Encyclopedia of the Incas

To rough out, cut, and dress stone, the Incas used simple river cobbles as
hammers. These tools and their fragments are found in abundance in the ancient
quarries scattered among roughed-out building blocks and in the quarrying
waste. The hammerstones are easily distinguished from other stones by both
their shape and their petrological characteristics; they are rounded stones of
materials significantly different from the quarried stone and the surrounding
bedrock. The hammers come in many sizes: some are as small as an egg and
weigh under 500 grams (17.6 ounces); others, the size of a football or larger,
weighing up to 8 kilograms (17.6 pounds) or more.
Stone masons used the largest of these hammerstones to break up and roughly
shape the raw stone, while the medium-sized ones served for dressing the faces
of the building blocks, and the smallest ones, for drafting and cutting their edges
and corners. The technique involved is exactly as the chronicler Garcilaso de la
Vega described it: “Stonemasons similarly worked their stone with some black
pebbles called hihuana [sic for hihuaya], with which they pounded rather than
cut” (Garcilaso 1966 [1609]).
When hitting the workpiece straight on with a hammerstone, the rock is
crushed, producing little more than dust. But if the angle of impact is increased
to about 15–20 degrees, little chips flake off. By further increasing the angle to
some 45 degrees by imparting a twist to the hammer just before impact, even
larger chips can be removed, accelerating the process considerably. The impact
of the hammer leaves a small pit mark on the workpiece. Such pit marks can be
observed on every face of every building block in every Inca wall of cut stone,
regardless of the building blocks’ material. Smaller, finer pit marks found along
the edges of the building blocks indicate that smaller stones were used to cut the
edges. This particular edge-cutting technique requires that the edge be shaped by
hitting the workpiece with grazing blows directed away from the workpiece,
resulting in corners with dihedral angles larger than 90 degrees. It is these obtuse
angles that account for the characteristic beveled joints of Inca cut stonemasonry.