Lost City of the Incas (Phoenix Press) by Hiram Bingham Page B

and tin occur in South America this is an untenable thesis, for such ores are not found in combination. It is well known that during World War II enormous quantities of tin were recovered from Bolivian mines and manufacturers were delighted to secure supplies to take the place of those that came from the Straits Settlements before the Japanese occupation. It is also well known that enormous deposits of copper are found in Peru and in Chile but not in combination with tin.
    My friend Professor Charles H. Matthewson of Yale University was the first modern metallurgist to make an exhaustive study of Inca bronzes. He discovered that the percentage of tin contained in Inca bronzes was not governed by the uses for which they were intended, but by the requirements of the ancient methods of manufacture. Everything that we know about Inca metallurgy is based on Professor Matthewson’s report.
    The Incas learned the interesting fact that bronze containing a high percentage of tin yields the best impression in casting,because during the process of solidifying it expands more than bronze having a low tin content. Hence the more delicate or ornamental pieces contain the highest percentage of tin, enabling artistic details to be more strikingly brought out in the finished product. Of course, had the Incas possessed steel graving tools the case would have been different. The Inca metallurgists learned that the operation of casting small delicate objects is facilitated when there is about 10 per cent of tin in the mixture. Such alloys retain their initial heat longer and so remain longer in a fluid condition. Since small objects tend to cool rapidly this knowledge was particularly useful in the manufacture of ornamental shawl-pins and ear spoons and accounts for the higher percentage of tin the Incas used in making them.
    Since these early metallurgists were unfamiliar with modern methods of heat treatment they were compelled to sacrifice the extra hardness and strength obtainable in casting axes and chisels by increasing the tin content in them. Such implements had to be frequently hammered and annealed. Since cold-working had to be depended upon to produce the final hardness of such objects, more than one heating was needed in forging the blades, and this process necessitated a low tin content. Necessarily they employed a formula for combined copper and tin which has impressed archaeologists familiar only with the chemical analysis of Inca bronzes, as being that which is unsuited for axes, chisels, and large knives. It was only after a metallographic study of Inca bronzes, involving the mutilation of the pieces examined, that Professor Matthewson learned the structure of such objects, the methods of their manufacture, and the reasons for the variation that has been found to exist. The Inca metallurgists cast their bronze knives generally in one piece and then cold-worked them. Such reheating as took place was solely for the purpose of softening the metal to facilitate cold-working, which was probably done at less than red heat. Some Inca bronzes are found to have been repeatedly hammered and reheated. This hammering might have been done with the stone tools with which the Incas were familiar.
    The knife blades appear to have been worked and hammeredso as to extend the metal more or less uniformly in several directions. Chisels and axes, on the other hand, were cast practically in the shape finally desired.
    The Inca metallurgists were sufficiently ingenious to use more than one variety of bronze in the construction of artistic knives. Their knives were in the shape of an inverted ‘t’. If it was desired to ornament the end of the handle with a llama’s head or attractive bird, the ornament would be made of bronze with a high content of tin. The metal of the blade and the lower part of the handle on the other hand was of bronze of lower tin content because the blades had to be cold-worked.
    The ornamental part of the knife handle was