Before revision: 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.
After revision: 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 flue in a Norman-period kitchen, clay or sand bound by a weak lime mortar would be used.