Cutting and Fitting Massive Stones in Inca Architecture
The cutting and fitting of massive stones is the most distinctive technical feature of Inca architecture and the basis of the imperial building style. The Inca cut stones of irregular polygonal shape with as many as twelve interior angles, fitted them together without mortar, and assembled them into walls and buildings of extraordinary precision. The most refined examples of the technique are found in the imperial core of Cusco, at the Coricancha, and at royal estates such as Machu Picchu and Ollantaytambo. The methods used by the Inca have been reconstructed from the surviving tool marks on the stones, from the accounts of colonial chroniclers, from the study of preserved and partially restored walls, and from the experimental work of modern engineers and archaeologists.
Quarries and the “fire and water” technique
Inca builders generally quarried their stone within a few kilometers of the construction site, although the largest blocks, including the Rumihuasi monoliths of Sacsayhuamán and the six monoliths of the Temple of the Sun at Ollantaytambo, were brought from quarries up to several kilometers away. The use of local stone, sometimes taken directly from the bedrock on which a building was to stand, had the advantage of reducing the labor of transport and of making the finished structure visually consistent with the surrounding landscape. Major imperial quarries are still visible in the Cusco region, including the bedrock quarry of Quorior above Chinchero, the quarries of Cachicata above Ollantaytambo, and the pinkish granite outcrops of the Huaccoto range.
The technique of detaching a block from the parent rock is often described as the “fire and water” method. The stone was first heated by a fire built against the rock face, and then cold water was poured over the heated surface. The thermal shock caused the rock to crack along existing lines of weakness, and the crack was then enlarged by driving in wooden wedges, which were subsequently soaked with water so that the expansion of the wood split the rock along controlled lines. The method has been verified experimentally by modern researchers who have reconstructed quarrying operations at several Andean sites since the 1960s, and by the Italian architect Enrico Cancian, who has published detailed studies of the pircado, the process of cutting and fitting the stones.
The “fire and water” technique was used for the larger blocks but was not the only method available. Smaller stones were detached with simple stone hammers, and the smallest were obtained from the surface as fieldstones. The largest blocks, such as the Sayri Titi of Sacsayhuamán, with dimensions of roughly 8.5 by 4.5 by 4 meters and a mass that modern estimates place in the range of 120 to 140 tons, required the cooperation of hundreds of laborers over a period of months, but the typical block of an imperial wall weighed between 100 kilograms and a few tons and could be reduced to shape in a few days.
Hammer dressing and abrasion
Once a block had been detached from the quarry, it was reduced to a rough shape by a series of stone hammers, of which the most common was a hard river cobble of granite or quartzite, weighing up to 4 kilograms. The knapper held the cobble and struck the block at the points where the excess stone needed to be removed, leaving a series of small depressions, or sillares, on the surface. The depressions, visible today on many of the most famous walls of Cusco, are often taken as a sign of hand-dressed stonework, and they are a regular feature of the imperial building style.
After the rough shape had been obtained, the block was dressed with abrasives. The most common abrasive was a sand made of crushed quartz or of volcanic ash, applied with water. The block was rubbed with the sand-water mixture until the surface was smooth and the joint faces were flat. The process could be carried out at the quarry, on a temporary platform at the construction site, or in a final fitting once the block had been placed in position in the wall.
The final fitting of the block was the most demanding part of the process. The stone was set temporarily in its place in the wall and tapped with a small hammer until the contact points with the neighboring stones were identified. The contact points were then carefully dressed, and the process was repeated until the stone settled fully into place, with the joints between it and its neighbors so close that a thin blade could not be inserted between them. The final fit could take several days for a single block and required a level of skill that was passed down within particular families and communities of stonemasons.
Tools and abrasives
The principal tools used by the Inca stonemason were stone hammers, copper and bronze chisels, and abrasives. Stone hammers, of which many examples survive, were used for the rough reduction of the block and for the initial shaping of the joint faces. Copper and bronze chisels, including the characteristic T-shaped Inca chisel, were used for the more refined work and for the cutting of small details. Abrasives included sand, gravel, and volcanic ash, often applied with water in a stone basin or on a flat slab.
The Inca did not have iron tools in the pre-conquest period, and the absence of iron has been the subject of much scholarly debate. Some early commentators suggested that the stones were softened with the juice of a plant, a claim without archaeological support. Modern studies of tool marks, including the work of Jean-Pierre Protzen and his students at the University of California, Berkeley, have shown that the techniques used by the Inca were technically demanding but entirely practical, and that the precision of the joints was the result of skilled labor and patient abrasion rather than of any unusual technology.
Scattered bond and the three-layer wall
The Inca did not lay walls in continuous horizontal courses. Instead, they used what modern engineers call “scattered bond” construction: stones of varying sizes and shapes were placed so that the joints between any two courses did not align vertically. This technique, combined with the polygonal shaping of the stones themselves, distributed loads through the wall and produced an extremely rigid structure.
Walls of the imperial type were typically built in three layers. The outer facing of carefully cut polygonal stones carried the architectural appearance and the principal structural load, and the second facing on the interior of the wall provided additional rigidity. Between the two facings lay a rubble and mud core, which absorbed minor irregularities in the cut of the facing stones and helped to bind the wall together. The core did not fully fill the gap between the two facings, and the resulting cavity allowed the wall to absorb seismic motion without losing integrity.
The thickness of the walls depended on the function of the building. Walls of the imperial palace, the Coricancha, and the principal temples were typically between 1 and 2 meters thick, while the lower courses of the cyclopean walls of Sacsayhuamán were as much as 8 meters thick. The inward batter of the wall face, which was a characteristic feature of imperial masonry, gave the walls additional stability and helped to channel rainwater away from the foundation.