Tin Smelting
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Tin Smelting 1 he influence of tin on ancient civilization cannot be overestimated. When alloyed with copper, tin forms bronze, a metal so important in the making of weapons and implements that an entire period of history (about 3000 to 1100 B.C., depending on location) is named "the Bronze Age." Tin is a soft, silvery-white metal now widely used in plating steel cans for food containers; it is also found in solder and metals used for bearings. Tin is malleable and adaptable to rolling, extrusion, spinning, and other kinds of cold-working. Its melting point (232°C) is low compared with common structural metals; its boiling point (2603°C), however, is high. The interval between these two temperatures is greater than for most other metals, which means that loss by volatilization from a liquid melt or during alloying is insignificant. Tin's chemical symbol, Sn, is derived from the Latin term stannum, which originally denoted an alloy of lead and silver. However, an imitation of this silver-lead alloy could be made substituting about 67% tin. This imitation proved so popular that by the 4th century A.D., stannum had come to mean tin. Tin exists in two common allotropes, or forms—white tin (also called beta tin) and gray tin (or alpha tin), also called "tin pest." Gray tin is powdery and of little use, but through a kinetic crystalline transformation, gradually changes to white tin at temperatures above 13.2°C. This transformation occurs particularly rapidly above 100°C. The undesirable reverse transformation—white tin gradually degenerating into gray tin—can occur at temperatures below 13.2°C, but is prevented by small additions of lead, copper, silver, gold, bismuth, or antimony. Aristotle had described gray tin in the 4th century B.C. Coming to the 1st century A.D., Roman historian Pliny mentioned the use of a lead-tin alloy—solder—for joining metals. The Romans also produced eating utensils and other articles of copper coated with tin, which is nontoxic.
The first use of tin in making bronze (usually 5-10% tin in copper) probably occurred in the Near East in the 4th millennium B.C., although accidental alloying of copper with tin impurities may have occurred before this. Early Egyptian copper implements contain as much as 2% tin, while early copper celts (axlike tools for dressing timbers) in Ireland contain up to 1% tin. Both of these alloys are probably accidental mixtures. At the site of the city of Ur, bronze articles dating from about 3500 B.C. have been found. In Iran, bronze weapons, ornaments, tools, and chariot fittings dating from as early as 3000 B.C. have been found. As a metal, bronze is harder and more easily cast in molds than pure copper. It was therefore preferred for the manufacture of tools, weapons, and ornaments. For thousands of years, tin bronze proved to be the best metal available for a wide variety of uses until it was superseded by iron. Bronze was not superseded as a corrosion-resistant alloy until the introduction of stainless steel in this century. Copper was widely available in the an
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