Centring
Encyclopedia
Centring or centering (US), is the structure upon which the stones of arch
es or vault
are laid during construction. Once the arch is complete, it supports itself, but until the keystone
is inserted, it has no strength and needs the centring to keep the voussoir
s in their correct relative positions.
The centring is normally made of wood
, which was a relatively straightforward structure in a simple arch or vault, but with more complex shapes, involving double curvature, such as a small dome
or the bottle-shaped flues
of the kitchens of some Norman
-period houses; clay
or sand
bound by a weak lime
mortar
mix could be used. The shaping of this sort of centring would probably be done by eye, perhaps with the help of a template and the stone
or brick
structure laid against it. On bigger work, like a 19th-century commercial pottery
kiln
, this was impractical. The structure would be built round a post acting as a datum, and each course of stonework would be set at a distance from the datum as measured by a stick or string.
On completion of the intended structure, the centring of whichever sort is removed, and work on pointing and other finishing continued.
Arch
An arch is a structure that spans a space and supports a load. Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture and their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures.-Technical aspects:The...
es or vault
Vault (architecture)
A Vault is an architectural term for an arched form used to provide a space with a ceiling or roof. The parts of a vault exert lateral thrust that require a counter resistance. When vaults are built underground, the ground gives all the resistance required...
are laid during construction. Once the arch is complete, it supports itself, but until the keystone
Keystone (architecture)
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. This makes a keystone very important structurally...
is inserted, it has no strength and needs the centring to keep the voussoir
Voussoir
A voussoir is a wedge-shaped element, typically a stone, used in building an arch or vault.Although each unit in an arch or vault is a voussoir, two units are of distinct functional importance: the keystone and the springer. The keystone is the center stone or masonry unit at the apex of an arch. A...
s in their correct relative positions.
The centring is normally made of wood
Wood
Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...
, which was a relatively straightforward structure in a simple arch or vault, but with more complex shapes, involving double curvature, such as a small dome
Dome
A dome is a structural element of architecture that resembles the hollow upper half of a sphere. Dome structures made of various materials have a long architectural lineage extending into prehistory....
or the bottle-shaped flues
Chimney
A chimney is a structure for venting hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside atmosphere. Chimneys are typically vertical, or as near as possible to vertical, to ensure that the gases flow smoothly, drawing air into the combustion in what is known as the...
of the kitchens of some Norman
Norman architecture
About|Romanesque architecture, primarily English|other buildings in Normandy|Architecture of Normandy.File:Durham Cathedral. Nave by James Valentine c.1890.jpg|thumb|200px|The nave of Durham Cathedral demonstrates the characteristic round arched style, though use of shallow pointed arches above the...
-period houses; clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
or sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
bound by a weak lime
Calcium hydroxide
Calcium hydroxide, traditionally called slaked lime, is an inorganic compound with the chemical formula Ca2. It is a colourless crystal or white powder and is obtained when calcium oxide is mixed, or "slaked" with water. It has many names including hydrated lime, builders lime, slack lime, cal, or...
mortar
Mortar (masonry)
Mortar is a workable paste used to bind construction blocks together and fill the gaps between them. The blocks may be stone, brick, cinder blocks, etc. Mortar becomes hard when it sets, resulting in a rigid aggregate structure. Modern mortars are typically made from a mixture of sand, a binder...
mix could be used. The shaping of this sort of centring would probably be done by eye, perhaps with the help of a template and the stone
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
or brick
Brick
A brick is a block of ceramic material used in masonry construction, usually laid using various kinds of mortar. It has been regarded as one of the longest lasting and strongest building materials used throughout history.-History:...
structure laid against it. On bigger work, like a 19th-century commercial pottery
Pottery
Pottery is the material from which the potteryware is made, of which major types include earthenware, stoneware and porcelain. The place where such wares are made is also called a pottery . Pottery also refers to the art or craft of the potter or the manufacture of pottery...
kiln
Kiln
A kiln is a thermally insulated chamber, or oven, in which a controlled temperature regime is produced. Uses include the hardening, burning or drying of materials...
, this was impractical. The structure would be built round a post acting as a datum, and each course of stonework would be set at a distance from the datum as measured by a stick or string.
On completion of the intended structure, the centring of whichever sort is removed, and work on pointing and other finishing continued.
See also
- falseworkFalseworkFalsework consists of temporary structures used in construction to support spanning or arched structures in order to hold the component in place until its construction is sufficiently advanced to support itself...