Centring[1],centre[2],centering[3][4], orcenter[5] is a type offalsework: the temporary structure upon which the stones of anarch orvault are laid during construction. Until thekeystone is inserted an arch has no strength and needs the centring to keep thevoussoirs in their correct relative positions. A simple centring without a truss is called acommon centring.[3] A cross piece connecting centring frames is called a lag or bolst.[6]
Centring is normally made ofwood timbers, a relatively straightforward structure in a simple arch or vault; but with more complex shapes involving double curvature, such as adome or the bottle-shapedflue in aNorman-period kitchen,clay orsand bound by a weaklimemortar would be used. Shaping could be done by eye, perhaps with the help of a template, thenstones orbricks laid against it. On larger works like a 19th-centurypotterykiln 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.
When the centring is removed (as in "striking the centring"),[3] pointing and other finishing continues.
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