History of Roman and Byzantine 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. Stone or brick ribs were usually flush with the inside surface of Roman domes where they would not have been visible. The audience halls of many imperial palaces were domed. Domes were "closely associated with senatorial, imperial, and state-sponsored patrons" and proliferated in the capital cities and other cities with imperial affiliations. Domes were also very common over polygonal garden pavilions. Depictions on late Roman coins suggest that wooden bulbous domes sheathed in metal were used on late Roman towers in the eastern portion of the empire. 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. A "so-called 'Athenian dome'" type had an octagonal drum with a corrugated cornice of semi-circular eaves and a thin marble column at each corner. Typically there was a single window on each side of the octagon.
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. 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. The "so-called " is a domed Greek cross structure dated to either the 1st century BC or the 1st century AD. The hemispherical dome was made from large stone ashlar blocks pierced by four holes with shafts extending diagonally up to the outside surface.
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 bath, dates to either the late Roman Republic, or the reign of the first emperor Augustus, 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.