VARIATIONS CAUSED IN THE HEATING CURVES OF GLASS BY HEAT TREATMENT1
Authors: Tool A. Q. , Eicitlin C. G. 
Published: June 2006

The variations produced in the heating curves of a glass by previously subjecting it to different heat treatments are often fully indicative of the nature of these previous treatments, and they also bear a relation to changes caused in numerous other characteristics of glass by this same means.

The differences in the heating curves arise from differences in the thermal properties of the glass and especially from changes in the magnitude and character of the exothermic and endothermic effects. The physico‐chemical processes causing the endothermic are the reverse of those producing the exothermic, i.e., the two effects are related as in the case of the similar and well‐known phenomena observed at an inversion point such as is found in many crystals. They are, however, not confined to the usual relatively narrow temperature ranges and are also much more subject to undercooling and superheating.

Apparently this resistance to the physico‐chemical processes increases rapidly as the temperature is lowered making it possible by rapid cooling to suppress the exothermic processes to a great extent and thereby prepare the way for relatively large exothermic effects on subsequent heating. Since the preparatory exothermic activity is insufficient, the endothermic effects in the heating curves of chilled glasses are never pronounced but may become quite large in these curves if in the annealing, the exothermic processes have the opportunity to continue their activity into the range of relatively low annealing temperatures.

During annealing the exothermic activity progresses more and more slowly toward a limit which may be advanced by reducing the annealing temperature. The physicochemical condition which the glass would have if the limit for any given annealing temperature were reached is termed the equilibrium condition of the glass for that temperature and such an equilibrium is presumably kinetic in character. The continuous series made up of all these equilibrium conditions corresponding to the various temperatures in the annealing range is called the equilibrium sequence. It is shown that about any portion of such a sequence, a glass may be made to follow any number of apparently closed cycles and that in traversing these cycles it passes through a series of intermediate conditions which in general do not coincide with the equilibrium sequence except where this is practically insured by very thorough annealing at constant temperatures. At ordinary atmospheric temperatures, the usual type of glass is always in such an intermediate series since in ordinary practice it is impossible to extend the realizable equilibrium sequence tci temperatures much below the lower limit of the practical annealing range. In this range, however, very low rates of cooling or heating will cause a glass to follow its equilibrium sequence with relative closeness after coincidence has once been practically established.

In view of the present insufficient knowledge concerning the constitution of glass, the particular processes involved in these changes of physico‐chemical condition are presumably too complex and varied for analysis. Apparently, however, the activity of the exothermic type gives rise to a series of molecular readjustments which often limit the ability of the glass to reach other conditions having possibly greater stability especially at relatively low temperatures. In some cases these readjustments may insure a greater permanency to the vitreous state as long as the glass is not heated above the temperatures producing them. A relatively clear conception of these readjustments and of the general behavior of glass in the annealing range may be gained from a consideration of the changing conditions in any physico‐chemical system, e.g., a solution, where the equilibrium condition depends upon the temperature but normally does not change simultaneously with it. From the standpoint of investigating certain properties and characteristics, systems like glass yield advantages since their reaction rates are low and give opportunity for many observations during any change of importance and also since the thermal effects caused by superheating and undercooling are readily detectable.

The relation which the changes in physico‐chemical condition bear to certain problems in practical annealing is also considered, and from the discussion it is evident that annealing procedures may often be manipulated so that the resultant changes in certain propertics (notably the refractivity) will cause a glass to meet standards of requirement which it would normally fail to reach if annealed according to a fixed schedule prescribed for its general type. Although the tolerances of present‐day standards are so lenient that adjustments of this character are seldom required or employed, it is believed that the time is rapidly approaching when an extended use will be made of them especially in those cases where a highly‐standardized production of optical instruments of great precision is attempted. More important from the optical instrument standpoint, however, is the need of always annealing a glass in such a way that the physicochemical condition is fully uniform throughout the whole piece or blank from which an optical part is produced. In many cases gradients no greater than a small fraction of a degree will, if they exist during annealing, produce nonuniformities which cause greater disturbances than strains of considerable magnitude.