Temporary framework used during construction to support arches and similar structures while the mortar or concrete is setting or the steel is being joined. As soon as the work is set and the structure is self-supporting, the centering is struck (carefully removed).
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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 nineteenth 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. Brunelleschi's construction of the Duomo at Florence was achieved in what was fundamentally, this way and without major scaffolding.
On completion of the intended structure, the centring of whichever sort is removed and work on pointing and other finishing continued.
see also: falsework