Akira Inman is a stone mason in Toronto who came to our Rocktober Festival in Grand Valley in 2009 , and then through having met with Thomas Lipps (who we sponsored to be there) was invited to be a translator in Ventura last January, for the Japanese stone masons who came over for the Stone Symposium in California, and were demonstrating the ancient Anoh tradition of dry stone construction.
Prior to the 20th Century, the support for a new arch was called centering. The arrangement was similar to the technology involved in the construction of a wheel. Another method used for smaller arches was to pile sand on a wooden platform to create the desired shape for the stonework above. It was quite simple then to remove the loose sand when building was complete.
Nowadays it is much more common to cut the required shape on at least two boards then to cover these with strong cross pieces or with bendable strong materials such as marine ply. It is essential that the former of whatever type is strong enough to bear the weight of not just the arch rocks but also any material which will be built on top before the former is removed. Any area of weakness can result in failure.
The support for the former should be resting on a solid base. Bricks or blocks are ideal for the support as they can be easily knocked away when required.
Care is necessary when arranging these support so that the former can safely be dropped down to allow its removal, without interfering with the stone work. I have heard of people being required to set fire to the former to remove it as it was not possible to get it out any other way!
With a half moon former it is possible and useful when placing the arch stones to mark lines on the outside to indicate the required slope of the builders and especially the risers.
When the desired arch is not a half circle, the lines should be drawn from the theoretical centre.