The Hagia Sophia was built using brick, stone, lime mortar, and an enormous variety of colored marbles sourced from across the Byzantine Empire. Completed in 537 CE under Emperor Justinian I, the building’s materials were chosen not just for beauty but for structural performance, and the specific combination of flexible mortar and lightweight brick is a major reason the structure has survived nearly 1,500 years of earthquakes.
Brick and Stone: The Structural Core
The walls, arches, and dome of the Hagia Sophia are primarily brick masonry. Byzantine builders used thin, flat bricks roughly 5 centimeters thick, layered with generous amounts of mortar. In fact, the mortar joints were often 1.1 times the thickness of the bricks themselves, meaning there is nearly as much mortar as brick in the walls. This unusually high mortar-to-brick ratio was intentional: the thick mortar layers gave the structure flexibility, allowing it to absorb and redistribute stress during seismic events rather than cracking apart.
The main piers that support the massive dome are a different story. These were built from large, rigid stone blocks bedded in mortar, providing the compressive strength needed to carry the dome’s enormous weight. So the building uses two complementary systems: stiff stone piers to bear vertical loads and flexible brick-and-mortar walls and arches to handle lateral forces from wind and earthquakes.
The Mortar That Held It All Together
The mortar used in the Hagia Sophia is one of its most remarkable materials. Byzantine builders made a crushed-brick-and-lime mortar, mixing powdered brick into a lime binder at ratios ranging from 1 part binder to 2 parts aggregate up to 1 part binder to 4 parts aggregate, depending on where in the structure it was applied. The crushed brick wasn’t just filler. It acted as a pozzolanic addite, meaning it reacted chemically with the lime to create a slow-curing, cement-like material with genuine hydraulic properties.
This mortar continued to harden and gain strength over centuries, which is why researchers classify it as an advanced cement-based composite rather than a simple lime paste. Tensile strength tests on original mortar-brick samples taken from a dome rib during 1949 repairs measured 0.4 to 0.5 megapascals, a respectable figure that confirms the mortar still maintains a strong bond after more than a millennium. Different areas of the building used different mortar recipes: hot-lime mortars with crushed brick in some zones, typical lime mortars in others, reflecting a sophisticated understanding of where the structure needed flexibility versus rigidity.
Marbles From Across the Empire
Justinian sourced decorative stone from quarries spanning the eastern Mediterranean, and the variety of marble inside the Hagia Sophia was meant to showcase imperial reach and wealth. Several distinct types appear throughout the building:
- Proconnesian marble: A white-to-gray stone quarried from Proconnesus Island (modern Marmara Island) in the Sea of Marmara, used extensively for columns, capitals, and wall cladding. This was the most readily available high-quality marble near Constantinople.
- Verde antico: A deep green marble from Thessaly in Greece, used for columns in the nave and in the prostoon (entrance area), where it was paired with red and white veined marbles for visual contrast.
- Porphyry: A distinctive purple-red stone from Egypt, prized across the Roman and Byzantine worlds as a symbol of imperial power.
- Iasian marble: A red-and-white stone from the Iasos region of southwestern Turkey, used for elements like the templon stylobate and column bases in the sanctuary area.
- Onyx: Honey-colored translucent stone panels that likely came from Egypt, though Cappadocia is another possible source. The 6th-century poet Paul the Silentiary described these panels in detail but never specified exactly where they originated.
- Breccia corallina: A pale yellow stone with swirling red patterns, only recently identified among the Hagia Sophia’s stone panels. Quarry discoveries near Manisa and the Karaburun peninsula in western Turkey suggest these slabs came from the ancient region of Lydia.
Many of these marbles were cut into thin slabs and arranged in mirror-image pairs on the walls, a technique called bookmatching that created symmetrical, almost organic patterns. The overall effect was a shimmering interior where every surface displayed a different color and texture of stone.
Gold Mosaics and Colored Glass
The upper walls, vaults, and dome of the Hagia Sophia were covered in mosaics made from glass tesserae, small cubes roughly a centimeter across. The backgrounds of these mosaics used gold and silver tesserae: tiny pieces of glass with a thin leaf of gold or silver sandwiched between two layers of clear glass. When set into the curved surfaces of domes and arches at slightly varying angles, these gold tesserae caught and scattered light, creating the famous shimmering glow that made the interior feel almost weightless.
For the figural and decorative portions of the mosaics, Byzantine craftsmen produced colored glass tesserae using a soda-lime-silica base glass. They achieved a wide range of colors by adding specific metallic elements: iron for browns and yellows, copper for greens and reds, cobalt for deep blues, and manganese for purples. To make opaque colors, they added tin-based compounds, a technique that became standard from the 4th century onward. Calcium phosphate was occasionally used as an opacifier starting in the 5th century. The combination of these coloring and opacifying agents gave mosaic artists a full palette to work with, from flesh tones to vivid blues and rich reds.
Lightweight Materials in the Dome
The dome of the Hagia Sophia spans roughly 31 meters and rises about 55 meters above the floor. To make this possible without the dome’s own weight collapsing the structure, builders used especially lightweight bricks, reportedly made from clay found on the island of Rhodes. Ancient sources describe these bricks as so light that twelve of them weighed the same as a single ordinary brick. While that ratio is likely exaggerated, the principle is confirmed by the structure itself: the dome bricks are thinner and less dense than those used in the walls below.
The mortar in the dome also played a structural role. Because the mortar layers were so thick relative to the bricks, a significant portion of the dome’s mass is actually mortar rather than fired clay. As this mortar cured slowly over decades and centuries, the dome effectively became a single, monolithic shell rather than a collection of individual bricks, distributing stress more evenly across its curved surface.
Iron and Lead Reinforcements
Metal played a supporting role in several parts of the structure. Iron clamps and ties were used to connect stone blocks in the main piers and to reinforce critical joints where arches met walls. Lead sheets served as cushioning layers between stone elements, absorbing minor movements and preventing hard stone-on-stone contact that could cause cracking. Lead was also poured molten into joints to seal and stabilize connections. The roof was originally covered with lead sheeting, which provided waterproofing and added a small amount of beneficial weight to help hold the structure in compression.