TABLE OF CONTENTS The earliest glasses used by man were found in nature. The ease of formation of sharp edges on obsidians, for example, allowed the production of knives, arrow heads, and other cutting tools. These naturally occurring glasses, which result from the cooling of molten rock, or lava, contain a wide variety of components, including alkali, alkaline earths, and transition metal oxides. In every case, however, silica is found to be the major constituent of these materials.
Since naturally occurring glasses proved to be so useful to early man, it is not surprising that the desire to produce glasses at will developed thousands of years ago. Furthermore, since all known glasses were silicates, it is also not surprising to find that the earliest man-made glasses were also silicates. In fact, very few non-silicate glasses were known prior to 1900. As a result, the first theories advanced to explain why some materials form glasses while others do not, were based heavily on the existing knowledge of the behavior of silicate melts and the structure of silicate crystals. These theories tend to assume that some unique feature of certain melts leads to glass formation for those materials, while the lack of these features prevents the formation of glasses from other materials. These theories are often grouped under the heading of Structural Theories of Glass Formation.
In recent years, we have recognized the existence of a vast number of non-silicate glasses. In fact, we now know that polymers and metals can also...
