The Incas' ingenious metalsmiths

When handling an alloy, metallurgists are careful not to poison the mix of metals with unwanted elements. Bismuth, for example, can make bronze brittle. However, it now appears that ancient Inca smiths of Machu Picchu learned how to use bismuth to give bronze new properties without such embrittlement.

This discovery adds a new dimension of sophistication to Inca metalwork, says Yale University metallurgist Robert B. Gordon.

It is more than an item of archaeological trivia. One of the overarching questions in anthropology is mankind's cultural evolution. Does it always proceed through a given sequence of stages inherent in human nature, or is the passage from primitive living to urban sophistication different each time?

The development of technical knowledge, such as metalworking, is an element of cultural evolution which specialists can study through archaeological remains. And since metallurgy developed independently in the Americas and in Eurasia, archaeologists have at least two examples of such cultural evolution to study.

''It is an opportunity to see how metallurgy developed independently,'' says Gordon, adding that this is ''what is remarkable about this study.''

Gordon, together with graduate student John W. Rutledge, recently reported discovery of the bismuth-containing bronze in Science. It forms a handle, shaped as a llama head, on a knife found in a cave at Machu Picchu, the Inca city high in the Peruvian Andes.

The bronze of this handle is much whiter than that of the knife blade and its shank. The latter have long been known to be made of a typical Inca copper-tin bronze alloy with 3 percent tin. The average composition of the handle turns out to contain 18 percent bismuth and 9 percent tin, an unusually high tin content for Inca bronze.

This appears to be the first known use of bismuth in bronze anywhere in the world. Moreover, it is used in a way that takes advantage of the new properties bismuth confers without suffering the drawback of making the alloy brittle.

Gordon and Rutledge explain that this reflects the microscopic structure of the alloy. The metal is made up of microscopic grains. Ordinarily, bismuth-containing grains penetrate grains of copper-tin - a penetration which makes the metal brittle.

x In modern copper-based bismuth-containing alloys such as brass the addition of zinc prevents such grain-boundary penetration. In the Inca knife handle, the addition of an unusually high amount of tin also inhibits grain-boundary penetration.

Thus the Inca metalsmiths discovered a subtle property of bronze alloys which they could put to practical use. And they did it without the benefit of high-power microscopes, handbooks of the properties of metals, or, as far as is known, a systematic process of research which we would recognize as a form of scientific thinking.

Gordon says he thinks the discovery of such a technique was part of a process that had been going on for a long time in the region. There is much evidence for long experimentation in the use of metals for decorative purposes. Metal artifacts appeared in the Peruvian area by 1000 BC. Bronze ornaments and some tools were widespread by 1,000 years ago. The knife handle of high-bismuth bronze is dated to AD 1476-1534, a period called Late Horizon Times by archaeologists.'

Gordon notes that someone could argue that the bismuth found its way into the bronze by accident, as a contaminant. But he considers that unlikely. The Inca smiths were skilled and careful workers. They may not have been ''scientific'' in their approach as we would use the term today. But they could learn much about the metals and alloys they handled by the feel of the metal as they worked it and by long observation.

Gordon says the Inca smith most likely added the bismuth deliberately to help make sounder castings, gain better adhesion between knife head and shank, or perhaps just to produce the unusual whitish color in the bronze.

Whatever the reason may have been, that smith left behind evidence of metalworking knowledge even more advanced than archaeologists had already recognized in those skillful people. Now Gordon wants to study other tools from that area to see if he can probe further into the ancient metalsmiths' trade secrets.