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Crown glass was an early type of window glass. 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 In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It is directed away from an axis which is paralle ...
, up to 5 or 6 feet (1.5 to 1.8 metres) in diameter. The glass was then cut to the size required. The thinnest glass was in a band at the edge of the disk, with the glass becoming thicker and more opaque toward the center. Known as a bullseye, the thicker center area around the pontil mark was used for less expensive windows. To fill large window spaces with the best glass, many small diamond shapes were cut from the edge of the disk, and then some might be halved into triangles. These were mounted in a lead lattice work and fitted into the window frame. Crown glass was one of the two most common processes for making window glass until the 19th century. The other was blown plate. Crown glass window panes with ceramic frames have been found at
Soba East Location of Soba in medieval times (below on the right) Soba is an archaeological site and former town in what is now central Sudan. Three kingdoms existed in medieval Nubia: Nobadia with the capital in Faras, Makuria with the capital in Do ...
, the medieval capital of
Alodia Alodia, also known as Alwa ( grc-gre, Aρουα, ''Aroua''; ar, علوة, ''ʿAlwa''), was a medieval kingdom in what is now central and southern Sudan. Its capital was the city of Soba, located near modern-day Khartoum at the confluence of t ...
. They are only in diameter and were probably used to provide light in storerooms. The process of making crown glass window panes was perfected by French glassmakers in the 1320s, notably around
Rouen Rouen (, ; or ) is a city on the River Seine in northern France. It is the prefecture of the region of Normandy and the department of Seine-Maritime. Formerly one of the largest and most prosperous cities of medieval Europe, the population ...
, and was a trade secret. Hence crown glass was not made in London until 1678. Crown glass is one of many types of hand-blown glass. Other methods include:
broad sheet Broad sheet is a type of hand-blown glass. It was first made in Sussex in 1226. Production It is made by blowing molten glass into an elongated tube shape with a blowpipe. Then, while the glass is still hot, the ends are cut off and the res ...
, blown plate,
polished plate Polished plate is a type of hand-made glass. It is produced by casting glass onto a table and then subsequently grinding and polishing the glass. This was originally done by hand, and then later by machine. It was an expensive process requiring a l ...
and cylinder blown sheet. These methods of manufacture lasted at least until the end of the 19th century. The early 20th century marks the move away from hand-blown to machine-manufactured glass such as rolled plate,
machine drawn cylinder sheet Machine drawn cylinder sheet was the first mechanical method for "drawing" window glass. Cylinders of glass 40 feet (12 m) high are drawn vertically from a circular tank. The glass is then annealed and cut into 7 to 10 foot (2 to 3  ...
, flat drawn sheet, single and twin ground polished plate and
float glass Float glass is a sheet of glass made by floating molten glass on a bed of molten metal, typically tin, although lead and other various low- melting-point alloys were used in the past. This method gives the sheet uniform thickness and very flat su ...
.


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{{Glass forming Glass production Glass types History of glass