The Determination Of Thermal Expansion Differences Between Glasses By The Measurement Of Birefringence In Seals

It is shown that the high sensitivity of the Senarmont compensator can be exploited in measurements on seals in which the retardation is very nonuniform if an appropriate masking device is employed; such measurements can then be very sensitive indicators of composition differences. An optical bench using a Senarmont compensator with half shade device is described which can give retardation measurements to ±1.5 nm across block seals. Measurements made on a set of soda-lime-silica glasses showed that expansion coefficient differences could be determined to ±0.3x10-8 deg C-1 which is as sensitive to composition as Preston density determinations. The results could not be compared accurately with the measured dilatometric curves owing to the uncertainty of the setting temperatures.