Keystone (architecture)

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Voussoir stones of an arch
Keystone from the palazzo Borgazzi (Milan, Italy)
Rib vault keystone with boss, in church of Virgin Mary in Chełmno
Dropped keystone at Colditz Castle

A keystone is the wedge-shaped stone piece at the apex of a masonry vault or arch, which is the final piece placed during construction and locks all the stones into position, allowing the arch to bear weight.[1][2][3] Although a masonry arch or vault cannot be self-supporting until the keystone is placed, the keystone experiences the least stress of any of the voussoirs, due to its position at the apex.[4] Old keystones can decay due to vibration, a condition known as bald arch.

In a rib-vaulted ceiling, keystones may mark the intersections of two or more arched ribs. For aesthetic purposes, the keystone is sometimes larger than the other voussoirs, or embellished with a boss. Mannerist architects of the 16th century often designed arches with enlarged and slightly dropped keystones, as in the "church house" entrance portal at Colditz Castle (see image). Numerous examples are found in the work of Sebastiano Serlio, a 16th-century Italian Mannerist architect.

Figurative use

The term is used figuratively to refer to the central supporting element of a larger structure, such as a theory or an organization, without which the whole structure would collapse.[3] Example: Trade is the keystone of modern civilization.

The U.S. state of Pennsylvania is called the "Keystone State" because of its central location and commercial and political importance among the 13 colonies. The notched keystone shape is sometimes used as a logo for the state, as most state-run agencies such as the Pennsylvania Lottery, Pennsylvania Department of Transportation, and the Pennsylvania Turnpike Commission use the keystone for their logo. From this, the keystone shape became the logo of the Pennsylvania Railroad (PRR), the shoulder sleeve insignia of the 28th Infantry Division of the Pennsylvania National Guard, Little League Baseball (founded in Williamsport), and of Heinz Ketchup (headquartered in Pittsburgh). The PRR's passenger and mixed traffic locomotives had keystone numberplates. Amtrak also uses the name for its Keystone Corridor through the state. The keystone shape is also used on Pennsylvania's state route markers, and except for the five-year period leading up to the United States Bicentennial (in which the Liberty Bell was used instead), Pennsylvania license plates have used the keystone as a number divider since 1958.

The Canadian province of Manitoba is nicknamed the "Keystone Province" due to its location in the centre of Canada, uniting the eastern and western domains. The phrase was first uttered by Lord Dufferin in a speech given in Winnipeg in 1877.

Gallery

  • The York Minster Chapter House ceiling

  • A boss depicting Jesus Christ decorates the keystone in the rib-vaulting at Chapel of St. Anne in Malbork, 14th century.

  • Ornamental keystone over portal

  • Bossed keystone in the ceiling of an apse chapel (Toulouse Cathedral)

  • Symbolic shape

  • Arches with equal-size voussoirs and keystones, Mosque of Cordoba, Spain

  • The keystone of the gateway to the Venetian arsenal at Gouvia in Corfu carries the inscription "ZBM ANNO MDCCLXXVlll" where "ZBM" are probably the initials of one of the ship captains who constructed the arsenal.

See also

Notes

  1. ↑ Ching, Francis D.K. (1995). A Visual Dictionary of Architecture. New York: John Wiley & Sons, Inc. p. 12. ISBN 0-471-28451-3. 
  2. ↑ "Glossary of Medieval Art and Architecture – Keystone". University of Pittsburgh. Retrieved 2007-06-25. 
  3. ↑ 3.0 3.1 "keystone". Merriam Webster. Retrieved 2007-06-25. 
  4. ↑ "Windows and More About Arches". Retrieved 2007-06-25. 

External links

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