Stone arch - Crossetted arch.

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A stone arch consists of stones specially cut to a wedge shape so that the joints between stones radiate from a common centre, the soffit is arched and the stones bond in with the surrounding walling. The individual stones of the arch are tenned ‘voussoirs’, the arched soffit the ‘intrados’ and the upper profile of the arch stones the ‘extrados’.

Figure 109 is an illustration of a stone arch whose soffit is a segment of a circle. The choice of the segment of a circle that is selected is to an extent a matter of taste, which is influenced by the appearance of strength. A shallow rise is often acceptable for small openings and a greater rise for larger, as the structural efficiency of the arch increases the more nearly the segment approaches a full half circle, The voussoirs of the segmental arch illustrated in Fig. 109 are cut with steps that correspond in height with stone courses, to which the stepped extrados is bonded.

The stones of an arch are cut so that there is an uneven number of voussoirs with a centre or key stone. The key stone is the last stone to be put in place as a key to the completion and the stability of the arch, hence the term key stone.

The majority of semi-circular arches are formed with stones cut to bond in with the surrounding stonework in the form of a stepped extrados similar to that shown for a segmental arch in Fig. 109.

Crossetted arch.
The semi-circular arch, illustrated in Fig. 110, is formed with stones that are cut to bond into the surrounding walling to form a stepped extrados and also to bond horizontally into the surrounding stones. The stones, voussoirs, are said to be crossetted, or crossed. This extravagant cutting of stone is carried out purely for appearance sake. This is not a structurally sound idea as a very slight settlement might cause the crossetted end of a stone to crack away from the main body of the stone, whereas with plain voussoirs the slight settlement would be taken up by the joints.

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