Throughout the early medieval period, the great majority of windows were unglazed. In timber-framed buildings they were simple openings in the structural frame (and for wider openings were often sub-divided into two or more ‘lights’ with plain or moulded mullions).Taller windows might be sub-divided horizontally with transoms. Vertical wood or iron bars were inserted to keep out intruders.
The reason for this was that glass was extremely expensive and rare and was not considered a fixture. As a conseqeuence, glass was typically only found in Royal Palaces and Royal lodges and for the majority of people, timber shutters were widely used for security, privacy and to reduce draughts. Even in the palaces, the available glass was small in size and this resulted in leaded-glass windows wiich survived into the 1930s. In England, these medieval timber shutters were often internal and either hinged or slid along a wooden upper and lower groove although thesee gave way to upper metal bar using runners. Although these early shutters have rarely survived, the runners sometimes remain.
The early method of producing sheet glass, Broad Glass. wasa widely used by the 12th century and was an early form of cylinder glass where the glassmaker swung a bubble of molten glass back and forth whilst blowing to produce an elongated balloon, and the balloon then laid on a very smooth surface. The two ends were quickly cut off to leave a tube, which was then sliced along its length with a pair of shears and flattened to form a small rectangular sheet of glass. With this technique it was difficult to achieve good glass and thee result was a very thick and heavy, distorting glass. As taxation was applied based on the weiight of the glass, it was very expensive.
In this process, glass was blown into a “crown” or hollow globe. This was then transferred from the blowpipe to a punty and then flattened by reheating and spinning out the bowl-shaped piece of glass (bullion) into a flat disk by centrifugal force, up to 5 or 6 feet (1.5 to 1.8 metres) in diameter. The glass was then cut to the size required
A new method of producing sheet glass, Crown Glass, was produced in which glass was blown into a “crown” or hollow globe and the bubble of molten glass then transferred onto a metal ‘punty rod’ or ‘pontil rod’, reheated and spun between the hands of the glass-blower and this spinning out of the bowl-shaped piece of glass (bullion) into a flat disk spinning caused the molten glass to form viia centrifugal force into a much lighter disc, sometimes as much as 1.5m in diameter, which was then carefully cut to size. The thinnest glass was in a band at the edge of the disk, with the glass becoming thicker and more opaque and distorting toward the centre which was known as a bullseye. The thicker center area of glass around the pontil mark (A pontil mark or punt mark is the scar where the pontil, punty or punt was broken from a work of blown glass) was used for less expensive windows. As can still be seen today in original Tudor windows, to fill large window spaces with the best glass, the edge of the glass disc was cut into many small diamond or triangular shapes which were mounted in a lead-lattice work (hence lead-work windows) and fitted into the window frame
The earliest known crown glass in England dates from the 1440s, although the process of making crown glass window panes perfected by French glassmakers in the 1320s around Rouen was for many years after that a trade secret.
It was a crown glass was widely used for windows until the mid-19th century, when taxation by weight ceased and when, as a result, cylinder glass became cheaper. Early examples were small but by the end of the 19th century the cylindrical bubbles could be as much as 1.5m long. Crown glass has not been manufactured since the early 20th century.
Polished plate glass is a glass that is cast onto a highly polished table of copper or cast iron and then ground and polished until flat and crystal clear. Developed in France, the process was used in England from the late-18th century until mechanisation in the mid-19th century made large sheets of highly finished plate glass much less expensive.
Modern glass was drawn flat sheet glass which started to be produced from early in the 20th century and involved drawing molten glass through a die into a flat continuous sheet rather than a slab or cylinder Float glass Float glass was invented in the late 1950s and involves flowing the molten material over a bath of molten tin. It is completely flat and therefore lacks the varied surface interest of earlier glass.
A more developed form of broad-glass manufacture arrived during the 19th Century with Cylinder Blown Glass, a type of hand-blown window glass which was created with a similar process to the much older broad sheet glass but using larger cylinders. Glass is blown into a cylindrical iron mold and the ends cut off. A cut is made down the side of the cylinder and the cut cylinder then placed in an oven where it unrolls into a flat glass sheet. This method caused surface damages on the glass due to the flattening and moving, and the sheet therefore had to be ground and polished. In 1839 the Chance Brothers invented the patent plate process where the glass plate was placed on a wet piece of leather and ground and polished to remove all the surface damage. The technique was pioneered by Chance Brothers and Company, a glassworks originally based in Spon Lane, Smethwick in the West Midlands/Staffordshire aand refined to use leather and a process of grinding the finished glass to create a finer plate glass, and later using a technique learned from the pioneers of the manufacture of high-precision lenses for observatory telescopes. This resulted in Chance glass being outstanding for plate and window glass, with other British glass-makers Pilkingtons, Hartleys and Cooksons also graadully adopting their techniques. (During world War II, the Chance company was still making glass and involved in production of cathode ray tubes for early radar sets, making up to 7,000 per week). The result of the Chance technique is much larger panes and improved surface quality over broad sheet, although still containing some imperfections.
The early 20th century marks the move away from hand-blown to machine manufactured glass such as rolled plate, machine drawn cylinder sheet, Fourcault process flat drawn sheet and sinigle and twin ground polished plate and float glass and it was Sir Alistair Pilkington who in the 1950s developed modern glass, created by pouring molten glass onto a tin plate which then levels out and forms strong edges, with a clear and mostly unblemished finish on both sides, then cooled with a Lehr oven creating flat glass.
If you could not afford glass, alternatives were sometimes iin place and windows were often covered with oiled fabric, nailed directly to the frame or stretched over a thin timber lattice and had the advantage of allowing ventilation whilst providing privacy and a barrier to flying insects, and full shutters were used during hours of darkness for security and to prevent insects being attracted by candles and oil lamps, even if these had the disaadvantage of creating a fume-ridden interior eenvironment.
As much of the plain glass and most if not all of the coloured glass used in England during the medieval period was imported from the continent
and thus prohibitively expensive for widespread domestic use, it was and remaiins rare and today medieval glass is irreplaceable as modern glass looks nothinig like the mediaeval glas. By the late 7th century, windows had beome more sophisticated with wooden tracery, moulded mullions and deep projecting cills, largely as glass was no longer quite as expensive as it started to be manufactured in England and therefore it started to be used for the upper class dwelliings and then graduating down to ordinary domestic buildings. There was also a process of retro-fitting existing timber houses with glass windows
