Centring, centre, centering, or center is a type of falsework: the temporary structure upon which the stones of an arch or vault are laid during construction. Until the keystone is inserted an arch has no strength and needs the centring to keep the voussoirs in their correct relative positions. A simple centering without a truss is called a common centering. A cross piece connecting centering frames is called a lag or bolst.
The centring is normally made of wood timbers, 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 called "...striking the centering...", and work on pointing and other finishing continued.
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- "Centring" def. 3. Oxford English Dictionary Second Edition on CD-ROM (v. 4.0). Oxford University Press 2009
- "Centre" def. 13. Oxford English Dictionary Second Edition on CD-ROM (v. 4.0). Oxford University Press 2009
- "Centering 2, Centring 2" def. 1. Whitney, William Dwight, and Benjamin E. Smith. The Century dictionary and cyclopedia. vol. 2. New York: Century Co., 1901. p. 885.
- "Center 2, Centre 2" def. 1. Whitney, William Dwight, and Benjamin E. Smith. The Century dictionary and cyclopedia. vol. 2. New York: Century Co., 1901. p. 885.
- Ching, Frank (1995). A visual dictionary of architecture. New York: Van Nostrand Reinhold. p. 3. ISBN 0471284513.
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