The keystone is the final stone located in the very top of the arch. The keystone is not more important than any other stone in the arch as far as function is concerned; after all, it is the whole line of stones in the arch that holds the structure together. However, as the final stone required to connect the two halves of the arch the keystone is important to ensure a solid structure. In this post, we will look at placing the keystone in a rubble arch; an ashlar arch will have all the stones cut to the right size to fit to begin with.
Choosing the Stones
Most arches will technically use several keystones. Unless the arch is very narrow or the keystone is an unusually long piece, it will require several stones to close the width of the arch. The main trick is to find a stone that is a hair too large to actually fit all the way down against the form without some help. Also, it is important to find stones that will break the joints reasonably well.
Preparing the Keystone
So now you have a stone that won’t quite fit all the way down against the form. To prepare it for inserting, it helps to cut a slight triangle on the bottom to help it slide in. This is done by using a chisel or stonecutting hammer to knock off the corners on the bottom of the stone, creating a wedge in the stone.
Placing the Keystone
Inserting the keystone is often done by pounding it in place with a hammer. To do this, place a board between the sledge and the stone (the board keeps the stone from shattering) and pound away until it is as tight as it can be. It may be necessary to adjust the angle of attack in order to keep the keystone heading the right direction rather than twisting. Pound until the stone hits the form, or until it stops moving. The keystone can be slightly above the form; this is a judgement call. However, if it simply won’t go down far enough, it will have to be removed and trimmed as needed. This is why the initial sizing is important. Slight gaps between stones is absolutely acceptable, though initial trimming to mitigate them is recommended.
Stone Arch Bridges 