Variations caused in heating curves of glass by heat treatment

This paper presents some of the variations that are produced in the heating curves of a glass by subjecting it previously to different heat treatments.The discussion incorporated shows that these variations are often fully indicative of the character of these previous treatments, and also that they bear a relation to the changes caused in numerous other characteristics of glass by the same means.The differences in the curves presented arise from differences in the thermal prop- erties of the glass, and especially from changes in the magnitude and character of the exothermic and endothermic effects.The physicochemical processes causing the latter effect are the reverse of those producing the exothermic; that is, 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 the resistance to the activity of the physicochemical processes causing these effects in glass increases very rapidly as the temperature is lowered.This makes it possible to suppress the exothermic 'processes to a great extent by rapid cooling and to prepare the way thereby for relatively large exothermic effects on subsequent heating.Since the preparatory exothermic activity is in- sufficient, the endothermic effects in the heating curves of chilled glasses are never pronounced, but they may become quite large in these curves after the glasses are annealed in a manner such that the exothermic processes have the opportunity to continue their activity into the range of relatively low annealing temperatures.;."'! Bureau of Standards Journal of Research [Vol.eof glass in the annealing range may be gained from a consideration of the changing conditions in any physicochemical system (for example, a solution) where the mdition depends upon the temperature, but normally does not imnltaneously with it.From the standpoint of investigating certain and characteristics, systems like glass yield advantages since their reaction rates are low and give opportunity for many observations during any ge of importance and also since the thermal effects caused by superheating and undercooling are readily detectable.The relation which the changes in physicochemical condition bear to certain problems in practical annealing is also considered, and from the discussion it will be evident that annealing procedures may often be manipulated so that the resultant changes in certain properties (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 prescribad for its general type.Although the tolerances of present-day standards are so lenient that adjustments of this rharacter 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 pre- cision is attempted.More important from the optical instrument standpoint, however, is the need of always annealing a glass in such a way that the physico- chemical condition is full}'' uniform throughout the whole piece or blank from which an optical part is produced.In many cases gradients no greater than a I] fraction of a degree will, if they exist during annealing, produce nonuniformities which cause greater disturbances than strains of considerable magnitude. CONTENTSPage I. Introduction 524 II.Method of obtaining and presenting the heating curves 527 III.General characteristics of the heating curves 528 1 \ .Annealing and "disannealing" 535 V. Detailed exposition of the curves 539 VI.General discussion and conclusions 548 I instruments it is evident thi tj lass besides possess- ing nigh degrees ol structural homogeneity should also be in a pbysi- .1920.