The Keystone Arch

"The Romans were the first builders in Europe, perhaps the first in the world, fully to appreciate the advantages of the arch, the vault and the dome." This statement, attributed to D.S. Robertson, author of Greek and Roman Architecture, 2nd edn., Cambridge 1943, p.231, is cited in Wikipedia's entry entitled "Arch."

Note, in this quotation, the correct use of an un-split infinitive, "fully to appreciate." Colloquially we may say, "to fully appreciate," but, if we have old-school training ("never split an infinitive"), we remember, somewhere in the back of our mind, that this usage is often seen as incorrect.

So, moving on.

A post-and-beam span, that is, an opening framed by two posts with a beam across the top, supports itself by distributing the weight of gravity from the center of the beam to the two sides and down the posts.

The keystone arch cannot support itself until the keystone, the wedge-shaped puzzle piece at the apex, is put into place. The arch is first built on top of a framework, called a "centering." The centering is removed when the arch becomes a true compression form with the placement of the keystone.

The arch supports itself by distributing the weight of gravity from the wedge-shaped keystone to the next wedge-shaped voussoir and the next, on down to the springers at the spring line. The spring line is defined by the two points at the base of the arch and the theoretical center point, "drawn" from the two lower points of the keystone and crossing at the spring line. The springers have trapezoidal tops and level bottoms to continue spreading the weight of gravity down the posts.

We have an example of an early keystone arch in the northern gate, one of seven, in the walls encircling Perugia, the capitol city of Umbria. The walls were built of limestone block between 600 and 300 BCE to protect the city from the enemy, Rome, about a hundred miles away.

The original settlers of Perugia, or in Latin, Perusia, were the Umbri, an ancient--and perhaps the most ancient--tribe in Italy.  They spoke Umbrian, an Italic language related to Latin and Oscan. But their neighbors in Etruria, the Etruscans, spoke an unrelated language, which some scholars now believe may be a Tyrsenian language like the Raetic language of the Alps and the Lemnian language of Lemnos.

By the time the Roman general Fabius Maximus Rullianus led a successful expedition against Perugia in 310 or 309 BCE, the Umbrians had intermingled with the Etruscans and Perugia counted itself as one of the twelve cities of the Etruscan League. According to Livy, our source in this matter, the Perugians lost the battle and were accorded a thirty-year truce with Rome.

In a rebellious move, Perugia took part in the Second Samnite War against Rome in 295 BCE and had to renegotiate the treaty. By 216 and 205 BCE, the city found it prudent to help Rome in the Second Punic War.

Then the city fell a third time, when Mark Antony's younger brother and supporter Lucius sought refuge there in 40 BCE. Octavian, the future emperor Augustus, besieged the city and Lucius was compelled to surrender. The city was burned to the ground, but, based on archaeological evidence, rebuilding began almost immediately.

Soon thereafter, as part of a public-works program, Octavian repaired the wall and the ancient northern gate of Perugia, whose Umbrian or Etruscan name, as far as I can tell, we do not know. He rebuilt the keystone arch that the Perugians had originally built and this gate soon became known as the Augustan Gate, Porta d'Augusta.

So, as they often did, the Romans adopted the keystone arch from the people they conquered and used it widely in bridges and aqueducts, triumphal arches and gates, vaults and domes.