Plain Glass:
A material often overlooked
Since starting the scholarship we have been introduced to a number of glass specialists. Through our time spent with these specialists we have not only been taught about the history of glass and it's varying methods of production, but we have also come to fully appreciate plain glasses contribution to the character of a historic building.
It is now impossible to look at a historic building, with modern glass, and not be upset by the intrusiveness of this modern material on the eye.
Glass is often taken for granted in the modern age, however it has only been in the last 400 years that plain glass has been used regularly to glaze window openings. Below is a brief history of plain glazed windows.
Note: This article is intended as a basic summary and by no means covers all types or styles. Date of windows in images is unknown, but examples are shown to indicate style.
Pre-16th century most domestic window openings were unglazed, they had shutters or were covered in thin sheets of either cloth/ paper or skin to keep out the weather. The first windows to be glazed were those in the highest status buildings. The first windows were constructed of small panes of glass-known as quarries (made from cylinder glass) and then held together in a lattice of lead strips, called cames. These windows were usually reinforced with iron bars, either in the horizontal (known as saddle bars) or in the vertical (known as stanchions).
Fig.1.: Leaded lights, Hampton Court- Authors own image
As prosperity increased in the Tudor period, windows began to have opening casements, often this would include a wrought iron frame set into stone or timber mullions. These opening casements would have the same leaded glazing, but this would be attached to the casement.
Fig.2.: Leaded casement window, during repair at Ben Sinclair's- Authors own image
Fig. 3.: Leaded casement windows, Hampton Court- Authors own image
In the 17th century everyday house glass was still rare but was slowly becoming more common. As the 17th century progressed, windows were increasing constructed of timber in the form of casement windows, to start these timber casements still had leaded lattice work.
Fig.4.: Timber casement leaded light, Cirencester- Authors own image
However as the production of crown glass began, panes of glass could become larger and the panes within casement windows could be held in with glazing bars.
Fig.5.: Timber casement windows, Launceston- Authors own image
The casement window was then replaced in the later part of the 17th century with the sash window. The sash window began life with just a bottom moving section, held in place by pegs or latches, however as the 17th century continued, the sash window became what we recognise today, with cords and a counter balance allowing both sections of the window to move.
Then in the 1770’s came the introduction of plate glass, which enabled the pane size of windows to increase, by the end of the 18th century most houses, including small workers cottages would have had sash windows. As we move into the 19th century, glazing bars started to be removed and by the mid- 19th century most windows didn't have glazing bars. Sash windows were still common and to compensate for the lack of glazing bars horns on sash windows were introduced.
Fig.6.: Traditional sash window, Edinburgh Castle- Authors own image
Fig.7. Traditional sash window, Wentworth Woodhouse- Authors own image
Fig.8.: An example of a sash window with large Panes of glass and ‘horns’, Cirencester - Authors own image
Glass Production (by order of general appearance with the UK):
• Slab Glass
Slab glass is the earliest form of window glass and is produced by casting molten glass into moulds on a flat surface.
• Cylinder Glass (or Broad glass)
Molten glass is blown and swung to form a cylinder, both ends are cut off and the cylinder is then cut lengthways, reheated and flattened into sheets. The glass is then annealed slowly. Cylinder glass is often easy to identify as it has a distorted, ripple effect across the glass with air bubbles and small imperfections.
Fig.9.: Dodsworth, Roger. (1984) Glass and Glass Making. UK: Shire Publications Ltd.
Fig.10.: Cylinder glass in production- Authors Own Image
• Crown Glass
Crown glass was first produced on the continent in the 14th century, however it was not produced in the UK until 1674 and then production ceased in the mid-nineteenth century. Crown glass is produced by molten glass being blown into a bubble and then spun into a disk, historically these disks could reach around 4ft in diameter. The glass is then cooled and cut into panes with the outer areas being the most desired as they were the thinnest. The central bullion or ‘bulls-eye’ where the rod was attached was then waste, however often appears in stained glass or leaded lights as decoration. Although crown glass allowed panes of glass to become larger, Crown glass was very expensive therefore it was beyond the reach of most of the population so leaded glazing remained popular throughout the 17th and much of the 18th centuries.
Fig.11.: Cylinder glass half way through production, at Jim Budds Workshop- Authors own image
Fig.12.: Crown Glass in Production- Dodsworth, Roger. (1984) Glass and Glass Making. UK: Shire Publications Ltd.
Fig.13. Crown Glass in Production- Dodsworth, Roger. (1984) Glass and Glass Making. UK: Shire Publications Ltd.
When trying to identify whether glass is crown glass, one should look for the curved sweeps within the glass, showing the marks of its production.
• Polished Plate Glass
Plate glass was first produced in Britain in 1773. The Chance Brothers industrialised the process in the 1830s, the process involves creating slab glass and then grinding down the surface, before being polished. This enabled the production of high quality relatively inexpensive flat glass. Plate glass allowed the rippled effect to be greatly reduced. Crystal Palace was built using plate glass.
• Drawn Glass
Drawn glass was invented in the early 20th century in Belgian, and first produced in the UK in 1919. The process essentially draws molten glass upwards, through and over rollers and into a cooling chamber. Drawn glass still has some movement in the vertical plane, as the drawing process is not fully consistent, it is often used to replicate hand made glass in historic buildings.
• Float Glass
Float glass is a 20th century creation invented by Pilkington in 1959. Float glass is produced by pouring molten glass onto a molten tin surface. The glass ‘floats’ on the tin and as the tin is cooled it creates an even sheet of glass which is perfectly smooth.
Historically the aim of glass manufacture was to produce a product which was as clear and thin as possible, with minimal ripples or imperfections. Pilkington's float glass has enabled us to now have achieved this- therefore one could argue that modern glass is superior to historical glass- however what we have lost within the evolution of glass is the depth and character which glazing once had. Although modern glass maybe suitable for modern buildings, retrofitting this into historic buildings greatly changes the character of the place, and is not in-keeping with the rest of the naturally imperfect vernacular materials.
Fig.14.: An example of large sections of lost glass within a sash window, Cirencester- Authors own image
Next time you look at a historic building, look at the windows- do they glisten in the sun or are they flat? How does this affect the feeling and charm of the overall building? With so many windows being replaced for ‘double glazing’ its time to take a step back and review the beauty of what we have and conserve this wherever possible.
Fig.15.: Historic plain glass in quarries, Hardwick Hall- Authors own image
Bibliography
Dodsworth, Roger. (1984) Glass and Glass Making. UK: Shire Publications Ltd. Anonymous (unknown)
London Crown Glass, Found At: http://londoncrownglass.com/History.html
Free, Simon (2020) Sash Window Specialists, Found At: https://sashwindowspecialist.com/blog/ history-of-window-glass/
Sharman, Jess (2021)The NBS, Found At: https://www.thenbs.com/knowledge/windows-glassglazing- a-brief-history