In the succession of prehistoric and primitive times the “bronze age” precedes the “iron age.” This fact is sometimes mentioned with surprise by persons not familiar with metallurgy because they suppose that a simple metal, like iron, would naturally come into use before a compound metal or alloy, such as bronze. No doubt this would have been the case, and the iron age could have preceded the bronze age but for the fact that it is difficult to obtain iron from its ores, a greater degree of heat being required for its smelting than the earliest metal workers could command in their rude furnaces built in the ground and heated with wood.
Iron, tin and copper
On the other hand, copper and tin, the two metals by whose combination bronze is produced, can be smelted with comparative ease. That iron would not have been so late in coming into use it it could have been readily obtained is shown by the eagerness with which primitive races have utilized native masses of nearly pure iron — like the huge meteorite found in Greenland now to be seen in the Museum of Natural History — for making knives and other implements.
The object of combining a certain percentage of tin with copper, thus making the alloy called bronze, was to obtain a metal harder than either of its two components, and so capable of being ground to a cutting edge. What primitive men especially wanted were sharp tools and weapons. As the arts progressed an additional reason for the preference of bronze is found in its superior beauty, and the variety of shades of color that may be imparted to it by varying the percentage of the alloy and in other ways.
The bronze age prevailed in many parts of the world and among many races, but it was by no means synchronous everywhere. Among other places where bronze was used before iron were Bolivia and Peru in the days of the Incas. It has long been a disputed question whether these people dwelling along the Andes invented bronze or merely obtained it by accident from copper ores that happened, as occasionally occurs, to be mingled with a little tin.
This problem seems at last to have been solved by analysis of prehistoric bronze and copper objects found in Peru and Bolivia, and possessed by the American Museum of Natural History and the Peabody Museum. Mr Charles W Mead, writing in the Museum Journal, shows how the conclusion that the Peruvians purposely mingled tin with copper, and did not stumble over the discovery, accords with the historic evidence furnished by the chroniclers of the Spanish conquest under Pizarro.
It appears that as much as 13 percent of tin was sometimes mingled with the copper, and the ancient Peruvians are credited with having possessed considerable metallurgical skill. In the ruins of the remarkable mountain town of Machu Picchu, believed to have been built two thousand years ago, then “lost” until its discovery a few years since by Dr Hiram Bingham, a large piece of pure tin was found, which had first been beaten out in a thick flat sheet and then rolled up “like a sandwich.”
From this mass of tin it is believed the metallurgists of Machu Picchu cut off pieces, as wanted, to smelt with copper in preparing bronze. Many bronze objects were found in the ruins possessing varying amounts of tin from 5 to 9 percent and showing artistic skill and taste in their fabrication. In accidental mixtures of tin with copper, the amount of tin averages about 2 percent, and there is often a slight intermixture of other metals.
Strictly speaking, Mr Mead says, the term “copper” should be applied to all implements which contain 96 percent or more of this metal, the remaining 4 percent being a mixture of two or more other metals. Assays of the Bolivian copper ores show that they did not contain tin, so that “accidental bronze” could not have been produced from them; while, on the other hand, the Bolivian cities are particularly rich in bronze objects.
These facts add greatly to the interest of the ancient civilization that gathered along the mighty mountain backbone of South America, and emphasize the assertion of Dr Bingham, the discoverer of Machu Picchu, that “there is no part of the world which offers to the scientfic explorer a more attractive field than the highlands of Peru and Bolivia.”
Top photo: Machu Picchu, often referred to as “The Lost City of the Incas.” Photo 2: Hiram Bingham at Machu Picchu in 1912