Centring

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. 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 voussoirs in their correct relative positions. A simple centering without a truss is called a common centering. The cross piece connecting centering frames are 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.