Symposium on Glass Papers preEented before the Division of Industrial and Engineering Chemistry a t the 85th Meeting of t h e American Chemical Society, Washington, D. C., March 26 t o 31, 1933.
Introduction F. C. FLIIVT,Hazel-Atlas Glass Company, Washington, Pa.
G
LASS is one of the oldest manufactured products of the world, and the industry has existed for thousands of years. Then it was purely as an art, and the workmen were artisans, not mechanics. The quantity produced was limited and the value of the product high, because each man then was limited in his own personal effort. The quality of the glass produced varied because there existed so many unknown factors in the production and use of the materials a t hand. In the last generation by the enlargement of units of manufacture, by the introduction of scientific study of the principles underlying the melting process, and by a greater knowledge of the qualities of the glass, the industry has exhibited an expansion in use of its product and a stability of quality that has amounted virtually to its rebirth. Kow its products are valued a t several hundred million dollars a year, and the amount of glass that is continuously under fire would represent a lake of molten glass waist deep and covering several acres. The largest industrial furnaces in the United States are used in the manufacture of glass. By the introduction of automatic machinery and the increase of the production from each unit, and by the production
of special glasses has a progress in science and engineering been forced on the industry that only reluctantly left its ruleof-thumb methods of former years. This new large-scale production, coupled with the widening use of glass, has demanded a new knowledge which must be accurate and diversified. Though glass might all answer to the same definition of “glassiness,” it must have properties more variable than the alloys of iron. It must be transparent to opaque, color!ees, multicolored, white or black, easily melted or “hard,” brilliant or dull, chemically resistant, or water-soluble, and all this with an accuracy of control worthy of the finest tools. To effect this knowledge and control has necessitated an unusual collaboration of physicists, chemists, and engineers. The problems have included the most difficult analyses, giant questions in fuel application, the construction of special equipment, and the meticulous control of chemical detail. Physical studies have been extremely difficult by the very nature of the substance, and the use of the product has brought to bear on it a phalanx of specialists studying and applying its special value in a multitude of fields.
Phase Equilibrium Relationships Determining Glass Compositions GEORGEW. AIo~EY, Geophysical Laboratory, Carnegie Institution of Washington, Washington, D. C.
M
ODERS glassware is made up of many clifferent types differing in their appearance, properties, uses, and compositions; but of all these types, those which are essentially mixtures of soda, lime, and silica make up an overwhelmingly preponderant proportion of the total production. hloreover, not only is most glassware largely composed of these three oxides, but their relative proportions always remain the same, within rather narrow limits. This has been true of glassware from the earliest days (4). DEFINITION OF GLASS It has not proved possible to set up a definition of glass in terms of specific physical properties which will properly classify divers types of glasslike materials and also place proper limits on the change of these properties with change of composition and temperature. It is feasible to define glass by a comparison with other substances, and in so doing we bring in the concept of phase equilibrium. For any glass composition there exists a temperature above which devitrification is impossible, and at which crystals already formed will be melted. This is the temperature of the beginning of crystallization on cooling or of the completion of melting on heating, and is known as the liquidus temperature. Above this temperature glass cannot
devitrify because the Crystalline products of devitrification dissolve in the liquid; below this temperature the liquid is unstable in respect t o the crystalline phases and, if given time, will change into a crystalline aggregate. From the liquidus temperature down to ordinary temperatures the devitrified condition is the thermodynamically stable one, and the glass persists in the unstable condition of an undercooled liquid only by reason of its great viscosity. There is every range of ease or difficulty with which the undercooled liquid may be obtained and with which it may be retained in the crystalline condition. Some glasses are practically impossible to crystallize even under the most favorable conditions without the aid of a flux or mineralizer; others can be made only in small quantities by the most rapid cooling possible. Similarly, some glasses persist indefinitely in the condition of an undercooled liquid a t ordinary temperatures, while others sooner or later spontaneously crystallize. It should be emphasized that in this discussion the term “liquid” is used in a highly technical sense. I n connection with phase equilibrium studies a liquid is a noncrystalline phase (not a gas) which limits the range of existence with increasing temperature of a crystalline phase, and the idea of fluidity or rigidity plays no part in this definition. The argument as to the propriety of calling a glass an undercooled
742
