Inca Stonework and Building Techniques

Inca stonework and building techniques constitute the technical core of the architectural tradition of the Sapa Inca and the imperial state that ruled the Andes from the early fifteenth century until the Spanish conquest of 1532. The techniques include the selection of stone, the cutting of blocks, the transport of megaliths, the assembly of walls without mortar, the design of foundations, and the engineering of roof structures. They are most clearly expressed in the polygonal ashlar masonry of imperial Cusco, in the cyclopean walls of Sacsayhuamán, in the great walls of the Coricancha, in the twelve-angled stone on Calle Hatunrumiyoc, and in the imperial estates of Machu Picchu, Ollantaytambo, and Choquequirao. The imperial building program is famous for the precision of the joints between adjacent stones, for the absence of mortar in the most important walls, and for the remarkable seismic resilience of the resulting structures.

The imperial building techniques were developed during the period of consolidation of the Inca state, particularly during the reign of the ninth Sapa Inca, Pachacuti Inca Yupanqui, in the mid-fifteenth century. They drew on a long Andean tradition of stone construction that included the Wari (c. 600–1000 CE), Tiwanaku (c. 500–1000 CE), Chimú, and the Killke, but they added a much higher degree of refinement in the cutting of individual stones and a deliberate imperial iconography expressed in the puma, condor, and serpent forms used in urban planning. The techniques were the work of master stonemasons, who in Quechua were called pirqaq, and they were applied under the supervision of state engineers known as camayoc. The works of the pirqaq have prompted modern study both for their aesthetic refinement and, especially since the 1950 Cusco earthquake, for their engineering performance under seismic stress.

The categories of Inca masonry

Inca masonry is conventionally divided into several categories, based on the size of the stones, the regularity of their shaping, and the type of bond. Rustic masonry uses stones of irregular size, bonded with mud or clay mortar, and was used for vernacular housing, terrace walls, and most of the rural infrastructure of the empire. Coursed ashlar uses rectangular stones laid in horizontal courses, often of varying height, and is found in some Inca coastal buildings and in works associated with the Chimú tradition. Polygonal masonry uses stones with multiple, irregular sides that interlock tightly with their neighbors, and is the characteristic style of imperial Cusco, of the Coricancha, and of most of the buildings of Machu Picchu. Cyclopean masonry describes walls built of very large, partially shaped blocks, used especially in the lower courses of Sacsayhuamán. Mosaic-style masonry is rare in the imperial tradition and is found mostly in the architecture of the Chachapoya, in the cloud forest of the eastern Andes.

The imperial style is dominated by the polygonal mode, in which stones are shaped to fit their neighbors without mortar, and in which the joints between stones are not laid in continuous courses but follow a “scattered bond” pattern. The result is a wall of great visual sophistication, in which the stones appear to be locked together by the geometry of their surfaces, and in which the wall as a whole is capable of absorbing deformation under stress without losing integrity. The most famous single example of the polygonal mode is the Twelve-Angled Stone, set into a wall of the Hatunrumiyoc palace in Cusco, although the wall in which the stone is set contains many stones of comparable complexity.

The polygonal mode is not, however, a single homogeneous style. The Italian architect Enrico Cancian and the French researcher Jean-Pierre Protzen have shown, in detailed studies of the walls of Ollantaytambo and the Coricancha, that there are subtle but consistent differences in the cutting techniques used in different parts of the empire. Some walls were cut with very high precision and very small joints, while others were cut with larger joints and rougher surfaces. The differences appear to reflect local traditions of stonemasonry, the relative importance of the building, the period of construction, and the type of stone used. The diversity within the polygonal mode is one of the reasons that the imperial building program is so difficult to summarize in a single technical formula.

Quarrying, transport, and cutting of stones

Inca builders generally quarried stone within a few kilometers of the construction site, although at some sites, particularly at Ollantaytambo and at Sacsayhuamán, the stone was brought from specific quarries up to several kilometers away. The principal quarries associated with imperial projects include the bedrock quarries above Ollantaytambo, from which the six monoliths of the Temple of the Sun were taken, the quarries of Raqchi, from which blocks for several state buildings were extracted, and the pinkish granite of the Quorior quarry near Chinchero. The use of local stone had two practical advantages: it reduced the labor of transport, and it made finished buildings visually consistent with the surrounding bedrock.

Stones were detached from the parent rock through a combination of natural jointing, the careful use of fire, and the insertion of wooden wedges that were soaked with water. The expansion of the wood as it absorbed water split the rock along controlled lines. This technique, often described as “fire and water” quarrying, has been verified experimentally and observed in several Andean quarries. Once a block was detached, it was reduced in stages. A knapper first removed the rough excess with harder stone hammers, and the stone was then dressed with abrasives, including sand, water, and possibly volcanic ash, until its final form was achieved. The cutting of polygonal stones, known in Quechua as pircado, was performed in three principal stages.

The first stage of cutting was the reduction of the block to a rough shape, with a series of grooves cut at the locations of the future joints. The stone was then fitted into a temporary position in the wall and tapped with a small hammer until the contact points with neighboring stones were identified. These contact points were then carefully hammered down, producing small circular depressions, or sillares, separated by a network of fine lines. The final fit was achieved by abrading the contact points with sand and water until the stone settled fully into place. This process could take several days for each individual stone and required a level of skill passed down within particular families and communities of stonemasons.

The transport of dressed stones presented considerable logistical challenges. The largest megalithic blocks, such as the three stones of the Rumihuasi at Sacsayhuamán, each estimated to weigh between 120 and 200 tons, were moved several kilometers from their quarries to the construction site. Colonial chroniclers recorded that thousands of laborers cooperated to drag such blocks on log rollers. More commonly, however, individual stones weighed between several hundred kilograms and a few tons, well within the range that could be moved by teams of 12 to 40 men using ropes, levers, and wooden skids. Recent reconstructions of the Sun Temple at Ollantaytambo have shown that the joint between vertical members of a wall was typically cut and partially dressed at ground level, with the upper stones fitted into place during lifting, which required accurate surveying so that each stone met the dimensions of its neighbors to within a few millimeters.

The Twelve-Angled Stone

The Twelve-Angled Stone is a single block of green diorite that forms part of a wall of the former Hatunrumiyoc palace, now a colonial residence on Calle Hatunrumiyoc in the historic center of Cusco. The stone is a celebrated example of imperial polygonal masonry and an emblem of the city. Although the wall in which the stone is set contains numerous stones of comparable complexity, the Twelve-Angled Stone has become a popular landmark and a symbol of Cusco and of Inca stonework in general. The stone is estimated to weigh approximately two tons and is fitted into a wall of polygonal stones of various sizes, each joint meeting its neighbors within fractions of a millimeter.

The stone has twelve interior angles and thirteen exterior sides, each fitted precisely against its neighbors. The green diorite of which the stone is made is the same material as the bedrock of the region and was quarried within a few kilometers of the city. The wall in which the stone is set was probably built in the mid-fifteenth century, during the reign of Pachacuti Inca Yupanqui, and it is one of several surviving examples of imperial polygonal masonry in the historic center of Cusco. The wall has been restored several times since the colonial period, but the imperial polygonal stones remain in their original positions.

The Twelve-Angled Stone is often used as an emblem of the technical achievements of Inca stonemasons. The cutting of the stone, with twelve distinct faces, none of them planar and each fitted to a different neighbor, is technically demanding. The stone is also a useful illustration of the imperial building program: the wall in which it is set is part of a former palace of the panaca of Inca Roca, one of the early rulers of the dynasty, and the wall was incorporated into a colonial residence after the Spanish conquest of 1533. The stone, in other words, has survived for more than five centuries in continuous use, embedded in the architecture of the imperial city.

Mortarless construction

Inca polygonal walls are built without mortar of any kind, with stones held together by their own weight, by gravity, and by the precise matching of their surfaces. This mortarless construction has a striking visual effect: the joints are sometimes so close that they appear to be cracks in a single piece of stone. The technique is, however, more than an aesthetic statement. By avoiding mortar, the Inca created walls that are capable of significant deformation under stress without losing integrity.

The technique depends on the geometry of the stones and on the inward batter of the wall face, which together produce a self-locking structure. The polygonal shaping of the stones means that each stone is held in place by the geometry of its neighbors, with the weight of the upper courses pressing the lower courses inward against the wall. The joints between stones are not fixed but are designed to permit a small amount of motion. When the wall is loaded laterally, as it is in an earthquake, the stones can rock on their foundations and the joints can open and close without losing contact. The wall as a whole deforms but does not collapse.

The advantages of mortarless construction were demonstrated dramatically in 1950, when an earthquake of magnitude approximately 6.0 on the Richter scale struck the Cusco region on 21 May 1950. The earthquake destroyed many colonial buildings, including a significant portion of the cathedral, but Inca polygonal walls, including those of the Coricancha and the Sacsayhuamán site, generally withstood the shaking with little damage. Engineers, including those associated with the Pontificia Universidad Católica del Perú and later the Getty Conservation Institute, have shown that Inca walls absorb seismic energy by rocking on their foundations and by allowing adjacent stones to slide and then resettle into their original positions. This characteristic has led to a renewed interest in Inca building methods in the twenty-first century, particularly in seismic zones such as Japan, Italy, and Peru.

Seismic resilience

The seismic resilience of Inca mortarless construction is one of the most studied aspects of the imperial building program. Civil engineers have analyzed the joints, the foundations, and the wall geometry in detail, and they have shown that several features work together to produce the observed resilience. The polygonal geometry distributes the load through the wall, the inward batter of the wall face locks the stones against lateral movement, the rubble core absorbs minor irregularities, and the joints themselves are designed to permit a small amount of motion. The key feature is that the joints are not fixed but are designed to allow the wall to deform under stress.

The contrast between Inca and colonial buildings during the 1950 earthquake is striking in many places. At the church of Santo Domingo, which was built directly atop the Coricancha following the Spanish conquest, the colonial walls cracked and partially collapsed, while the Inca foundations remained intact. At the church of San Cristóbal in Cusco, built directly over an Inca foundation, the colonial structure failed in 1950, but the underlying Inca walls remained structurally sound. This pattern has been repeated in subsequent earthquakes, including the 1986 Cusco event and the 2007 Pisco earthquake, in which adobe and colonial masonry buildings suffered extensive damage while Inca constructions, where they were still standing, generally performed well.

The study of Inca joints has led to a renewed interest in the design of dry-stone walls in contemporary engineering. The Italian architect Enrico Cancian has proposed that the principles of Inca construction could be applied to the design of retaining walls in seismic zones, while Japanese engineers have studied Inca joints as a model for the design of base-isolated structures. The principles are not directly transferable to mass construction, in which the time and skill required to cut polygonal stones are not practical, but they have informed a broader understanding of the seismic performance of masonry.