Indentation regions were formed in glass by pressing a hardened steel ball into the surface using loads below the fracture limit. These indented regions were found to recover with time and the process was studied with optical interference methods. A linear relationship was found between the logarithm of the depth of the indentation and the logarithm of the elapsed time after the load removal. The maximum load studied was about 5 × 105 psi. The indentation effect could still be observed at loads around 1 × 105 psi.
W. C. LEVENGOOD and T. S. VONG, "Observations Concerning Delayed Elastic Effects in Glass," J. Opt. Soc. Am. 49, 61-66 (1959)