the glass where it could equilibrate internal pressure with external pressure at a temperature at which some seed regrowth could occur in example 9 the glass was at a temperature above its softening point for about 2 5 hours whereas in example 8 the glass was at a temperature above 1580 u00b0 c for only about 18 minutes and above 1000 u00b0 c for about 26 minutes the slow cooling of example 9 would provide adequate time for some defect regrowth particularly if there is some residual gas within the seeds thus rapid cooling i e gt 10 u00b0 c minute constitutes the preferred practice this problem can be solved however by employing higher pressures of helium or using appropriate pressures of argon which gas does not diffuse into the glass as rapidly as helium helium with its small atomic diameter is the preferred gas for collapsing defects ranging in diameter up to about 300 u03bcm because it does not impart any gradient refractive index properties to the glass argon may be more useful where the defects are larger inasmuch as its larger atomic diameter hence lower diffusion rate results in a gradient in pressure for a longer interval of time than helium before diffusion of the argon into the.