© Ivan W. Watkins, Professor of Geoscience in the Department Of Earth Sciences, St. Cloud State University, Minnesota

[The following is the text only, without the diagrams, of an article that appeared in Rocks and Minerals magazine, Vol. 65, Nov/Dec 1990. My thanks to correspondent Wayne Van Kirk for bringing it to my attention and forwarding a copy. See also here for an excellent photographic example of Inca masonry.]

At its height the Inca Empire stretched from Quito, Equador, to Santiago, Chile, along the Andes Mountains of South America. In 1532 Pizarro and the Conquistadores captured the last Inca, Atahualpa, putting an end to this great empire. The Spaniards then proceeded to loot the empire of its vast riches of gold and silver; they melted the precious metal objects and cast them into ingots, which they shipped back to Spain. The "great golden dish two men across" found in the temple of the sun was cut into pieces for gambling chips before being melted (Garcilaso de la Vega, 1961). The Conquistadores did nor ask for what purposes the gold was used; they knew only that the metals were of great value in ingot form. Surely those savages could not have used the gold for anything but ornaments and barbarous religious trinkets; surely they couldn't have used it in their magnificent stonemasonry projects that remain at Machu Picchu and other Inca ruins (figure 1).

Several hypotheses have been advanced to explain how the Inca, stonemasons could cut stones to fit together so tightly that not even a piece of paper can be slid between them. In fact, some of the stones fit so tightly that I could not even blow air between them. Protzen (1986), following Bingham (1913), concluded that the stonework had been done by pounding with quartzite hammers. Jessup (1934) and Goetz (1942) championed grinding and polishing with sand and water. Bingham (1913) and Frank (1980) thought that the stones had been cut and shaped by wedging, using wood, metal, freezing water, or the expansion of heated vermiculite. Arnold (1983) proposed the use of organic acids.


Hammering: If one tries to shape a stone with a hammer (Bingham, 1913; Protzen, 1986), the smallest inside corner that can be produced must have a radius larger than the smallest radius of the hammer. The mason must be able to strike the material that is to be removed. Therefore, an observer must not see inside comers with radii smaller than than the size of the hammers used to produce those corners. Figure 2 shows one of many inside comers that have very small radii. To fashion these, the stone hammers would have to have been extremely small chisels. Furthermore, when a rock is hammered it tends to break selectively along planes of weakness, such as mineralized fractures. Even if it does not break completely, the rock will chip out at the intersection of the cut surface and the fracture, producing a groove at the intersection. Thus, fractures as shown in figure 3 would not exist. It is concluded that the blocks could not have been cut and shaped by hammering.

Grinding and polishing: This process, suggested by Goetz (1942), could produce inside comers of varying radii as well as cut surfaces intersecting low-angle fractures. However, if a rock containing quartz is abraded with river sand, which is mostly quartz, the softer minerals in the rock will be sanded out, while the quartz grains will hardly be scratched. Indeed, sanding granite with quartz should result in quartz grains standing in relief above the rest of the sanded surface. While I have no illustration to show that this relationship does not exist, I did observe that there were no quartz grains standing above the rest of the minerals on the surface. Thus, I conclude that those beautiful Inca stone surfaces could not have been produced by sanding.

Wedging: If wedging (Bingham, 1913; Frank, 1980) had been used, neither inside comers nor low-angle fractures intersecting cut surfaces could be present. When wedging, the mason cannot stop a crack at the desired point, nor can he produce inside comers of diverse radii. During wedging, rock will selectively break along any preexisting low-angle fracture. Thus, there can be no low-angle fractures. Therefore, one can conclude that wedging could not have been used to produce those surfaces.

Chemical processes: Most minerals in rock may be slowly changed by chemical reaction with organic acids and may produce compounds that are soluble in water. Cutting with organic acids (Amold, 1983) might be possible if the acid could be kept in contact with the rock long enough for the reactions to take place. A water solution is required for ion transfer to take place during the reaction. A string wetted with the acid and placed against the stone would then make a reaction line from which the soluble compounds could be washed. The water solution, however, would surely get into preexisting fractures so that the rock would hardly have cut surfaces intersecting low-angle mineralized fractures such as the one shown in figure 2. Furthermore, since most acids do not react with quartz, it seems likely that the quartz grains would stand above the surface. It is concluded that the rocks could not have been cut and polished by acids.

A New Hypothesis

David Lindroth (personal communication, 1986) has been working for years with the thermal disaggregation of rock at the U.S. Bureau of Mines, Twin Cities Research Center. Lindroth has shown that with 100 watts of light energy focused to a circle about 2 mm in diameter any rock can be cut. While each kerf is only about 2 mm deep, a rock of any size can be cut by repeated passes. He has also found that quartzite spalls very easily, while a rock like basalt does not spall, but melts.

The Cold Spring Granite Company plant in Cold Spring, Minnesota, uses a flame to produce surfaces on some of the tiles they sell. Nearly all the quartz, which has a high thermal expansivity compared to the other minerals in granite, is spalled from the surface. To rid the spalled material from flamed surfaces, a stream of water is sprayed away from the flame. When the spalled material is not removed, it melts and forms a glaze on the surface. Several quarries use a fuel oil and compressed air torch to quarry large blocks of granite.

The rock surfaces on Inca stones are similar to those that have been thermally disaggregated. Indeed, some of. the slick surfaces on the Inca building stones are glazed, so it becomes apparent that the Incas must have used thermal disaggregation. But what was the source of the energy?

Garcilaso de la Vega (1961) wrote about the Festival of the Sun that still takes place each year in Cuzco, Peru.

The fire used for this sacrifice had to be fresh or, as they said, given to them by the hand of the Sun. For this, they took a large bracelet, belonging to the high priest, and similar in form to that usually worn on their left wrists by the Incas. The central motif of this bracelet was a very carefully polished concavity as big as half an orange. They turned this to the sun to capture its rays, which they then concentrated on a small wisp of very dry, fluffy cotton, that caught fire instantly.

So, it seems possible, if not probable, that the Incas used sunlight for their energy source.

The Incas to whom Garcilaso refers were actually only the emperors, who obviously knew how to concentrate solar energy. Peru is largely high and dry and just south of the equator, where sunlight is available all year long. The great god of their culture was Inti, the sun god, and the Incas mined and used vast amounts of gold. Because gold is a wonderful reflector of solar energy, it seems likely that the thermal energy source used by the Incas to cut the stones was solar energy concentrated by gold mirrors.

Consequently, I propose the following hypothesis to explain how the Incas cut and polished their incredible stonework. First, the energy used was solar energy focused with large parabolic gold mirrors (figure 4). The reflectors could be made of any focal length, or combined with plane mirrors to cut in any direction at essentially any distance from the large primary reflector (Watkins, 1986). The depth and width of each kerf would be determined by the size, arrangement, and speed of motion of the reflectors. Second, the fitting together of the stones could be accomplished by using a thin beam of light that would leave an equal distance between the stones, regardless or the shape of the cut. This would be similar to the way modern builders of log cabins run chain saws between logs to make them fit tightly together. On the last cut, they could leave any spalled material so that it would melt to form a glass cement (glaze) that would fill and seal any remaining irregularities.

How did the Incas create such beautiful stonemasonry? After much observation and careful thought, my conclusion is the Incas created their beautiful stonemasonry using sunlight focused with gold parabolic reflectors.

Additional Considerations

At one of the quarry sites at Machu Picchu there is a semicylinder where the fractures in the granite are healed by a glassy material. That surface had to have been severely heated. It would have been extremely difficult to build a fire to get that pattern. Reflected solar energy would have done the trick nicely.

The post at Machu Picchu (figure 4) and a similar post at Pisac were probably used to support the heavy gold disk that was used for the primary reflector. It would have been very tiring to control such a disk without some support.

The glazed slippery slide Rodadero, at Sacsayhuaman (figure 5), a "porcelaneous surface" (Ericksen, 1987) on the dacite country rock; a smaller glazed slide found at Machu Picchu cut into the granite country rock; and the slide in metamarl at Ollantaytambo all display smooth glazed surfaces that could be achieved by the high temperatures easily obtained with large solar reflectors.

The very fine inside corners with a fracture right through one of the corners (figure 1) that were carved into the red granite used at Ollantaytambo could be cut with a focused beam of light but would be very difficult to make any other way.

Source References

Arnold, D. E. 1983. Stoneworking in ancient Peru: Reply to Frank. Archaoeometry 25: 87-90.

Bingham, H. 1913. In the wonderland of Peru. National Geographic 24: 487-494.

Ericksen. G. 1987. Letter. Geology 15:96.

Frank, E. 1980. Stoneworking in ancient Peru. Archaeometry 22:211-212.

Garcilaso de la Vega, 1961. The Incas. New York: The Orion Press Inc.

Goetz, D. 1942. The Incas. Baltimore: Pan American Union Press.

Jessup, M. K. 1934. Inca masonry at Cuzco. American Anthropologist 36: 239-241.

Lindroth, D. 1986. U. S. Dept. of the Interior, Bureau of Mines, Twin Cities Research Center, personal communication.

Protzen, J.-P. l986. Inca stonemasonry. Scientific Amer. 254: 94-105.

Watkins, I. 1986. Solar powered focusing and directing apparatus for cutting, shaping, and polishing, U.S. Patent No. 4,611,857.