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
Glassblowing is a glassforming technique that involves inflating molten glass into a bubble (or parison) with the aid of a blowpipe (or blow tube). A person who blows glass is called a ''glassblower'', ''glassmith'', or ''gaffer''. A '' lampworke ...
and then flattened by reheating and spinning out the bowl-shaped piece of glass (bullion) into a flat disk by
centrifugal force
Centrifugal force is a fictitious force in Newtonian mechanics (also called an "inertial" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It appears to be directed radially away from the axi ...
, 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
A pontil mark or punt mark is the scar where the pontil, punty or punt was broken from a work of blown glass. The presence of such a scar indicates that a glass bottle or bowl was blown freehand, while the absence of a punt mark suggests either ...
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, the medieval capital of
Alodia. 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 northwestern France. It is in the prefecture of Regions of France, region of Normandy (administrative region), Normandy and the Departments of France, department of Seine-Maritime. Formerly one ...
, 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,
blown plate,
polished plate 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,
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 of a low melting point, typically tin, although lead was used for the process in the past. This method gives the sheet uniform thickness and a very flat surfa ...
.
References
External links
{{Glass forming
Glass production
Glass types
History of glass