History of Roman and Byzantine domes

Domes were a characteristic element of the architecture of Ancient Rome and of its medieval continuation, the Byzantine Empire. They had widespread influence on contemporary and later styles, from Russian and Ottoman architecture to the Italian Renaissance and modern revivals. The domes were customarily hemispherical, although octagonal and segmented shapes are also known, and they developed in form, use, and structure over the centuries. Early examples rested directly on the rotunda walls of round rooms and featured a central oculus for ventilation and light. Pendentives became common in the Byzantine period, provided support for domes over square spaces.

Early wooden domes are known only from a literary source, but the use of wooden formwork, concrete, and unskilled labor enabled domes of monumental size in the late Republic and early Imperial period, such as the so-called "Temple of Mercury" bath hall at Baiae. Nero introduced the dome into Roman palace architecture in the 1st century and such rooms served as state banqueting halls, audience rooms, or throne rooms. The Pantheon's dome, the largest and most famous example, was built of concrete in the 2nd century and may have served as an audience hall for Hadrian. Imperial mausolea, such as the Mausoleum of Diocletian, were domed beginning in the 3rd century. Some smaller domes were built with a technique of using ceramic tubes in place of a wooden centering for concrete, or as a permanent structure embedded in the concrete, but light brick became the preferred building material over the course of the 4th and 5th centuries. Brick ribs allowed for a thinner structure and facilitated the use of windows in the supporting walls, replacing the need for an oculus as a light source.

Christian baptisteries and shrines were domed in the 4th century, such as the Lateran Baptistery and the likely wooden dome over the Church of the Holy Sepulchre. Constantine's octagonal church in Antioch may have been a precedent for similar buildings for centuries afterward. The first domed basilica may have been built in the 5th century, with a church in southern Turkey being the earliest proposed example, but the 6th century architecture of Justinian made domed church architecture standard throughout the Roman east. His Hagia Sophia and Church of the Holy Apostles inspired copies in later centuries.

Cruciform churches with domes at their crossings, such as the churches of Hagia Sophia in Thessaloniki and St. Nicholas at Myra, were typical of 7th and 8th century architecture and bracing a dome with barrel vaults on four sides became the standard structural system. Domes over windowed drums of cylindrical or polygonal shape were standard after the 9th century. In the empire's later period, smaller churches were built with smaller diameter domes, normally less than after the 10th century. Exceptions include the 11th century domed-octagons of Hosios Loukas and Nea Moni, and the 12th century Chora Church, among others. The cross-in-square plan, with a single dome at the crossing or five domes in a quincunx pattern, as at the Church of St. Panteleimon, was the most popular type from the 10th century until the fall of Constantinople in 1453.

Overview

Rounded arches, vaults, and domes distinguish Roman architecture from that of Ancient Greece and were facilitated by the use of concrete and brick. By varying the weight of the aggregate material in the concrete, the weight of the concrete could be altered, allowing lighter layers to be laid at the top of concrete domes. But concrete domes also required expensive wooden formwork, also called shuttering, to be built and kept in place during the curing process, which would usually have to be destroyed to be removed. Formwork for brick domes need not be kept in place as long and could be more easily reused. The mortar and aggregate of Roman concrete was built up in horizontal layers laid by hand against wooden form-work with the thickness of the layers determined by the length of the workday, rather than being poured into a mold as concrete is today. Roman concrete domes were thus built similarly to the earlier corbel domes of the Mediterranean region, although they have different structural characteristics. The aggregate used by the Romans was often rubble, but lightweight aggregate in the upper levels served to reduce stresses. Empty " vases and jugs" could be hidden inside to reduce weight. The dry concrete mixtures used by the Romans were compacted with rams to eliminate voids, and added animal blood acted as a water reducer. Because Roman concrete was weak in tension, it did not provide any structural advantage over the use of brick or stone. But, because it could be constructed with unskilled slave labor, it provided a constructional advantage and facilitated the building of large-scale domes.

Roman domes were used in baths, villas, palaces, and tombs. Oculi were common features. They were customarily hemispherical in shape and partially or totally concealed on the exterior. In order to buttress the horizontal thrusts of a large hemispherical masonry dome, the supporting walls were built up beyond the base to at least the haunches of the dome and the dome was then also sometimes covered with a conical or polygonal roof. A variety of other shapes, including shallow saucer domes, segmental domes, and ribbed domes were also sometimes used. The audience halls of many imperial palaces were domed. Domes were also very common over polygonal garden pavilions. Domes were "closely associated with senatorial, imperial, and state-sponsored patrons" and proliferated in the capital cities and other cities with imperial affiliations. Construction and development of domes declined in the west with the decline and fall of the western portion of the empire.

In Byzantine architecture, a supporting structure of four arches with pendentives between them allowed the spaces below domes to be opened up. Pendentives allowed for weight loads to be concentrated at just four points on a more practical square plan, rather than a circle. Until the 9th century, domes were low with thick buttressing and did not project much into the exterior of their buildings. Drums were cylindrical when used and likewise low and thick. After the 9th century, domes were built higher and used polygonal drums decorated with engaged columns and arcades. Exterior dome decoration was more elaborate by the 12th century and included engaged columns along with niches, blind arcades, and string courses. Multiple domes on a single building were normal.

Domes were important elements of baptisteries, churches, and tombs. They were normally hemispherical and had, with occasional exceptions, windowed drums. Roofing for domes ranged from simple ceramic tile to more expensive, more durable, and more form-fitting lead sheeting. The domes and drums typically incorporated wooden tension rings at several levels to resist deformation in the mortar and allow for faster construction. Metal clamps between stone cornice blocks, metal tie rods, and metal chains were also used to stabilize domed buildings. Timber belts at the bases of domes helped to stabilize the walls below them during earthquakes, but the domes themselves remained vulnerable to collapse. The surviving ribbed or pumpkin dome examples in Constantinople are structurally equivalent and those techniques were used interchangeably, with the number of divisions corresponding to the number of windows. Aided by the small scale of churches after the 6th century, such ribbed domes could be built with formwork only for the ribs. Pumpkin domes could have been built in self-supporting rings and small domical vaults were effectively corbelled, dispensing with formwork altogether.

History

Late Republic and early Imperial period

Roman baths played a leading role in the development of domed construction in general, and monumental domes in particular. Modest domes in baths dating from the 2nd and 1st centuries BC are seen in Pompeii, in the cold rooms of the Terme Stabiane and the Terme del Foro. These domes are very conical in shape, similar to those on an Assyrian bas-relief found in Nineveh. At a Roman era tepidarium in Cabrera de Mar, Spain, a dome has been identified from the middle of the 2nd century BC that used a refined version of the parallel arch construction found in an earlier Hellenistic bath dome in Sicily. According to Vitruvius, the temperature and humidity of domed warm rooms could be regulated by raising or lowering bronze discs located under an oculus. Domes were particularly well suited to the hot rooms of baths circular in plan to facilitate even heating from the walls. However, the extensive use of domes did not occur before the 1st century AD.

Varro's book on agriculture describes an aviary with a wooden dome decorated with the eight winds that is compared by analogy to the eight winds depicted on the Tower of the Winds, which was built in Athens at about the same time. This aviary with its wooden dome may represent a fully developed type. Wooden domes in general would have allowed for very wide spans. Their earlier use may have inspired the development and introduction of large stone domes of previously unprecedented size. Complex wooden forms were necessary for dome centering and support during construction, and they seem to have eventually become more efficient and standardized over time.

Domes reached monumental size in the Roman Imperial period. Although imprints of the formwork itself have not survived, deformations from the ideal of up to at the so-called "Temple of Mercury" in Baiae suggest a centering of eight radiating frames, with horizontal connectors supporting radial formwork for the shallow dome. The building, actually a concrete ''frigidarium'' pool for a bathhouse, dates to either the late Roman Republic,Baiae, historic site, Italy
. '' Britannica.com''. Accessed 6 June 2021. or the reign of the first emperor Augustus (27 BC – 14 AD), making it the first large Roman dome. There are five openings in the dome: a circular oculus and four square skylights. The dome has a span of and is the largest known dome built before that of the Pantheon. It is also the earliest preserved concrete dome.

First century

While there are earlier examples in the Republican period and early Imperial period, the growth of domed construction increased under Emperor Nero and the Flavians in the 1st century AD, and during the 2nd century. Centrally planned halls become increasingly important parts of palace and palace villa layouts beginning in the 1st century, serving as state banqueting halls, audience rooms, or throne rooms. Formwork was arranged either horizontally or radially, but there is not enough surviving evidence from the 1st and 2nd centuries to say what was typical.

The opulent palace architecture of the Emperor Nero (54 – 68 AD) marks an important development. There is evidence of a dome in his Domus Transitoria at the intersection of two corridors, resting on four large piers, which may have had an oculus at the center. In Nero's Domus Aurea, or "Golden House", planned by Severus and Celer, the walls of a large octagonal room transition to an octagonal domical vault, which then transitions to a dome with an oculus. This is the earliest known example of a dome in the city of Rome itself.

The Domus Aurea was built after 64 AD and the dome was over in diameter. This octagonal and semicircular dome is made of concrete and the oculus is made of brick. The radial walls of the surrounding rooms buttress the dome, allowing the octagonal walls directly beneath it to contain large openings under flat arches and for the room itself to be unusually well lit. Because there is no indication that mosaic or other facing material had ever been applied to the surface of the dome, it may have been hidden behind a tent-like fabric canopy like the pavilion tents of Hellenistic (and earlier Persian) rulers. The oculus is unusually large, more than two-fifths the span of the room, and it may have served to support a lightweight lantern structure or tholos, which would have covered the opening. Circular channels on the upper surface of the oculus also support the idea that this lantern, perhaps itself domed, was the rotating dome referred to in written accounts.

According to Suetonius, the Domus Aurea had a dome that perpetually rotated on its base in imitation of the sky. It was reported in 2009 that newly discovered foundations of a round room may be those of a rotating domed dining hall. Also reported in contemporary sources is a ceiling over a dining hall in the palace fitted with pipes so that perfume could rain from the ceiling, although it is not known whether this was a feature of the same dome. The expensive and lavish decoration of the palace caused such scandal that it was abandoned soon after Nero's death and public buildings such as the Baths of Titus and the Colosseum were built at the site.

The only intact dome from the reign of Emperor Domitian is a wide example in what may have been a nymphaeum at his villa at Albano. It is now the church of . Domitian's 92 AD Domus Augustana established the apsidal semi-dome as an imperial motif. Square chambers in his palace on the Palatine Hill used pendentives to support domes. His palace contained three domes resting over walls with alternating apses and rectangular openings. An octagonal domed hall existed in the domestic wing. Unlike Nero's similar octagonal dome, its segments extended all the way to the oculus. The dining hall of this private palace, called the ''Coenatio Jovis'', or Dining Hall of Jupiter, contained a rotating ceiling like the one Nero had built, but with stars set into the simulated sky.

Second century

During the reign of Emperor Trajan, domes and semi-domes over exedras were standard elements of Roman architecture, possibly due to the efforts of Trajan's architect, Apollodorus of Damascus, who was famed for his engineering ability. Two rotundas in diameter were finished in 109 AD as part of the Baths of Trajan, built over the Domus Aurea, and exedras wide were built as part of the markets north-east of his forum. The architecture of Trajan's successor, Hadrian, continued this style. Three wide exedras at Trajan's Baths have patterns of coffering that, as in the later Pantheon, align with lower niches only on the axes and diagonals and, also as in the Pantheon, that alignment is sometimes with the ribs between the coffers, rather than with the coffers themselves.

The Pantheon in Rome, completed by Emperor Hadrian as part of the Baths of Agrippa, has the most famous, best preserved, and largest Roman dome. Its diameter was more than twice as wide as any known earlier dome. Although considered an example of Hadrianic architecture, there is brickstamp evidence that the rebuilding of the Pantheon in its present form was begun under Trajan. Speculation that the architect of the Pantheon was Apollodorus has not been proven, although there are stylistic commonalities between his large coffered half-domes at Trajan's Baths and the dome of the Pantheon. Other indicators that the designer was either Apollodorus or someone in his circle who was "closer in artistic sensibility to Trajan’s era than Hadrian’s" are the monumental size and the incorporation of tiny passages in the structure. The building's dimensions seem to reference Archimedes' treatise ''On the Sphere and Cylinder'', the dome may use rows of 28 coffers because 28 was considered by the Pythagoreans to be a perfect number, and the design balances its complexity with underlying geometrical simplicity. Dating from the 2nd century, it is an unreinforced concrete dome wide resting on a circular wall, or rotunda, thick. This rotunda, made of brick-faced concrete, contains a large number of relieving arches and voids. Seven interior niches and the entrance way divide the wall structurally into eight virtually independent piers. These openings and additional voids account for a quarter of the rotunda wall's volume. The only opening in the dome is the brick-lined oculus at the top, in diameter, that provides light and ventilation for the interior.

The shallow coffering in the dome accounts for a less than five percent reduction in the dome's mass, and is mostly decorative. The aggregate material hand-placed in the concrete is heaviest at the base of the dome and changes to lighter materials as the height increases, dramatically reducing the stresses in the finished structure. In fact, many commentators have cited the Pantheon as an example of the revolutionary possibilities for monolithic architecture provided by the use of Roman pozzolana concrete. However, vertical cracks seem to have developed very early, such that in practice the dome acts as an array of arches with a common keystone, rather than as a single unit. The exterior step-rings used to compress the "haunches" of the dome, which would not be necessary if the dome acted as a monolithic structure, may be an acknowledgement of this by the builders themselves. Such buttressing was common in Roman arch construction. The cracks in the dome can be seen from the upper internal chambers of the rotunda, but have been covered by re- rendering on the inside surface of the dome and by patching on the outside of the building. The Pantheon's roof was originally covered with gilt bronze tiles, but these were removed in 663 by Emperor Constans II and replaced with lead roofing.

The function of the Pantheon remains an open question. Strangely for a temple, its inscription, which attributes this third building at the site to the builder of the first, Marcus Agrippa, does not mention any god or group of gods. Its name, ''Pantheon'', comes from the Greek for "all gods" but is unofficial, and it was not included in the list of temples restored by Hadrian in the ''Historia Augusta''. Circular temples were small and rare, and Roman temples traditionally allowed for only one divinity per room. The Pantheon more resembles structures found in imperial palaces and baths. Hadrian is believed to have held court in the rotunda using the main apse opposite the entrance as a tribune, which may explain its very large size. Later Roman buildings similar to the Pantheon include a (c. 145) in the old Hellenistic city of Pergamon and the so-called "Round Temple" at Ostia (c. 230–240), which may have been related to the Imperial cult. The Pergamon dome was about 80 Roman feet wide, versus about 150 for the Pantheon, and made of brick over a cut stone rotunda. The Ostia dome was 60 Roman feet wide and made of brick-faced concrete. No later dome built in the Imperial era came close to the span of the Pantheon. It remained the largest dome in the world for more than a millennium and is still the world's largest unreinforced concrete dome.

Use of concrete facilitated the complex geometry of the octagonal domed hall at the 2nd century Small Thermal Baths of Hadrian's Villa in Tivoli. The vaulting has collapsed, but a virtual reconstruction suggests that the walls of the octagonal hall, which alternate flat and convex, merged into a spherical cap. Segmented domes made of radially concave wedges, or of alternating concave and flat wedges, appear under Hadrian in the 2nd century and most preserved examples of the style date from this period. Hadrian's villa has examples at the Piazza D'Oro and in the semidome of the Serapeum. Recorded details of the decoration of the segmented dome at the Piazza D'Oro suggests it was made to evoke a billowing tent, perhaps in imitation of the canopies used by Hellenistic kings. Other examples exist at the Hadrianic baths of Otricoli and the so-called "Temple of Venus" at Baiae. This style of dome required complex centering and radially oriented formwork to create its tight curves, and the earliest surviving direct evidence of radial formwork is found at the caldarium of the Large Baths at Hadrian's villa. Hadrian was an amateur architect and it was apparently domes of Hadrian's like these that Trajan's architect, Apollodorus of Damascus, derisively called "pumpkins" prior to Hadrian becoming emperor. According to Dio Cassius, the memory of this insult contributed to Hadrian as emperor having Apollodorus exiled and killed.

In the middle of the 2nd century, some of the largest domes were built near present-day Naples, as part of large bath complexes taking advantage of the volcanic hot springs in the area. At the bath complex at Baiae, there are remains of a collapsed dome spanning , called the "Temple of Venus", and a larger half-collapsed dome spanning called the "Temple of Diana". The dome of the "Temple of Diana", which may have been a nymphaeum as part of the bath complex, can be seen to have had an ogival section made of horizontal layers of mortared brick and capped with light tufa. It dates to the second half of the 2nd century and is the third largest dome known from the Roman world. The second largest is the collapsed "Temple of Apollo" built nearby along the shore of Lake Avernus. The span cannot be precisely measured due to its ruined state, but it was more than in diameter.

Octagonal rooms of the Baths of Antoninus in Carthage were covered with cloister vaults and have been dated to 145–160.

In the second half of the 2nd century in North Africa, a distinctive type of nozzle tube shape was developed in the tradition of the terracotta tube dome at the Hellenistic era baths of Morgantina, an idea that had been preserved in the use of interlocking terracotta pots for kiln roofs. This tube could be mass-produced on potter's wheels and interlocked to form a permanent centering for concrete domes, avoiding the use of wooden centering altogether. This spread mainly in the western Mediterranean.

Although rarely used, the pendentive dome was known in 2nd century Roman architecture and possibly earlier, in funerary monuments such as the ''Sedia dei Diavolo'' and the ''Torracio della Secchina'' on the Via Nomentana. Pendentive domes would be used much more widely in the Byzantine period. A "Roman tomb in Palestine at Kusr-en-Nêuijîs" had a pendentive dome over the square intersection of cruciform barrel vaults and has been dated to the 2nd century. A small dome on spherical pendentives at Beurey-Beauguay on the Côte-d'Or

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