Various attempts to check simply the thermal expansion of glass without laborious measuring by dilatometer have been reported in a series of papers. The method described here is the one which makes use of the cylindrical glass-to-glass seals and observes the deviation of fringes in the photo-elastic stress pattern corresponding to the difference of expansion between two glasses.
It does not concern whether the tested glass is sealed inside or outside of the standard glass, inasmuch as the fact that the glass which shows higher expansion, introduces compression in the other and simultaneously tension in its own as is usually the case. The displacement of the fringe pattern is so sensitive to the residual stress that the slight difference of expansion is frequently detected even when the commonly used optical lever type dilatometer is unable to differentiate the expansion curves.
The experimental fact that the soda-lime glass is more sensitive than the soft lead glass for the same order of the difference of expansion will probably be attributed to the considerable difference of activation energy necessary for the release of stresses by viscous flow during cooling.
Since the displacement of the fringe is linearly proportional to the stress, we can calculate the residual stresses easily, the values of which should always be kept within a certain limit.
The method presented herein has been used in our laboratory and found to be practical for routine checking of the thermal expansion of glass.