over a roller until it lays horizontally.
Glass produced by both of these methods was marked with ripples where it has been pulled and then rolled and, as with earlier processes, the glass had to be ground and polished afterwards.
The process for making float glass was introduced by Alastair Pilkington in 1959, and it is still the industry standard today. With this process, the molten glass is poured onto a bed of molten tin. Floating on the tin, the molten glass spreads out to form a level surface.
Originally, Pilkington’s process enabled glass only to be made at 6.8 mm thick, but today it can be as thin as 0.4 mm or as thick as 25 mm. While the principles of the process remain unchanged, the surface quality of the glass has greatly improved, providing an end product devoid of distortions and/or flaws.
The introduction of the float process opened the doors to an architectural revolution that allowed very large panes of perfect glass to be created. Additional improvements since then have enabled increased and varied functionality and the further development of what is known as intelligent glazing.