In 1561, the vice regal court of Los Reyes (now Lima), Peru, heard testimony from the town of Xauxa, listing the items that the Spanish conquistador Francisco Pizarro and his soldiers had taken or looted from the Inca imperial storehouses. The list was read not from a manuscript or a tablet, but from a bundle of knotted strings called a khipu, which the Incas used for a wide variety of record-keeping, from the state census to the genealogies of Inca kings.
On Tuesday, May 2, a giant facsimile of a segment of the Xauxa khipu was unveiled in the fifth-floor lobby of Building 16. More than 50 students and faculty gathered for the opening, which was presided over by Heather Lechtman, Professor of Archaeology and Ancient Technology in the Department of Materials Science and Engineering.
The khipu was one of several products made by the students in Course 3.094, "Materials in Human Experience," a class that Lechtman taught jointly this semester with Professor Linn Hobbs. The course deals with how people select, manipulate, and use materials.
"It puts people at center stage," Lechtman said. In each of three case studies -- in metal, fiber and ceramics -- the students not only studied the materials themselves, but also the human context in which people "turn materials of nature into items of culture."
Lechtman points out that there are many unsolved issues still surrounding the khipu. "The cords were made according to strict rules and protocols, the colors and the use of Z or S knots were deliberately chosen," she says. The position and type of each knot and the color and twist of each cord conveyed meaning, but that meaning is for the most part lost.
Archaeologists are currently investigating some 600 remaining khipu, in an effort to determine whether any of the khipu may have constituted a system of writing. Nevertheless, it is clear that the khipu played a central role in the cohesion of the Inca state, "the last Andean empire and the largest in the Americas before the Spanish invasion," Lechtman observes. It allowed important information to be communicated in a society that apparently had no writing system, she adds.
In all of this, the maker and interpreter of the khipu -- called a khipu kamayuq -- emerges as a central figure. "The maker was also its guardian," she says. It is known that the Incas calculated using base 10: In the MIT khipu, the top row of knots on the pendant cords represents items numbering 104, the next row 103, and so on down to single units.
Sean Morton, a sophomore in materials science and engineering, was on hand to demonstrate the students' cord-making technique. For the khipu project, students studied the mechanical properties, such as the tensile strength of the fibers favored by the Incas, which included cotton and camelid wool. They learned how to create different types of cord, with different color patterns and thicknesses, as well as the various kinds of knots the khipu kamayuq used to distinguish among many categories and numbers of recorded items. Through experimentation, Morton explained, they calculated that you lose 20 percent to 30 percent of the length of a cord when you ply cords together, and that the strength is proportional to the ply.
But beyond addressing the physical properties of khipu materials, the course also delved into their cultural meanings. Fiber was to the Incas what metal or stone was to other societies, such as the ancient Greeks and Romans. "When Andean peoples had a problem that needed to be engineered," says Lechtman, "they turned to fiber." Thus the Incas used fiber to make suspension bridges, hunting and fishing gear, roofing, portage containers, weapons, and even armor -- and, of course, khipu -- so fiber objects consequently became "tools of power and politics" in the Inca state.
Morton may speak for many of the 3.094 students in saying he was attracted by the "hands-on methods" used in the course. In the metal-case study, students made copper-silver alloys from malachite (a copper ore) and silver metal. They first "won the copper from the ore," as Lechtman puts it, by smelting it in a crucible with charcoal, then alloyed it with silver metal to form an ingot. They hammered and annealed the ingot many times to change its shape and pickled the surface with dilute sulfuric acid after each anneal to remove the dark copper oxide scale.
"We're pretty sure that naturally occurring corrosive minerals were used by the Andean peoples," says Lechtman, "but we used sulfuric acid." After repeated annealing and pickling, sufficient copper was removed so that an enriched silver layer formed at the surface of the alloy ingot, and the metal displayed a silver color. The copper-silver alloy system was one of the most important techniques Andean smiths developed before the Spanish invaded.
A version of this article appeared in MIT Tech Talk on May 10, 2006 (download PDF).