Arch/keystones

[Musivarius]

I'm sure this was discussed before but I can't find it on a search. Did the Romans develop the use of the keystone in arches?

The reason I ask is that one of the setting rules for Roman mosaics is that when you have a line of tesserae following the line of a curve you don't use a triangle to fill any gaps but always cut a keystone. You can to a degree just open the gaps slightly but at some point the gaps on the outside of the curve will be too wide.

I'm trying to figure if this was just an artistic technique for mosaic or if it did somehow relate to their architecture. Usually you do avoid using triangles as these imply direction.

Hope this makes sense, I'm too dim to figure out how to post an image :wink:

Lawrence

[Athena Areias]

I just googled "Roman Arches" in Google Images. They all have keystones.
We use keystone bricks here at work all the time. They are not triangular because the forces exerted on them from the arch sides will pop the triangle up and out - collapsing the arch. It's an engineering thing. I don't know if the Romans discovered keystones or appropriated them from someone else. They are a very important architectural/engineering principle.

[Eleatic Guest]

I've done some research into the matter, and, while arches and vaults appeared very early in the Mesopotamian area, these were never widely employed, mostly confined to underground tombs (where the problem of lateral thrust is negligible) and of uncertain forms.

The regular use of triangular keystones seems to appear first in the Italian peninsula. Usually it is attributed to the Etruscans, but two recent and notable monographs on Roman bridges have questioned the Etruscan origin of many extant arches, dating them now into the Roman period.


Further reading:
*Van Beek, Gus W. (1987): Arches and vaults in the Ancient Near East, Scientific American, July, pp.78-85

*Boyd, D. T. 1976. The arch and vault in Greek Architecture. Dissertation. Indiana University

*Thomas D. Boyd: “The Arch and the Vault in Greek Architecture“, ''American Journal of Archaeology'', Vol. 82, No. 1 (1978), pp. 83–100

*Lancaster, Lynne: Concrete Vaulted Construction in Imperial Rome

*Besenval, R., 1984, Technologie de la Voute dans L’Orient Ancien, Èditions de Recherche sur les civilisations, Paris

[SigniferOne]

Excellent links there, thanks Eleatic Guest!